├── .github └── workflows │ └── ci.yml ├── .gitignore ├── CMakeLists.txt ├── LICENSE.txt ├── README.md ├── doc └── man │ └── man1 │ └── copycat.1 ├── flake.lock ├── flake.nix ├── src ├── bin │ ├── ld_preload.c │ ├── ld_preload.h │ ├── main.c │ └── seccomp │ │ ├── seccomp_exec.c │ │ ├── seccomp_exec.h │ │ ├── seccomp_trap.c │ │ ├── seccomp_trap.h │ │ ├── util.c │ │ └── util.h └── lib │ ├── copycat.c │ ├── copycat.h │ └── syscalls │ ├── open.c │ ├── open.h │ ├── openat.c │ ├── openat.h │ ├── openat2.c │ └── openat2.h └── tests ├── CMakeLists.txt ├── benchmark.c ├── run-tests.sh └── tests_general.c /.github/workflows/ci.yml: -------------------------------------------------------------------------------- 1 | name: Continuous Integration 2 | on: [push, pull_request] 3 | jobs: 4 | build: 5 | runs-on: ubuntu-latest 6 | steps: 7 | - uses: actions/checkout@v4 8 | - uses: cachix/install-nix-action@v27 9 | with: 10 | nix_path: nixpkgs=channel:nixos-unstable 11 | - run: nix flake check --print-build-logs 12 | -------------------------------------------------------------------------------- /.gitignore: -------------------------------------------------------------------------------- 1 | build/ 2 | .cache/ 3 | tests_general 4 | benchmark 5 | .copycat.conf 6 | 7 | # Created by https://www.toptal.com/developers/gitignore/api/c,cmake 8 | # Edit at https://www.toptal.com/developers/gitignore?templates=c,cmake 9 | 10 | ### C ### 11 | # Prerequisites 12 | *.d 13 | 14 | # Object files 15 | *.o 16 | *.ko 17 | *.obj 18 | *.elf 19 | 20 | # Linker output 21 | *.ilk 22 | *.map 23 | *.exp 24 | 25 | # Precompiled Headers 26 | *.gch 27 | *.pch 28 | 29 | # Libraries 30 | *.lib 31 | *.a 32 | *.la 33 | *.lo 34 | 35 | # Shared objects (inc. Windows DLLs) 36 | *.dll 37 | *.so 38 | *.so.* 39 | *.dylib 40 | 41 | # Executables 42 | *.exe 43 | *.out 44 | *.app 45 | *.i*86 46 | *.x86_64 47 | *.hex 48 | 49 | # Debug files 50 | *.dSYM/ 51 | *.su 52 | *.idb 53 | *.pdb 54 | 55 | # Kernel Module Compile Results 56 | *.mod* 57 | *.cmd 58 | .tmp_versions/ 59 | modules.order 60 | Module.symvers 61 | Mkfile.old 62 | dkms.conf 63 | 64 | ### CMake ### 65 | CMakeLists.txt.user 66 | CMakeCache.txt 67 | CMakeFiles 68 | CMakeScripts 69 | Testing 70 | Makefile 71 | cmake_install.cmake 72 | install_manifest.txt 73 | compile_commands.json 74 | CTestTestfile.cmake 75 | _deps 76 | 77 | ### CMake Patch ### 78 | # External projects 79 | *-prefix/ 80 | 81 | # End of https://www.toptal.com/developers/gitignore/api/c,cmake 82 | -------------------------------------------------------------------------------- /CMakeLists.txt: -------------------------------------------------------------------------------- 1 | cmake_minimum_required(VERSION 3.23) 2 | project(copycat VERSION 0.1) 3 | 4 | set(LIB_TARGET "${PROJECT_NAME}") 5 | set(BIN_TARGET "${PROJECT_NAME}-bin") 6 | 7 | set(CMAKE_C_STANDARD 23) 8 | set(CMAKE_C_STANDARD_REQUIRED ON) 9 | set(CMAKE_C_EXTENSIONS OFF) 10 | 11 | 12 | file(GLOB_RECURSE LIB_SRCS "src/lib/*.c") 13 | file(GLOB_RECURSE BIN_SRCS "src/bin/*.c") 14 | 15 | add_library(${LIB_TARGET} SHARED ${LIB_SRCS}) 16 | target_include_directories(${LIB_TARGET} PUBLIC "src/lib") 17 | 18 | add_executable(${BIN_TARGET} ${BIN_SRCS}) 19 | target_include_directories(${BIN_TARGET} PRIVATE "src/bin") 20 | set_target_properties(${BIN_TARGET} PROPERTIES RUNTIME_OUTPUT_NAME "${PROJECT_NAME}") 21 | target_link_libraries(${BIN_TARGET} ${LIB_TARGET}) 22 | 23 | # install 24 | include(GNUInstallDirs) 25 | install(TARGETS ${LIB_TARGET}) 26 | install(TARGETS ${BIN_TARGET}) 27 | install(DIRECTORY "${CMAKE_SOURCE_DIR}/doc/man/" TYPE MAN) 28 | 29 | # testing 30 | if (BUILD_TESTING) 31 | enable_testing() 32 | add_subdirectory(tests) 33 | endif() 34 | -------------------------------------------------------------------------------- /LICENSE.txt: -------------------------------------------------------------------------------- 1 | GNU GENERAL PUBLIC LICENSE 2 | Version 3, 29 June 2007 3 | 4 | Copyright (C) 2007 Free Software Foundation, Inc. 5 | Everyone is permitted to copy and distribute verbatim copies 6 | of this license document, but changing it is not allowed. 7 | 8 | Preamble 9 | 10 | The GNU General Public License is a free, copyleft license for 11 | software and other kinds of works. 12 | 13 | The licenses for most software and other practical works are designed 14 | to take away your freedom to share and change the works. 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Such new versions will 567 | be similar in spirit to the present version, but may differ in detail to 568 | address new problems or concerns. 569 | 570 | Each version is given a distinguishing version number. If the 571 | Program specifies that a certain numbered version of the GNU General 572 | Public License "or any later version" applies to it, you have the 573 | option of following the terms and conditions either of that numbered 574 | version or of any later version published by the Free Software 575 | Foundation. If the Program does not specify a version number of the 576 | GNU General Public License, you may choose any version ever published 577 | by the Free Software Foundation. 578 | 579 | If the Program specifies that a proxy can decide which future 580 | versions of the GNU General Public License can be used, that proxy's 581 | public statement of acceptance of a version permanently authorizes you 582 | to choose that version for the Program. 583 | 584 | Later license versions may give you additional or different 585 | permissions. However, no additional obligations are imposed on any 586 | author or copyright holder as a result of your choosing to follow a 587 | later version. 588 | 589 | 15. Disclaimer of Warranty. 590 | 591 | THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY 592 | APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT 593 | HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY 594 | OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, 595 | THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 596 | PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM 597 | IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF 598 | ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 599 | 600 | 16. Limitation of Liability. 