├── .gitignore ├── LICENSE ├── README.md ├── linker_example.cpp └── runme.py /.gitignore: -------------------------------------------------------------------------------- 1 | format_data* 2 | random_data* 3 | linker_example 4 | linker_example.o 5 | -------------------------------------------------------------------------------- /LICENSE: -------------------------------------------------------------------------------- 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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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 | 635 | Copyright (C) 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 | Copyright (C) 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 | # Linking tutorial 2 | A step-by-step tutorial for low-level C++ linkage on Linux, meant for people with 0 C++ / low level knowledge. 3 | 4 | Make sure you have a C++ compiler installed. On Ubuntu, you need to 5 | ```sudo apt-get install build-essential``` 6 | You also need `Python>=3.8`. Should be fine on any recent Ubuntu. 7 | 8 | Run `./runme.py`. 9 | 10 | At every step, the script will print the shell command used and explain what it's doing / why. 11 | 12 | It will: 13 | - generate a file containing binary data 14 | - visualize its contents 15 | - build a cpp file 16 | - embed the data in the executable 17 | - disassemble the executable 18 | - show where the data is 19 | - verify that everything is correct ☺ 20 | -------------------------------------------------------------------------------- /linker_example.cpp: -------------------------------------------------------------------------------- 1 | #include // std::cout 2 | #include // std::setfill, std::setw 3 | #include // size_t 4 | #include // std::invalid_argument 5 | 6 | /* Define an array of bytes. The "extern" modifier tells the compiler to not assign any value to this. 7 | * Think of the linker as the Python importer, it's the thing that links functions to their implementation. 8 | * In C++ you can declare that a function exists, but it's implemented by someone else. 9 | * "extern" will ask the linker to fill in this symbol. */ 10 | extern char _binary_random_data_bin_start[]; 11 | extern char _binary_format_data_bin_start[]; 12 | 13 | int main( 14 | int argc /* number of cli arguments, "argument count" */, 15 | char* argv[] /* vector of pointers to arguments as char arrays, "argument vector" */ ) 16 | { 17 | if(argc <=1 ) 18 | { 19 | throw std::invalid_argument("Usage: ./test "); 20 | } 21 | 22 | /* Get first command line argument, argv[1]; 23 | * argv[0] is always the program name in Unix systems */ 24 | size_t num_bytes = std::stoull(argv[1]); 25 | 26 | 27 | size_t line_size = _binary_format_data_bin_start[0]; 28 | /* See below for std::cout */ 29 | std::cout << "_binary_random_data_bin_start is at address " 30 | << std::hex << static_cast(_binary_random_data_bin_start) 31 | << std::endl; 32 | std::cout << "_binary_format_data_bin_start is at address " 33 | << std::hex << static_cast(_binary_format_data_bin_start) 34 | << std::endl; 35 | std::cout << "Printing " << line_size << " bytes per line." << std::hex << std::endl; 36 | 37 | size_t print_cnt = 0; /* Count printed characters */ 38 | bool first_line = true; 39 | for (size_t array_idx = 0; array_idx < num_bytes; array_idx++) 40 | { 41 | /* You can rewrite part of the line below as: 42 | * 43 | * std::cout.operator<<(std::hex).operator<<( (size_t) binddata[i] ).operator<<(std::endl); 44 | * 45 | * operator<< returns the object it is called on, similar to what happens in JavaScript. 46 | * It's called "operator chaining". 47 | * Note that some overloads are defined outside the class, in order to print a string literal 48 | * for example: 49 | * 50 | * operator<< ( std::cout, "hello" ); 51 | * 52 | * std::setfill sets the padding character 53 | * std::setw sets the space occupied by the entry 54 | * std::hex modifies the stream to format what future input to hexadecimal 55 | * std::endl is a portable end of line terminator. It's different across Windows and Linux 56 | * 57 | * This kinda makes sense overall, but yes, streams in C++ are kinda cursed. 58 | * If you ever write production quality C++, use libfmt. 59 | */ 60 | std::cout 61 | << std::setfill('0') 62 | << std::setw(2) 63 | << std::hex /* parse to hex */ 64 | << static_cast(_binary_random_data_bin_start[array_idx] & 0xFF) /* Don't interpret it as ASCII */ 65 | << " "; 66 | /* Align with hexdump output. ++print_cnt is a combined increment-use. 67 | * print_cnt++ would be use-increment. */ 68 | if (++print_cnt % line_size == 0 && !first_line ) 69 | { 70 | std::cout << std::endl /* Portable newline */; 71 | } 72 | first_line = false; 73 | } 74 | } 75 | -------------------------------------------------------------------------------- /runme.py: -------------------------------------------------------------------------------- 1 | #!