GccLibcPolyPatch

中文说明

Patch libc.so.6 to make gcc link to the lower version symbols by default.

This project only resolves symbol missing issues for libc.so.6 . For symbol missing issues in C++ (libstdc++.so), please refer to my other project.

This project is highly experimental and tested on Ubuntu 24.04 x86_64.

This project is sponsored by "Qihoo 360" and "Henan University of Science and Technology". Copyright (c) fgfxf, Qihoo360. All rights reserved.

For example:

cat /lib/x86_64-linux-gnu/libc.so
/* GNU ld script
   Use the shared library, but some functions are only in
   the static library, so try that secondarily.  */
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /lib/x86_64-linux-gnu/libc.so.6 /usr/lib/x86_64-linux-gnu/libc_nonshared.a  AS_NEEDED ( /lib64/ld-linux-x86-64.so.2 ) )

nm -D /lib/x86_64-linux-gnu/libc.so.6  | grep start_main
000000000002a200 T __libc_start_main@@GLIBC_2.34
000000000002a200 T __libc_start_main@GLIBC_2.2.5
nm -D /lib/x86_64-linux-gnu/libc.so.6  | grep memcpy

00000000000ba870 T memcpy@GLIBC_2.2.5
00000000000b1720 i memcpy@@GLIBC_2.14

Two @@ indicate the default when gcc/g++ linking. This means that when GCC links ELF files, it defaults to linking to __libc_start_main@@GLIBC_2.34 and memcpy@@GLIBC_2.14. On some older Linux systems, the nm command does not display @GLIBC, but this feature still works.

/lib64/libc.so.6: version `GLIBC_2.14' not found
/lib64/libc.so.6: version `GLIBC_2.38' not found

objdump  -T ./tool  | grep GLIBC_2.34
0000000000000000      DF *UND*	0000000000000000  GLIBC_2.34  __libc_start_main

objdump  -T ./tool  | grep GLIBC_2.14
0000000000000000      DF *UND*	0000000000000000  GLIBC_2.14  memcpy

On older systems without this symbol, the program cannot run.

git clone --recursive https://github.com/fgfxf/GccLibcPolyPatch.git
cd GccLibcPolyPatch
mkdir build
cd build
cmake ..
make -j 4

• Copy the system's libc.so.6 to build directory and then execute the patch program.
• Modify the path string of /lib/x86_64-linux-gnu/libc.so.6 in /lib/x86_64-linux-gnu/libc.so to the patched libc.so.6.patched path. DO NOT modify the system's libc.so.6 (/lib/x86_64-linux-gnu/libc.so.6), otherwise, it will cause the CPU soft fault.
• GCC compile and link

When linking ELF files with GCC later, it will default to using symbols from an older version. Copying the executable program to an older GLIBC system will still work, without the need to include the patch file.

nm -D libc.so.6.patched
...
00000000000ba870 T memcpy@@GLIBC_2.2.5
00000000000b1720 i memcpy@GLIBC_2.14
000000000002a200 T __libc_start_main@GLIBC_2.34
000000000002a200 T __libc_start_main@@GLIBC_2.2.5
...

gcc test.cpp  libc.so.6.patched

nm a.out  | grep start_main
                 U __libc_start_main@GLIBC_2.2.5

A case study is provided in the "example" folder, which involves compiling a C code on Ubuntu 24.04 and copying it to CentOS 5.11 for execution.

If the minimum version of the symbols in libc.so.6 is lower than the target system, this method cannot be resolved. For example:

nm -D /lib/x86_64-linux-gnu/libc.so.6  | grep getentropy
0000000000048710 T getentropy@@GLIBC_2.25
(getentropy is not yet available in GLIBC 2.2.5)

However, these new symbols will only be introduced if they are used in the code. You can refer to my other project, which involves partial static linking of libc.a/libstdc++.a and partial dynamic linking to libc-2.5.so / libstdc++.so.6.0.8 .