601 | 602 | IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING 603 | WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS 604 | THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY 605 | GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE 606 | USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF 607 | DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD 608 | PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), 609 | EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF 610 | SUCH DAMAGES. 611 | 612 | 17. Interpretation of Sections 15 and 16. 613 | 614 | If the disclaimer of warranty and limitation of liability provided 615 | above cannot be given local legal effect according to their terms, 616 | reviewing courts shall apply local law that most closely approximates 617 | an absolute waiver of all civil liability in connection with the 618 | Program, unless a warranty or assumption of liability accompanies a 619 | copy of the Program in return for a fee. 620 | 621 | END OF TERMS AND CONDITIONS 622 | 623 | How to Apply These Terms to Your New Programs 624 | 625 | If you develop a new program, and you want it to be of the greatest 626 | possible use to the public, the best way to achieve this is to make it 627 | free software which everyone can redistribute and change under these terms. 628 | 629 | To do so, attach the following notices to the program. It is safest 630 | to attach them to the start of each source file to most effectively 631 | state the exclusion of warranty; and each file should have at least 632 | the "copyright" line and a pointer to where the full notice is found. 633 | 634 | {one line to give the program's name and a brief idea of what it does.} 635 | Copyright (C) {year} {name of author} 636 | 637 | This program is free software: you can redistribute it and/or modify 638 | it under the terms of the GNU General Public License as published by 639 | the Free Software Foundation, either version 3 of the License, or 640 | (at your option) any later version. 641 | 642 | This program is distributed in the hope that it will be useful, 643 | but WITHOUT ANY WARRANTY; without even the implied warranty of 644 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 645 | GNU General Public License for more details. 646 | 647 | You should have received a copy of the GNU General Public License 648 | along with this program. If not, see . 649 | 650 | Also add information on how to contact you by electronic and paper mail. 651 | 652 | If the program does terminal interaction, make it output a short 653 | notice like this when it starts in an interactive mode: 654 | 655 | {project} Copyright (C) {year} {fullname} 656 | This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 657 | This is free software, and you are welcome to redistribute it 658 | under certain conditions; type `show c' for details. 659 | 660 | The hypothetical commands `show w' and `show c' should show the appropriate 661 | parts of the General Public License. Of course, your program's commands 662 | might be different; for a GUI interface, you would use an "about box". 663 | 664 | You should also get your employer (if you work as a programmer) or school, 665 | if any, to sign a "copyright disclaimer" for the program, if necessary. 666 | For more information on this, and how to apply and follow the GNU GPL, see 667 | . 668 | 669 | The GNU General Public License does not permit incorporating your program 670 | into proprietary programs. If your program is a subroutine library, you 671 | may consider it more useful to permit linking proprietary applications with 672 | the library. If this is what you want to do, use the GNU Lesser General 673 | Public License instead of this License. But first, please read 674 | . 675 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # Copycat 2 | 3 | [![Continuous Integration](https://github.com/vimpostor/copycat/actions/workflows/ci.yml/badge.svg)](https://github.com/vimpostor/copycat/actions/workflows/ci.yml) 4 | 5 | This library allows you to overwrite system calls of arbitrary binaries in an intuitive way. 6 | For example the following snippet tricks `cat` into opening another file than was given: 7 | ```bash 8 | echo "a" > /tmp/a 9 | echo "b" > /tmp/b 10 | COPYCAT="/tmp/a /tmp/b" copycat -- cat /tmp/a # this will print "b" 11 | # Success! cat was tricked into opening /tmp/b instead of /tmp/a 12 | ``` 13 | 14 | Internally `copycat` uses a modern [Seccomp Notifier](https://man7.org/linux/man-pages/man2/seccomp_unotify.2.html) implementation to reliably intercept system calls. 15 | This is more elegant and much faster than usual `ptrace`-based implementations. However due to this relatively new Linux Kernel feature, `copycat` only works on **Linux 5.9** or higher. Additionally, due to a [Linux kernel bug not notifying the supervisor when a traced child terminates](https://lore.kernel.org/all/20240628021014.231976-2-avagin@google.com/), it is recommended to use **Linux 6.11** or higher. 16 | 17 | # Building 18 | 19 | Note: Arch users can install the [copycat-git](https://aur.archlinux.org/packages/copycat-git) AUR package. 20 | 21 | `copycat` is built with `cmake`: 22 | ```bash 23 | cmake -B build 24 | cmake --build build 25 | 26 | # Usage 27 | COPYCAT="source destination" build/copycat -- /path/to/program 28 | 29 | # To install 30 | cmake --install build 31 | ``` 32 | 33 | # How does this work? 34 | 35 | Historically, system call interception was done using `ptrace()`. This has the disadvantage of being very slow, as `ptrace()` will trigger twice per system call. 36 | Using this method it is also incredibly cumbersome to overwrite system call arguments, and one quickly has to deal with architecture-specific quirks. 37 | 38 | Recent advancements in the [Seccomp Notifier](https://people.kernel.org/brauner/the-seccomp-notifier-cranking-up-the-crazy-with-bpf) API have made it possible to intercept any system call in a much more elegant way. 39 | This also offers significant speed improvements, now the performance impact is closer to running the application in a container. 40 | 41 | For a more detailed explanation see the [accompanying blog post](https://blog.mggross.com/intercepting-syscalls/). 42 | 43 | # Rules format 44 | 45 | Rules can be supplied via the `$COPYCAT` environment variable. Alternatively create a file with the name `.copycat.conf` and add the rules, one rule per line. 46 | 47 | Rules contain a source and destination that are split by a space. If the source ends with a trailing slash, the rule is recursive, i.e. the source is interpreted as directory and all folders and files within this directory are redirected. 