/bin/env python3 2 | import subprocess 3 | 4 | LINE_SIZE = 16 5 | NUM_BYTES = 64 6 | 7 | 8 | def print_wait_run(cmds): 9 | cmd_list = isinstance(cmds, list) 10 | if not cmd_list: 11 | cmds = [cmds] 12 | outputs = [] 13 | for cmd in cmds: 14 | print() 15 | input(cmd) 16 | proc = subprocess.run(cmd, shell=True, executable="/bin/bash", stdout=subprocess.PIPE) 17 | if (proc.returncode != 0): 18 | raise Exception("Failed with code: ", proc.returncode) 19 | output = proc.stdout.decode() 20 | print(output) 21 | outputs.append(output) 22 | 23 | return outputs if cmd_list else outputs[0] 24 | 25 | 26 | def validate_output(program_output_cmd, expected_output_cmd): 27 | proc = subprocess.run(program_output_cmd, shell=True, stdout=subprocess.PIPE) 28 | output = proc.stdout 29 | proc = subprocess.run(expected_output_cmd, shell=True, 30 | stdout=subprocess.PIPE) 31 | expected_output = proc.stdout 32 | 33 | # Decode hex bytes to their decimal values 34 | output = [ 35 | int(byte, 16) 36 | # Split lines, ignore the first lines with other info 37 | for line in output.decode().split('\n')[3:] 38 | for byte in line.split(' ') 39 | if byte # Ignore empty bytes 40 | ] 41 | 42 | expected_output = [ 43 | int(byte, 16) 44 | for line in expected_output.decode().split('\n') 45 | for byte in line.split(' ') 46 | if byte 47 | ] 48 | 49 | return all([lhs == rhs for lhs, rhs in zip(output, expected_output)]) 50 | 51 | 52 | def compile(): 53 | input("Press enter to advance\n") 54 | 55 | print(f"""Let's generate {NUM_BYTES} bytes of pseudorandom data. 56 | 57 | dd copies data from block devices. 58 | if is the input device 59 | /dev/urandom is a kernel device that returns pseudorandom data, seeded from /dev/random 60 | of is the output device/file 61 | bs is the block size — 1 byte 62 | count is how many blocks we want to copy""") 63 | print_wait_run( 64 | f"dd if=/dev/urandom of=random_data.bin bs=1 count={NUM_BYTES}") 65 | 66 | print(f"""Let's take a look at them. 67 | 68 | Print {LINE_SIZE} groups of 1 byte ({LINE_SIZE}/1), 69 | formatted (%) as 0-padded (0) 2 characters wide (2) hexadecimal (x) (%02x), 70 | followed by a newline (\\n).""") 71 | expected_output_cmd = "hexdump -e '" + \ 72 | f'{LINE_SIZE}/1'+' "%02x " "\\n" '+"' random_data.bin" 73 | print_wait_run(expected_output_cmd) 74 | 75 | print(f"""Let's store how many bytes we print per line ({LINE_SIZE}, hexadecimal: 0x{LINE_SIZE:x}), 76 | so the C++ program can format it properly.""") 77 | print_wait_run(f"echo -ne \"\\x{LINE_SIZE:x}\" > format_data.bin") 78 | 79 | print("""Let's make an object file out of that that we can link. 80 | 81 | ld links object files. 82 | -b binary tells the linker the input format is binary data 83 | -r means relocatable — tells the linker that the output is not an executable, 84 | but an object file, that will be linked in another invocation, in other words, 85 | the output will be \"relocated\" to the final binary. 86 | -o is the name of the output file.""") 87 | print_wait_run( 88 | ["ld -b binary -r -o random_data.o random_data.bin", 89 | "ld -b binary -r -o format_data.o format_data.bin"]) 90 | 91 | print("""Let's see what the symbols are named. 92 | 93 | -C means \"demangle\" — symbols have some garbage around them 94 | to prevent conflicts when two functions have the same name in different classes.""") 95 | print_wait_run( 96 | ["nm -C random_data.o", 97 | "nm -C format_data.o"]) 98 | 99 | print("""Now run the compiler. the -c flag means to skip linking for now. 100 | This will output a compiled object file named linker_example.o.""") 101 | print_wait_run("g++ -c linker_example.cpp") 102 | 103 | print("""Let's check if our symbols have been defined. 104 | 105 | Only look for ours, cause the binary will name a lot of symbols. 106 | Those are the standard library functions we have #include-d """) 107 | 108 | print_wait_run("nm -C linker_example.o | grep _binary") 109 | 110 | print("""There they are. \"U\" means undefined, as expected. 111 | They're waiting for the linker to define them. 112 | 113 | Let's link; let g++ call ld for us, it will set up paths for the standard library functions.""") 114 | print_wait_run( 115 | "g++ random_data.o format_data.o linker_example.o -o linker_example") 116 | 117 | print("Now let's run the program") 118 | program_output_cmd = f"./linker_example {NUM_BYTES}" 119 | print_wait_run(program_output_cmd) 120 | 121 | print("Do they match? Let's look at the file we linked again:") 122 | print_wait_run(expected_output_cmd) 123 | 124 | matches = validate_output(program_output_cmd, expected_output_cmd) 125 | if matches: 126 | print("(I think they do… ☺)") 127 | else: 128 | print("Mmm… something looks off…") 129 | raise Exception("Output doesn't match!") 