├── .gitignore
├── LICENSE
├── README.md
├── linker_example.cpp
└── runme.py
/.gitignore:
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1 | format_data*
2 | random_data*
3 | linker_example
4 | linker_example.o
5 |
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/LICENSE:
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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 |
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()
--------------------------------------------------------------------------------