48 | If the destination also ends with a trailing slash, then a directory to directory mapping is created and the prefix is always replaced. If only the source ends with a trailing slash, then all files are mapped to the same location. 49 | Otherwise the rule matches source literally, i.e. the rule matches only the single file with the exact name like source. 50 | 51 | ## Examples 52 | 53 | ```bash 54 | # Redirect /tmp/a to /tmp/b 55 | /tmp/a /tmp/b 56 | # Redirect all files and folders in /tmp/f recursively to files and folders in /etc/f 57 | /tmp/f/ /etc/f/ 58 | # Redirect all files and folders in /tmp/f to the single file /etc/f 59 | /tmp/f/ /etc/f 60 | ``` 61 | 62 | # Related work 63 | 64 | - [kafel](https://github.com/google/kafel) - This uses a similar approach for higher-level policy based filtering. It does not support modifying arguments of the system calls. 65 | -------------------------------------------------------------------------------- /doc/man/man1/copycat.1: -------------------------------------------------------------------------------- 1 | .TH "copycat" 1 "02 May 2022" "" "copycat Documentation" 2 | 3 | .SH NAME 4 | copycat \- Intercept system calls 5 | 6 | .SH SYNOPSIS 7 | .B copycat 8 | [\-hn] \-\- 9 | .I command 10 | 11 | .SH DESCRIPTION 12 | 13 | .P 14 | This command intercepts system calls for a program given via the 15 | .I command 16 | argument. This is mostly used to redirect hardcoded files in a binary. 17 | 18 | .P 19 | Redirecting rules are defined via the 20 | .I COPYCAT 21 | environment variable. For example to redirect all system calls accessing /tmp/a.txt to /tmp/b.txt, you would use 22 | .I COPYCAT="/tmp/a.txt /tmp/b.txt" 23 | to redirect them without needing to do any change to the binary. 24 | 25 | .TP 26 | .B \-h 27 | Show usage information. 28 | 29 | .TP 30 | .B \-n 31 | Do not use seccomp, but an alternative 32 | .I LD_PRELOAD 33 | implementation to intercept system calls. This alternative implementation has minimal performance impact, but does not work with all binaries, thus this option is disabled by default. 34 | Example binaries that do not work with this method include statically linked binaries and binaries that call system calls directly instead of through the libc interface. 35 | 36 | .SH EXIT STATUS 37 | The exit status will be passed through from the supervised process. 38 | 39 | .SH EXAMPLES 40 | The following example tricks cat into opening a different file than was given. 41 | .PP 42 | .in +2n 43 | .EX 44 | $ COPYCAT="/tmp/a.txt /tmp/b.txt" \fBcopycat\fP \-\- cat /tmp/a.txt 45 | .EE 46 | .in 47 | .PP 48 | 49 | .SH HOMEPAGE 50 | https://github.com/vimpostor/copycat 51 | 52 | Please report bugs and feature requests in the issue tracker. 53 | -------------------------------------------------------------------------------- /flake.lock: -------------------------------------------------------------------------------- 1 | { 2 | "nodes": { 3 | "nixpkgs": { 4 | "locked": { 5 | "lastModified": 1734424634, 6 | "narHash": "sha256-cHar1vqHOOyC7f1+tVycPoWTfKIaqkoe1Q6TnKzuti4=", 7 | "owner": "NixOS", 8 | "repo": "nixpkgs", 9 | "rev": "d3c42f187194c26d9f0309a8ecc469d6c878ce33", 10 | "type": "github" 11 | }, 12 | "original": { 13 | "owner": "NixOS", 14 | "ref": "nixos-unstable", 15 | "repo": "nixpkgs", 16 | "type": "github" 17 | } 18 | }, 19 | "quartz": { 20 | "inputs": { 21 | "nixpkgs": "nixpkgs" 22 | }, 23 | "locked": { 24 | "lastModified": 1734721134, 25 | "narHash": "sha256-1nZFWCuT8k9siQoIxo4lFzjiFfs3lx2f9bVZjtChdw0=", 26 | "owner": "vimpostor", 27 | "repo": "quartz", 28 | "rev": "87e0bd6813cb9cc66b353bb286b4032f447620c2", 29 | "type": "github" 30 | }, 31 | "original": { 32 | "owner": "vimpostor", 33 | "repo": "quartz", 34 | "type": "github" 35 | } 36 | }, 37 | "root": { 38 | "inputs": { 39 | "quartz": "quartz" 40 | } 41 | } 42 | }, 43 | "root": "root", 44 | "version": 7 45 | } 46 | -------------------------------------------------------------------------------- /flake.nix: -------------------------------------------------------------------------------- 1 | { 2 | description = "A library for intercepting system calls"; 3 | inputs = { 4 | quartz.url = "github:vimpostor/quartz"; 5 | }; 6 | 7 | outputs = { self, quartz }: quartz.lib.eachSystem (system: 8 | let 9 | pkgs = quartz.inputs.nixpkgs.legacyPackages.${system}; 10 | stdenvs = [ { name = "gcc"; pkg = pkgs.gcc14Stdenv; } { name = "clang"; pkg = pkgs.llvmPackages_18.stdenv; } ]; 11 | defaultStdenv = (builtins.head stdenvs).name; 12 | makeStdenvPkg = env: env.mkDerivation { 13 | pname = "copycat"; 14 | version = quartz.lib.cmakeProjectVersion ./CMakeLists.txt; 15 | 16 | src = ./.; 17 | 18 | nativeBuildInputs = with pkgs; [ 19 | cmake 20 | libseccomp 21 | ]; 22 | }; 23 | in { 24 | packages = { 25 | default = self.outputs.packages.${system}.${defaultStdenv}; 26 | } // builtins.listToAttrs (map (x: { name = x.name; value = makeStdenvPkg x.pkg; }) stdenvs); 27 | checks = { 28 | format = pkgs.runCommand "format" { src = ./.; nativeBuildInputs = [ pkgs.clang-tools pkgs.git ]; } "mkdir $out && cd $src && find . -type f -path './*\\.[hc]pp' -exec clang-format -style=file --dry-run --Werror {} \\;"; 29 | } // builtins.listToAttrs (map (x: { name = "tests-" + x.name; value = (makeStdenvPkg x.pkg).overrideAttrs (finalAttrs: previousAttrs: { 30 | doCheck = true; 31 | cmakeFlags = ["-DBUILD_TESTING=ON"]; 32 | } 33 | ); }) stdenvs); 34 | } 35 | ); 36 | } 37 | -------------------------------------------------------------------------------- /src/bin/ld_preload.c: -------------------------------------------------------------------------------- 1 | #include "ld_preload.h" 2 | 3 | #define MAX_ENV_SIZE 256 4 | 5 | int ld_exec(const char *file, char *const argv[]) { 6 | // copy environment to add our own variable 7 | char *env[MAX_ENV_SIZE]; 8 | size_t env_size = 0; // holds the size of the dynamically allocated part, the real size is 2 greater 9 | for (size_t i = 0; i < MAX_ENV_SIZE - 2 && environ[i] != NULL; ++i) { 10 | env[i] = strdup(environ[i]); 11 | env_size++; 12 | } 13 | env[env_size] = "LD_PRELOAD=/usr/lib/libcopycat.so"; // TODO: Fix the hardcoded path 14 | env[env_size + 1] = NULL; 15 | 16 | int status_code = execvpe(file, argv, env); 17 | 18 | // if we reach this, then execvp failed 19 | perror(file); 20 | 21 | // free environment again 22 | for (size_t i = 0; i < env_size; ++i) { 23 | free(env[i]); 24 | } 25 | return status_code; 26 | } 27 | -------------------------------------------------------------------------------- /src/bin/ld_preload.