130 | print() 131 | 132 | def disassemble(): 133 | print("""Now let's disassemble the executable we got. 134 | First, let's see where the symbols are located""") 135 | res = print_wait_run("nm -C linker_example | grep \"_binary.*_start\"") 136 | lines = [ line for line in res.split("\n") if line ] 137 | for line in lines: 138 | section = line.split()[1] 139 | if section != "D": 140 | raise Exception(f"Found symbol in invalid section: {section}") 141 | print("'D' here means that the symbols are in the .data section.\n") 142 | 143 | print("""Let's get the offset of the .data section in the ELF executable. 144 | 145 | objdump is the disassembler. 146 | -h dumps the program header. 147 | 148 | We care about the "file offset" field, which is the location of the section in the file, 149 | and the VMA (Virtual Memory Address) field. 150 | 151 | When programs are run they are copied into memory within their virtual address space. 152 | 153 | They think they're the only thing existing in memory, and that memory starts from address 0. 154 | This is of course not how memory works, but the kernel (Linux) hides what's actually happening. 155 | Every time they query an address, this address is assumed within the program's virtual address space. 156 | The kernel fetches the actual value from the actual memory address (physical memory address). 157 | """) 158 | print_wait_run('objdump -h linker_example') 159 | 160 | res = subprocess.run('objdump -h linker_example | grep "\.data "',shell=True, stdout=subprocess.PIPE).stdout.decode() 161 | cols = res.split() 162 | data_section_mem_offset = int(cols[3],16) 163 | data_section_file_offset = int(cols[5],16) 164 | 165 | input() 166 | print(f"From this line:") 167 | print(res) 168 | print(f"""The data section offset in the file in this case is 0x{data_section_file_offset:x}, \ 169 | while the offset in memory is 0x{data_section_mem_offset:x} 170 | """) 171 | 172 | print("""Let's see how the data section looks like. 173 | 174 | -t dumps symbol information 175 | -j .data specifies to only look at the .data section """) 176 | res = print_wait_run("objdump -t -j .data linker_example | grep \"_binary.*_start\"") 177 | lines = [ line for line in res.split("\n") if line ] 178 | symbol_offsets = dict() 179 | for line in lines: 180 | cols = line.split(' ') 181 | symbol = cols[-1] 182 | vma = int(cols[0],16) 183 | symbol_offsets[symbol] = vma-data_section_mem_offset 184 | print(f"{symbol} is at memory address 0x{vma:x}, so at offset 0x{symbol_offsets[symbol]:x} in the .data section") 185 | print() 186 | 187 | num_hexdump_lines = NUM_BYTES//LINE_SIZE + 1 188 | print("""Let's look at the file. 189 | 190 | Add the offset (_a) in hexadecimal format (x) at the beginning of each line. 191 | This is printed by hexdump if called with no arguments. 192 | However that would print 2-byte sequences, whereas we want single bytes. 193 | 194 | Then we grep enough lines after the address we're looking for to check the output.""") 195 | 196 | def grep_offset_cmd(symbol,n_lines): 197 | off = symbol_offsets[symbol]+data_section_file_offset 198 | off = off - (off % LINE_SIZE) # Align offset down to line size 199 | cmd = "hexdump -e '" + \ 200 | f'"%05_ax: " {LINE_SIZE}/1'+' "%02x " "\\n" '+"' linker_example" + \ 201 | f" | grep -A{n_lines-1} {off:x}:" 202 | return off,cmd 203 | symbol = "_binary_random_data_bin_start" 204 | off,cmd = grep_offset_cmd(symbol,num_hexdump_lines) 205 | print(f"Look for {symbol} at 0x{off:x}") 206 | res = print_wait_run(cmd) 207 | lines = [ line for line in res.split('\n') if line ] 208 | found_random_data = [ int(b,16) for line in lines for b in line.split(' ')[1:] if b ][:NUM_BYTES] 209 | 210 | symbol = "_binary_format_data_bin_start" 211 | off,cmd = grep_offset_cmd(symbol,1) 212 | print(f"Look for {symbol} at 0x{off:x}") 213 | ret = print_wait_run(cmd) 214 | found_line_size = int(ret.split(' ')[1],16) 215 | 216 | print("Do these check out?") 217 | if (found_line_size!=LINE_SIZE): 218 | raise Exception("Executable seems to report LINE_SIZE={line_size_found} instead of {LINE_SIZE}!") 219 | expected_random_data = [] 220 | with open("random_data.bin",'rb') as f: 221 | while byte := f.read(1): 222 | expected_random_data.append(int.from_bytes(byte, "little")) 223 | if not all( lhs == rhs for lhs,rhs in zip(found_random_data,expected_random_data)): 224 | raise Exception("Executable _binary_random_data_bin_start doesn't match random_data.bin") 225 | print("They look good to me ☺") 226 | 227 | 228 | if __name__ == "__main__": 229 | compile() 230 | disassemble() --------------------------------------------------------------------------------