Is it possible to change the libc for cpp program without source?

Question

I have built and installed another glibc from source, and I want to have existing executables written in c++ to run with the custom glibc for experimental purpose. In order to do this, i tried to change the loader of the executable. Firstly, a link named ld_linux-x86-64.so.2 was created under /lib64 , with its path pointing to the new loader

sudo ln /home/ubuntu/glibc-2.27-amd64/lib/ld-2.27.so /lib64/ld_linux-x86-64.so.2

Secondly, the loader path in the executable was modified via text editor, changing '/lib64/ld-linux-x86-64.so.2' into '/lib64/ld_linux-x86-64.so.2'. I launched the executable and got the following error:

./demo_cpp: error while loading shared libraries: libstdc++.so.6: cannot open shared object file: No such file or directory

The patched cpp program failed to run and it seems that the c++ std lib is missing. However, this method did work for program written by pure C. Using the method mentioned here, i tried the following command and get the exactly same error:

patchelf --set-interpreter /home/ubuntu/glibc-2.27-amd64/lib/ld-linux-x86-64.so.2 --set-rpath /home/ubuntu/glibc-2.27-amd64/lib demo_cpp

So I wonder if it is possible to change glibc for a cpp program? Do i need to build cpp std lib from source too?

Loading an alternative shared library should not require installation of the entire system loader. That's something that's completely irrelevant. — Sam Varshavchik, Mar 27 '21 at 14:48
@SamVarshavchik Does that mean i can load modified glibc without changing the loader of the executable? How to do that? — kayochin, Mar 27 '21 at 14:51
This is explained in the `ld.so` manual page. it is unlikely that the executable will explicitly include the default library directories (`/lib` and `/usr/lib`) in it's `DT_RPATH`, if it has one. As such, the search path for shared libraries is overridable as explained in the manual page. The only exception to that is a suid program, which you did not mention. — Sam Varshavchik, Mar 27 '21 at 14:54
Read documentation of [GCC](http://gcc.gnu.org/), and compile with `g++ -Wall -Wextra -g -H demo.cpp -o demo_cpp` then run `ldd ./demo_cpp` — Basile Starynkevitch, Mar 27 '21 at 14:56
@BasileStarynkevitch I'm sorry the cpp code is causing misunderstanding. It is removed from question. Actually, my attempt is to patch glibc for existing cpp program without source — kayochin, Mar 27 '21 at 15:07
Sorry, I don't understand your question. Either improve it (by adding several paragraphs, with some [mre]) or send me an email (in English or in Russian) about it. Explain in several paragraphs how and why did you built and installed another glibc. See also https://linuxfromscratch.org/ — Basile Starynkevitch, Mar 27 '21 at 15:08
Are you allowed to use [strace(1)](https://man7.org/linux/man-pages/man1/strace.1.html) ? Are you allowed to use [schroot](https://linux.die.net/man/1/schroot) ? Please improve your question, and explain, in written English, what `schroot` and `strace` gave you. I am not sure you technically can patch glibc for existing cpp program without source, and in many countries that could be illegal — Basile Starynkevitch, Mar 27 '21 at 15:14
I guess you want to hijack some library functions. That's what LD_PRELOAD is good for. It's quite common topic on SO, especially hijacking malloc/calloc/realloc/free — Zsigmond Lőrinczy, Mar 28 '21 at 05:37
@ZsigmondLőrinczy thx. I will check the LD_PRELOAD trick, and see if and how it works for a cpp program — kayochin, Mar 28 '21 at 08:43

Basile Starynkevitch · Answer 1 · 2021-03-27T17:16:21.633

You need to read books about operating systems, compilers and linkers.

I recommend reading

Operating systems, three easy pieces, a good textbook about OSes.
the Dragon book, a good textbook about compilers.
Levine's Linkers and loaders book.
the documentation of GCC
the documentation of GNU binutils
the documentation of GDB

Once you have read all the above books, consider reading documentation of several Linux man pages, e.g:

strace(1), gcc(1), ld(1), ldd(1), pmap(1), ps(1), readelf(1)
syscalls(2), execve(2), fork(2), waitpid(2), mmap(2), chroot(2)
errno(3), dlopen(3), dlsym(3)
elf(5), proc(5)
credentials(7)
ld.so(8)

Study also the documentation and the source code of GNU libc and of the Linux kernel

Since they are free software !

So I wonder if it is possible to change glibc for a cpp program?

Yes it is possible, no it is not easy!

Is it possible to change the libc for cpp program without source?

1 Answers1

You need to read books about operating systems, compilers and linkers.

Study also the documentation and the source code of GNU libc and of the Linux kernel