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | // needed for execvpe 4 | #define _GNU_SOURCE 5 | #include 6 | #include 7 | #include 8 | #include 9 | 10 | 11 | int ld_exec(const char *file, char *const argv[]); 12 | -------------------------------------------------------------------------------- /src/bin/main.c: -------------------------------------------------------------------------------- 1 | #include 2 | 3 | #include "copycat.h" 4 | #include "ld_preload.h" 5 | #include "seccomp/seccomp_exec.h" 6 | 7 | void show_usage() { 8 | printf("Usage: copycat /path/to/program\n"); 9 | } 10 | 11 | int main(int argc, char *argv[]) 12 | { 13 | // parse args 14 | bool use_seccomp = true; 15 | bool show_help = false; 16 | int opt; 17 | static struct option long_opts[] = { 18 | { "help", no_argument, NULL, 'h' }, 19 | { "no-seccomp", no_argument, NULL, 'n' }, 20 | { NULL, 0, NULL, 0 } 21 | }; 22 | while ((opt = getopt_long(argc, argv, "hn", long_opts, NULL)) != -1) { 23 | switch (opt) { 24 | case 'h': 25 | show_help = true; 26 | break; 27 | case 'n': 28 | use_seccomp = false; 29 | break; 30 | case '?': 31 | show_help = true; 32 | break; 33 | default: 34 | fprintf(stderr, "?? getopt returned character code 0%o ??\n", opt); 35 | show_help = true; 36 | } 37 | } 38 | 39 | int status_code = EXIT_SUCCESS; 40 | if (show_help || argv[optind] == NULL) { 41 | show_usage(); 42 | } else { 43 | // actually run the executable 44 | const char *program = argv[optind]; 45 | char **const program_args = argv + optind; 46 | if (use_seccomp) { 47 | // seccomp 48 | status_code = seccomp_exec(program, program_args); 49 | } else { 50 | // LD_PRELOAD 51 | status_code = ld_exec(program, program_args); 52 | } 53 | } 54 | 55 | return status_code; 56 | } 57 | -------------------------------------------------------------------------------- /src/bin/seccomp/seccomp_exec.c: -------------------------------------------------------------------------------- 1 | #include "seccomp_exec.h" 2 | 3 | #include 4 | #include 5 | #include 6 | #include 7 | #include 8 | #include 9 | #include 10 | #include 11 | #include 12 | 13 | #include "syscalls/openat2.h" 14 | 15 | // list of all syscalls to trap 16 | static const int scalls[] = { 17 | __NR_open, 18 | __NR_openat, 19 | __NR_openat2, 20 | }; 21 | 22 | void handle_child_exit(int) { 23 | // This hacky workaround is only needed for old Linux kernel versions. With latest Linux, 24 | // SECCOMP_IOCTL_NOTIF_RECV will return to the caller properly once the supervised child exits. 25 | // But with older kernel versions there is a bug, where the call will hang indefinitely, 26 | // so we have to watch for the child exiting and just terminate from here as a workaround. 27 | // Otherwise the program would never terminate. 28 | exit(0); 29 | } 30 | 31 | int seccomp_child(const char *file, char *const argv[], struct seccomp_state *state) { 32 | // agree to not gain any new privs, see man 2 seccomp section SECCOMP_SET_MODE_FILTER 33 | prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); 34 | 35 | state->listener = user_trap_syscalls(scalls, ARRAY_SIZE(scalls), SECCOMP_FILTER_FLAG_NEW_LISTENER); 36 | // check if syscall trap setup was successful 37 | if (state->listener < 0) { 38 | perror("user_trap_syscalls"); 39 | return -1; 40 | } 41 | 42 | // send the listener to the parent; also serves as synchronization 43 | if (send_fd(state->sk_pair[1], state->listener) < 0) { 44 | return -1; 45 | } 46 | close(state->listener); 47 | 48 | // replace with target process 49 | int res = execvp(file, argv); 50 | 51 | // if we reach this, then execvp failed 52 | perror(file); 53 | return res; 54 | } 55 | 56 | int seccomp_parent(struct seccomp_state *state) { 57 | int exit_code = EXIT_FAILURE; 58 | 59 | // get the listener from the child 60 | state->listener = recv_fd(state->sk_pair[0]); 61 | if (state->listener < 0) { 62 | return -1; 63 | } 64 | 65 | // first get a pidfd 66 | int pidfd = pidfd_open(state->task_pid, 0); 67 | if (pidfd < 0) { 68 | perror("pidfd_open"); 69 | return -1; 70 | } 71 | 72 | // check for old Linux kernel version 73 | struct utsname uts; 74 | if (!uname(&uts)) { 75 | // to work around a Linux kernel bug, we may need to install a signal handler so that we can quit the supervisor, 76 | // once the target has terminated. This is not needed with Linux >= 6.11 anymore, as the bug has been fixed: 77 | // https://lore.kernel.org/all/20240628021014.231976-2-avagin@google.com/ 78 | const char *version = uts.release; 79 | if (strverscmp(version, "6.11") < 0) { 80 | fprintf(stderr, "WARNING: You are using an old Linux kernel version %s, installing a fallback workaround to terminate on SIGCHLD...\n", version); 81 | struct sigaction sa; 82 | sa.sa_handler = handle_child_exit; 83 | sa.sa_flags = 0; 84 | sigemptyset(&sa.sa_mask); 85 | if (sigaction(SIGCHLD, &sa, NULL) == -1) { 86 | perror("sigaction(SIGCHLD)"); 87 | return -1; 88 | } 89 | } 90 | } 91 | 92 | // setup supervisor 93 | struct seccomp_notif *req; 94 | struct seccomp_notif_resp *resp; 95 | struct seccomp_notif_sizes sizes; 96 | 97 | if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) { 98 | perror("seccomp(GET_NOTIF_SIZES)"); 99 | return -1; 100 | } 101 | req = malloc(sizes.seccomp_notif); 102 | resp = malloc(sizes.seccomp_notif_resp); 103 | memset(resp, 0, sizes.seccomp_notif_resp); 104 | 105 | while (true) { 106 | memset(req, 0, sizes.seccomp_notif); 107 | if (ioctl(state->listener, SECCOMP_IOCTL_NOTIF_RECV, req)) { 108 | if (errno == ENOENT) { 109 | // When the child exits, SECCOMP_IOCTL_NOTIF_RECV will return with ENOENT, 110 | // in which case we can also just quit the supervisor. 111 | // But first we have to retrieve the child's exit code 112 | int wstatus; 113 | pid_t wait = waitpid(state->task_pid, &wstatus, WUNTRACED | WCONTINUED); 114 | if (wait == state->task_pid && WIFEXITED(wstatus)) { 115 | // Hand over the same exit code 116 | exit_code = WEXITSTATUS(wstatus); 117 | } 118 | } else { 119 | perror("ioctl recv"); 120 | } 121 | break; 122 | } 123 | if (handle_req(req, resp, state->listener, pidfd) < 0) { 124 | break; 125 | } 126 | } 127 | 128 | // cleanup 129 | free(resp); 130 | free(req); 131 | close(pidfd); 132 | close(state->listener); 133 | exit(exit_code); 134 | } 135 | 136 | int seccomp_exec(const char *file, char *const argv[]) { 137 | struct seccomp_state state; 138 | if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, state.sk_pair) < 0) { 139 | perror("socketpair"); 140 | return -1; 141 | } 142 | 143 | state.task_pid = fork(); 144 | if (state.task_pid) { 145 | return seccomp_parent(&state); 146 | } else { 147 | return seccomp_child(file, argv, &state); 148 | } 149 | } 150 | 151 | bool cookie_valid(int listener, struct seccomp_notif *req) { 152 | return ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req->id) == 0; 153 | } 154 | 155 | int pidfd_open(pid_t pid, unsigned int flags) { 156 | errno = 0; 157 | return syscall(__NR_pidfd_open, pid, flags); 158 | } 159 | 160 | int pidfd_getfd(int pidfd, int targetfd, unsigned int flags) { 161 | errno = 0; 162 | return syscall(__NR_pidfd_getfd, pidfd, targetfd, flags); 163 | } 164 | 165 | int handle_req(struct seccomp_notif *req, 166 | struct seccomp_notif_resp *resp, int listener, int pidfd) 167 | { 168 | char path[PATH_MAX]; 169 | int ret = -1, mem; 170 | 171 | int dirfd = -1, proxy_dirfd = -1; 172 | char pathname[PATH_MAX]; 173 | const char *proxy_pathname = NULL; 174 | int flags; 175 | mode_t mode; 176 | struct open_how how; 177 | 178 | int nr = req->data.nr; 179 | 180 | resp->id = req->id; 181 | resp->error = -EPERM; 182 | resp->val = 0; 183 | resp->flags = 0; 184 | 185 | /* 186 | * Ok, let's read the task's memory to see what they wanted to open 187 | */ 188 | snprintf(path, sizeof(path), "/proc/%d/mem", req->pid); 189 | mem = open(path, O_RDONLY); 190 | if (mem < 0) { 191 | perror("open mem"); 192 | return -1; 193 | } 194 | 195 | /* 196 | * Now we avoid a TOCTOU: we referred to a pid by its pid, but since 197 | * the pid that made the syscall may have died, we need to confirm that 198 | * the pid is still valid after we open its /proc/pid/mem file. We can 199 | * ask the listener fd this as follows. 200 | * 201 | * Note that this check should occur *after* any task-specific 202 | * resources are opened, to make sure that the task has not died and 203 | * we're not wrongly reading someone else's state in order to make 204 | * decisions. 205 | */ 206 | if (!cookie_valid(listener, req)) { 207 | fprintf(stderr, "task died before we could map its memory\n"); 208 | goto out; 209 | } 210 | 211 | /* 212 | * Phew, we've got the right /proc/pid/mem. Now we can read it. Note 213 | * that to avoid another TOCTOU, we should read all of the pointer args 214 | * before we decide to allow the syscall. 215 | */ 216 | 217 | // the arguments are shifted one to the right for all syscalls but open 218 | const int argoffset = nr != __NR_open; 219 | ret = pread(mem, pathname, sizeof(pathname), req->data.args[argoffset]); 220 | if (ret < 0) { 221 | perror("pread"); 222 | goto out; 223 | } 224 | // Get the redirected file path 225 | if (!find_match(&proxy_pathname, pathname)) { 226 | // continue the syscall normally if there is no match 227 | resp->flags |= SECCOMP_USER_NOTIF_FLAG_CONTINUE; 228 | resp->error = 0; 229 | resp->val = 0; 230 | if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 && errno != ENOENT) { 231 | ret = -1; 232 | perror("ioctl send"); 233 | } 234 | goto out; 235 | } 236 | 237 | if (nr == __NR_openat2) { 238 | // read the special how struct 239 | ret = pread(mem, &how, sizeof(how), req->data.args[2]); 240 | if (ret < 0) { 241 | perror("pread"); 242 | goto out; 243 | } 244 | } 245 | 246 | if (nr != __NR_open) { 247 | dirfd = ls_int(req->data.args[0]); 248 | } 249 | // Pass-through dirfd from supervised process, in case it is needed for openat. 250 | // This is the case if: 251 | // - pathname is not absolute 252 | // - dirfd is NOT AT_FDCWD 253 | // 254 | // For more info see man openat(2) 255 | if (dirfd >= 0 && pathname[0] != '/') { 256 | // duplicate the file descriptor 257 | ret = pidfd_getfd(pidfd, dirfd, 0); 258 | if (ret < 0) { 259 | perror("pidfd_getfd"); 260 | goto out; 261 | } else { 262 | printf("Duplicating relative openat dirfd %d...", dirfd); 263 | proxy_dirfd = ret; 264 | } 265 | } 266 | 267 | if (!cookie_valid(listener, req)) { 268 | perror("post-read TOCTOU"); 269 | goto out; 270 | } 271 | 272 | flags = ls_int(req->data.args[argoffset + 1]); 273 | mode = (mode_t) ls_int(req->data.args[argoffset + 2]); 274 | 275 | // Make the final system call 276 | // This will resolve to our overloaded syscall 277 | if (nr == __NR_open) { 278 | ret = open(proxy_pathname, flags, mode); 279 | } else if (nr == __NR_openat) { 280 | ret = openat(proxy_dirfd, proxy_pathname, flags, mode); 281 | } else if (nr == __NR_openat2) { 282 | ret = openat2(proxy_dirfd, proxy_pathname, &how, sizeof(struct open_how)); 283 | } 284 | 285 | if (ret == -1) { 286 | ret = 0; 287 | // task got a signal and restarted the syscall 288 | if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 && errno != ENOENT) { 289 | perror("ioctl send"); 290 | goto out; 291 | } 292 | } else { 293 | // inject the file descriptor into the target process 294 | struct seccomp_notif_addfd addfd = {}; 295 | addfd.id = req->id; 296 | addfd.flags = SECCOMP_ADDFD_FLAG_SEND; // add the fd and return it, atomically 297 | addfd.srcfd = ret; 298 | resp->val = ret; 299 | // note that this branch does not need the SECCOMP_IOCTL_NOTIF_SEND, because this ADDFD call already includes it due to the SECCOMP_ADDFD_FLAG_SEND flag 300 | ret = ioctl(listener, SECCOMP_IOCTL_NOTIF_ADDFD, &addfd); 301 | if (ret == -1) { 302 | perror("SECCOMP_IOCTL_NOTIF_ADDFD"); 303 | goto out; 304 | } 305 | resp->error = 0; 306 | // we need to close the fd on our side, it will still be open on the target side, since we already sent it above 307 | close(addfd.srcfd); 308 | } 309 | out: 310 | if (proxy_dirfd >= 0) { 311 | close(proxy_dirfd); 312 | } 313 | close(mem); 314 | return ret; 315 | } 316 | -------------------------------------------------------------------------------- /src/bin/seccomp/seccomp_exec.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | #define _GNU_SOURCE 4 | #include 5 | #include 6 | #include 7 | #include 8 | 9 | #include "seccomp_trap.h" 10 | 11 | struct seccomp_state { 12 | int sk_pair[2]; 13 | int listener; 14 | pid_t task_pid; 15 | }; 16 | 17 | void handle_child_exit(int); 18 | int seccomp_child(const char *file, char *const argv[], struct seccomp_state *state); 19 | int seccomp_parent(struct seccomp_state *state); 20 | int seccomp_exec(const char *file, char *const argv[]); 21 | int pidfd_open(pid_t pid, unsigned int flags); 22 | int pidfd_getfd(int pidfd, int targetfd, unsigned int flags); 23 | int handle_req(struct seccomp_notif *req, struct seccomp_notif_resp *resp, int listener, int pidfd); 24 | -------------------------------------------------------------------------------- /src/bin/seccomp/seccomp_trap.c: -------------------------------------------------------------------------------- 1 | #include "seccomp_trap.h" 2 | 3 | #include 4 | #include 5 | #include 6 | #include 7 | #include 8 | 9 | // the maximum size of the BPF filter code 10 | #define MAX_FILTER_SIZE 16 11 | #define X32_SYSCALL_BIT 0x40000000 12 | 13 | int seccomp(unsigned int op, unsigned int flags, void *args) 14 | { 15 | errno = 0; 16 | return syscall(__NR_seccomp, op, flags, args); 17 | } 18 | 19 | int send_fd(int sock, int fd) 20 | { 21 | struct msghdr msg = {}; 22 | struct cmsghdr *cmsg; 23 | char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c'; 24 | struct iovec io = { 25 | .iov_base = &c, 26 | .iov_len = 1, 27 | }; 28 | 29 | msg.msg_iov = &io; 30 | msg.msg_iovlen = 1; 31 | msg.msg_control = buf; 32 | msg.msg_controllen = sizeof(buf); 33 | cmsg = CMSG_FIRSTHDR(&msg); 34 | cmsg->cmsg_level = SOL_SOCKET; 35 | cmsg->cmsg_type = SCM_RIGHTS; 36 | cmsg->cmsg_len = CMSG_LEN(sizeof(int)); 37 | *((int *)CMSG_DATA(cmsg)) = fd; 38 | msg.msg_controllen = cmsg->cmsg_len; 39 | 40 | if (sendmsg(sock, &msg, 0) < 0) { 41 | perror("sendmsg"); 42 | return -1; 43 | } 44 | 45 | return 0; 46 | } 47 | 48 | int recv_fd(int sock) 49 | { 50 | struct msghdr msg = {}; 51 | struct cmsghdr *cmsg; 52 | char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c'; 53 | struct iovec io = { 54 | .iov_base = &c, 55 | .iov_len = 1, 56 | }; 57 | 58 | msg.msg_iov = &io; 59 | msg.msg_iovlen = 1; 60 | msg.msg_control = buf; 61 | msg.msg_controllen = sizeof(buf); 62 | 63 | if (recvmsg(sock, &msg, 0) < 0) { 64 | perror("recvmsg"); 65 | return -1; 66 | } 67 | 68 | cmsg = CMSG_FIRSTHDR(&msg); 69 | 70 | return *((int *)CMSG_DATA(cmsg)); 71 | } 72 | 73 | int user_trap_syscalls(const int *nrs, size_t length, unsigned int flags) { 74 | struct sock_filter filter[MAX_FILTER_SIZE]; 75 | int i = 0; 76 | 77 | // load arch 78 | filter[i++] = (struct sock_filter) BPF_STMT(BPF_LD+BPF_W+BPF_ABS, offsetof(struct seccomp_data, arch)); 79 | 80 | // check arch 81 | filter[i++] = (struct sock_filter) BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, AUDIT_ARCH_X86_64, 0, 2); 82 | 83 | // load the number of the current syscall 84 | filter[i++] = (struct sock_filter) BPF_STMT(BPF_LD+BPF_W+BPF_ABS, offsetof(struct seccomp_data, nr)); 85 | 86 | // for the x32 ABI, all system call numbers have bit 30 set 87 | filter[i++] = (struct sock_filter) BPF_JUMP(BPF_JMP+BPF_JGE+BPF_K, X32_SYSCALL_BIT, 0, 1); 88 | 89 | // terminate the process if one of the earlier checks jumped here 90 | filter[i++] = (struct sock_filter) BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL_PROCESS); 91 | 92 | // now with the syscall nr still loaded, dynamically add checks for all syscall nrs we want to intercept 93 | for (int j = 0; j < length; ++j) { 94 | // jump if equal 95 | filter[i].code = (unsigned short) BPF_JMP+BPF_JEQ+BPF_K; 96 | // if equal (found a matching syscall), jump to early SECCOMP_RET_USER_NOTIF return 97 | // the jump distance is the difference between the current index and the last index of this for loop series (which has index len - 1), plus two to jump from the last index inside the for loop to the actual intended location, but again minus one, because BPF does one implicit jump forward on each instruction 98 | filter[i].jt = (length - 1) - j + 2 - 1; 99 | // if not equal (no matching syscall yet), try the next syscall nr in the next line (so don't jump at all) 100 | filter[i].jf = 0; 101 | // match against the syscall nr 102 | filter[i++].k = nrs[j]; 103 | } 104 | 105 | // didn't find a matching syscall, so return allow 106 | filter[i++] = (struct sock_filter) BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW); 107 | 108 | // this is the jump target. If we found a matching syscall, we return SECCOMP_RET_USER_NOTIF 109 | filter[i++] = (struct sock_filter) BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF); 110 | 111 | struct sock_fprog prog = { 112 | // i points to the next (still unused) index, so it is equal to the actual length 113 | .len = (unsigned short) i, 114 | .filter = filter, 115 | }; 116 | return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog); 117 | } 118 | -------------------------------------------------------------------------------- /src/bin/seccomp/seccomp_trap.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | /** 4 | * This file (seccomp_trap) is directly based on "samples/seccomp/user-trap.c" from the Linux kernel source tree. 5 | * The source code can be retreived at git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 6 | * The Linux kernel is licensed under the GPLv2 license, which is compatible with the GPLv3 license of this project. 7 | */ 8 | 9 | #define _GNU_SOURCE 10 | 11 | #include 12 | #include 13 | #include 14 | #include 15 | #include 16 | #include 17 | #include 18 | #include 19 | #include 20 | 21 | #include "util.h" 22 | 23 | #define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x))) 24 | 25 | int seccomp(unsigned int op, unsigned int flags, void *args); 26 | int send_fd(int sock, int fd); 27 | int recv_fd(int sock); 28 | int user_trap_syscalls(const int *nrs, size_t length, unsigned int flags); 29 | -------------------------------------------------------------------------------- /src/bin/seccomp/util.c: -------------------------------------------------------------------------------- 1 | #include "util.h" 2 | 3 | int ls_int(unsigned long long val) { 4 | return (int) val & 0x0f; 5 | } 6 | -------------------------------------------------------------------------------- /src/bin/seccomp/util.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | /** 4 | * Returns the least significant 32 bit part of a 64 bit integer as int 5 | * 6 | * seccomp returns 64 bit integer arguments, but only the 32 lowest bits contain the actual value 7 | * the upper 32 bits should be zeroed out 8 | * 9 | * Read man 2 seccomp for more details 10 | */ 11 | int ls_int(unsigned long long val); 12 | -------------------------------------------------------------------------------- /src/lib/copycat.c: -------------------------------------------------------------------------------- 1 | #include "copycat.h" 2 | 3 | #include 4 | 5 | #define COPYCAT_ENV "COPYCAT" 6 | char *copycat_env; 7 | 8 | #define COPYCAT_CONFIG ".copycat.conf" 9 | 10 | char path_buffer[PATH_MAX]; 11 | 12 | #define MAX_RULES_SIZE 64 13 | struct rule_t { 14 | const char *source; 15 | const char *dest; 16 | bool match_prefix; 17 | bool replace_prefix_only; 18 | }; 19 | struct rules_t { 20 | size_t size; 21 | struct rule_t table[MAX_RULES_SIZE]; 22 | } rules = {0}; 23 | 24 | /* 25 | * Adds a rule to the rule table that maps source to destination 26 | * This function assumes that source and destination are not empty strings 27 | */ 28 | void add_rule(char *source, char *destination) { 29 | char *src = strdup(source); 30 | char *dest = strdup(destination); 31 | bool match_prefix = false; 32 | bool replace_prefix_only = false; 33 | 34 | if (src[strlen(src) - 1] == '/') { 35 | // recursive mapping rule for directories 36 | // remove the trailing slash but set the match_prefix flag 37 | src[strlen(src) - 1] = '\0'; 38 | match_prefix = true; 39 | } 40 | 41 | if (dest[strlen(dest) - 1] == '/') { 42 | // trailing slash in destination 43 | // replace prefix only 44 | dest[strlen(dest) - 1] = '\0'; 45 | replace_prefix_only = true; 46 | } 47 | 48 | // actually add the rule 49 | rules.table[rules.size].source = src; 50 | rules.table[rules.size].dest = dest; 51 | rules.table[rules.size].match_prefix = match_prefix; 52 | rules.table[rules.size].replace_prefix_only = replace_prefix_only; 53 | rules.size++; 54 | } 55 | 56 | void parse_rule(char *line) { 57 | // split line into source and destination 58 | char *dest = strchr(line, ' '); 59 | if (dest != NULL) { 60 | // add a terminating null byte for the source string instead of the whitespace 61 | *dest = '\0'; 62 | // skip ahead from the whitespace to the actual first character of destination 63 | dest++; 64 | 65 | // guard against empty strings 66 | if (*dest && *line) { 67 | add_rule(line, dest); 68 | } 69 | } 70 | } 71 | 72 | void parse_rules(char *rls) { 73 | char *line = strtok(rls, "\n"); 74 | // split rules into lines 75 | while (line != NULL) { 76 | parse_rule(line); 77 | line = strtok(NULL, "\n"); 78 | } 79 | } 80 | 81 | void read_config() { 82 | FILE *f = fopen(COPYCAT_CONFIG, "r"); 83 | if (f == NULL) { 84 | return; 85 | } 86 | char *line = NULL; 87 | ssize_t read; 88 | size_t len; 89 | while ((read = getline(&line, &len, f)) != -1) { 90 | if (read > 0 && line[read - 1] == '\n') { 91 | // remove new line character from line 92 | line[read - 1] = '\0'; 93 | } 94 | parse_rule(line); 95 | } 96 | 97 | free(line); 98 | fclose(f); 99 | } 100 | 101 | // Returns true if a match was found 102 | bool find_match(const char **match, const char *query) { 103 | for (size_t i = 0; i < rules.size; ++i) { 104 | size_t rulesrc_len = strlen(rules.table[i].source); 105 | // check if we have a recursive rule (match only prefix) or if we have to check for a literal match 106 | size_t chars_to_compare = rulesrc_len; 107 | if (!rules.table[i].match_prefix) { 108 | chars_to_compare = MAX(chars_to_compare, strlen(query)); 109 | } 110 | 111 | // does the rule match? 112 | if (!strncmp(query, rules.table[i].source, chars_to_compare)) { 113 | char *result = (char *) rules.table[i].dest; 114 | if (rules.table[i].replace_prefix_only) { 115 | // extend result with rest of the input source 116 | // this means we have just replaced the prefix 117 | strcpy(path_buffer, result); 118 | result = path_buffer; 119 | strcat(result, query + rulesrc_len); 120 | } 121 | *match = result; 122 | return true; 123 | } 124 | } 125 | *match = query; 126 | return false; 127 | } 128 | 129 | void init() { 130 | copycat_env = getenv(COPYCAT_ENV); 131 | if (copycat_env != NULL) { 132 | parse_rules(copycat_env); 133 | } else { 134 | // read config file instead 135 | read_config(); 136 | } 137 | } 138 | 139 | void fini() { 140 | // free rules 141 | for (size_t i = 0; i < rules.size; ++i) { 142 | free((char *) rules.table[i].source); 143 | free((char *) rules.table[i].dest); 144 | } 145 | } 146 | -------------------------------------------------------------------------------- /src/lib/copycat.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | // needed for dlfcn.h RTLD_NEXT definition 4 | #define _GNU_SOURCE 5 | 6 | #include 7 | #include 8 | #include 9 | #include 10 | #include 11 | #include 12 | #include 13 | #include 14 | #include 15 | 16 | void add_rule(char *source, char *destination); 17 | void parse_rule(char *line); 18 | void parse_rules(char *rls); 19 | void read_config(); 20 | bool find_match(const char **match, const char *query); 21 | 22 | void init() __attribute__((constructor)); 23 | void fini() __attribute__((destructor)); 24 | -------------------------------------------------------------------------------- /src/lib/syscalls/open.c: -------------------------------------------------------------------------------- 1 | #include "open.h" 2 | 3 | int open(const char *pathname, int flags, mode_t mode) { 4 | return syscall(SYS_open, pathname, flags, mode); 5 | } 6 | -------------------------------------------------------------------------------- /src/lib/syscalls/open.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | #include "copycat.h" 4 | 5 | int open(const char *pathname, int flags, mode_t mode); 6 | -------------------------------------------------------------------------------- /src/lib/syscalls/openat.c: -------------------------------------------------------------------------------- 1 | #include "openat.h" 2 | 3 | int openat(int dirfd, const char *pathname, int flags, mode_t mode) { 4 | return syscall(SYS_openat, dirfd, pathname, flags, mode); 5 | } 6 | -------------------------------------------------------------------------------- /src/lib/syscalls/openat.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | #include "copycat.h" 4 | 5 | int openat(int dirfd, const char *pathname, int flags, mode_t mode); 6 | -------------------------------------------------------------------------------- /src/lib/syscalls/openat2.c: -------------------------------------------------------------------------------- 1 | #include "openat2.h" 2 | 3 | long openat2(int dirfd, const char *pathname, struct open_how *how, size_t size) { 4 | /** 5 | * glibc currently does not wrap openat2 6 | * Therefore we must manually implement it via syscall 7 | */ 8 | return syscall(SYS_openat2, dirfd, pathname, how, size); 9 | } 10 | -------------------------------------------------------------------------------- /src/lib/syscalls/openat2.h: -------------------------------------------------------------------------------- 1 | #pragma once 2 | 3 | #include "copycat.h" 4 | 5 | long openat2(int dirfd, const char *pathname, struct open_how *how, size_t size); 6 | -------------------------------------------------------------------------------- /tests/CMakeLists.txt: -------------------------------------------------------------------------------- 1 | add_executable(tests tests_general.c) 2 | 3 | add_executable(benchmark benchmark.c) 4 | target_link_libraries(benchmark m) 5 | 6 | add_test(NAME test COMMAND "${BIN_TARGET}" -- $) 7 | set_property(TEST test PROPERTY ENVIRONMENT "COPYCAT=/tmp/a /tmp/b") 8 | -------------------------------------------------------------------------------- /tests/benchmark.c: -------------------------------------------------------------------------------- 1 | #define _GNU_SOURCE 2 | 3 | #include 4 | #include 5 | #include 6 | #include 7 | #include 8 | #include 9 | #include 10 | #include 11 | 12 | #define EXPECT(cond) if (!(cond)) { fprintf(stderr, "Failed assert: %s\n", #cond); exit(EXIT_FAILURE); } 13 | 14 | void check_correct_fd(int fd) { 15 | EXPECT(fd >= 0); 16 | static char c; 17 | 18 | // we should be able to read from the file descriptor 19 | ssize_t r = read(fd, &c, 1); 20 | EXPECT(r); 21 | 22 | // the read char should be either 'a' or 'b' or 'c', depending on whether system calls are intercepted and which file is read 23 | EXPECT((c == 'a') || (c == 'b') || (c == 'c')); 24 | 25 | // close the descriptor again 26 | int a = close(fd); 27 | EXPECT(!a); 28 | } 29 | 30 | int do_openat(const char *filename) { 31 | return openat(0, filename, O_RDONLY); 32 | } 33 | 34 | // we need to do a benchmark without system calls involved, to check for general performance overhead 35 | int is_prime(int n) { 36 | for (int i = 2; i <= sqrt(n); ++i) { 37 | if ((n % i) == 0) { 38 | return 0; 39 | } 40 | } 41 | return 1; 42 | } 43 | 44 | int main(int argc, char *argv[]) 45 | { 46 | const char hot_filename[] = "/tmp/a"; 47 | const char cold_filename[] = "/tmp/c"; 48 | int f = -1; 49 | size_t n = 100000; 50 | struct timespec start, end; 51 | 52 | // hot path, every call is intercepted and changed 53 | clock_gettime(CLOCK_MONOTONIC_RAW, &start); 54 | for (size_t i = 0; i < n; ++i) { 55 | f = do_openat(hot_filename); 56 | check_correct_fd(f); 57 | } 58 | clock_gettime(CLOCK_MONOTONIC_RAW, &end); 59 | printf("openat (intercepted and modified): %lu.%lu s\n", end.tv_sec - start.tv_sec, end.tv_nsec - start.tv_nsec); 60 | 61 | // cold path, every call is intercepted but no arguments are changed (the BPF filter does not trigger) 62 | clock_gettime(CLOCK_MONOTONIC_RAW, &start); 63 | for (size_t i = 0; i < n; ++i) { 64 | f = do_openat(cold_filename); 65 | check_correct_fd(f); 66 | } 67 | clock_gettime(CLOCK_MONOTONIC_RAW, &end); 68 | printf("openat (intercepted and not modified): %lu.%lu s\n", end.tv_sec - start.tv_sec, end.tv_nsec - start.tv_nsec); 69 | 70 | // passive path, no call is intercepted (no system calls) 71 | int primes = 0; 72 | clock_gettime(CLOCK_MONOTONIC_RAW, &start); 73 | for (int i = 2; i < n; ++i) { 74 | primes += is_prime(i); 75 | } 76 | clock_gettime(CLOCK_MONOTONIC_RAW, &end); 77 | printf("path with no system calls: Found %d primes in %lu.%lu s\n", primes, end.tv_sec - start.tv_sec, end.tv_nsec - start.tv_nsec); 78 | 79 | return EXIT_SUCCESS; 80 | } 81 | -------------------------------------------------------------------------------- /tests/run-tests.sh: -------------------------------------------------------------------------------- 1 | #!/usr/bin/env bash 2 | 3 | set -e 4 | 5 | COPYCAT="/tmp/a /tmp/b" copycat -- tests 6 | 7 | echo -e "\nRunning benchmark without interception:" 8 | benchmark 9 | echo -e "\nRunning benchmark with interception:" 10 | COPYCAT="/tmp/a /tmp/b" build/copycat -- benchmark 11 | echo -e "\nRunning benchmark with strace:" 12 | strace --quiet=all -e openat -- benchmark 2>/dev/null 13 | echo -e "\nRunning benchmark with strace --seccomp-bpf:" 14 | strace -f --seccomp-bpf --quiet=all -e openat -- benchmark 2>/dev/null 15 | -------------------------------------------------------------------------------- /tests/tests_general.c: -------------------------------------------------------------------------------- 1 | #define _GNU_SOURCE 2 | 3 | #include 4 | #include 5 | #include 6 | #include 7 | #include 8 | #include 9 | 10 | #define EXPECT(cond) if (!(cond)) { fprintf(stderr, "Failed assert: %s\n", #cond); exit(EXIT_FAILURE); } 11 | 12 | void check_correct_fd(int fd) { 13 | static char c; 14 | 15 | // we should be able to read from the file descriptor 16 | ssize_t r = read(fd, &c, 1); 17 | EXPECT(r); 18 | 19 | // the read char should be 'b', and not 'a' 20 | EXPECT(c != 'a'); 21 | EXPECT(c == 'b'); 22 | 23 | // close the descriptor again 24 | int a = close(fd); 25 | EXPECT(!a); 26 | } 27 | 28 | void setup() { 29 | FILE *f = fopen("/tmp/b", "w"); 30 | EXPECT(f); 31 | fprintf(f, "b"); 32 | fclose(f); 33 | } 34 | 35 | int do_open(const char *filename) { 36 | return open(filename, O_RDONLY); 37 | } 38 | 39 | int do_openat(const char *filename) { 40 | return openat(0, filename, O_RDONLY); 41 | } 42 | 43 | int do_openat2(const char *filename) { 44 | struct open_how how = {O_RDONLY, 0, 0}; 45 | return syscall(SYS_openat2, 0, filename, &how, sizeof(struct open_how)); 46 | } 47 | 48 | int main(int argc, char *argv[]) 49 | { 50 | setup(); 51 | 52 | const char filename[] = "/tmp/a"; 53 | int f = -1; 54 | 55 | // open() 56 | f = do_open(filename); 57 | check_correct_fd(f); 58 | 59 | // openat() 60 | f = do_openat(filename); 61 | check_correct_fd(f); 62 | 63 | // openat2() 64 | f = do_openat2(filename); 65 | check_correct_fd(f); 66 | 67 | printf("All tests passed!\n"); 68 | return EXIT_SUCCESS; 69 | } 70 | --------------------------------------------------------------------------------