What is the role of program interpreters in executable files?-CodePudding

I was going through disassembly of elf executables and understanding the elf format. In there, I saw lib64/ld-linux-x86-64.so.2 used as program interpreter in the generated executable.

My guess is: I had used printf in the source code, which had to be dynamically linked. When I checked through dynamic section, I was able to find a reference to libc.so.6 shared library (tag:DT_NEEDED). In my system, I found multiple files with that name in different directories:

sourav@ubuntu-VirtualBox:/$ sudo find / -name libc.so.6
/usr/lib/x86_64-linux-gnu/libc.so.6
find: ‘/run/user/1000/doc’: Permission denied
find: ‘/run/user/1000/gvfs’: Permission denied
/snap/snapd/13170/lib/x86_64-linux-gnu/libc.so.6
/snap/snapd/11107/lib/x86_64-linux-gnu/libc.so.6
/snap/core18/1988/lib/i386-linux-gnu/libc.so.6
/snap/core18/1988/lib/x86_64-linux-gnu/libc.so.6
/snap/core18/2128/lib/i386-linux-gnu/libc.so.6
/snap/core18/2128/lib/x86_64-linux-gnu/libc.so.6

So, I guess purpose of program interpreter is to resolve these names to the proper libraries and load them during execution. Is this correct?

It seems, we can also have executables with no program interpreter (which is the case for program interpreter itself). In that case, does system/os itself loads the shared library? If so, how does it resolves the path of library?

Is it possible to generate executable with no program interpreter using gcc? My gcc version is 'gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)'.

CodePudding user response：

So, I guess purpose of program interpreter is to resolve these names to the proper libraries and load them during execution. Is this correct?

Yes, but that that's a bit minimalistic. Loading dynamic libraries involves locating them, loading or mapping them into memory if necessary, and resolving dynamic symbols within, possibly lazily, for multiple kinds of relocations. It involves recursively loading the libraries' own needed libraries. Also, in a dynamically linked executable, the program interpreter provides the program entry point (from the kernel's perspective), so it is also responsible for setting up and entering the program-specific entry point (for example, main() in a C or C program).

It seems, we can also have executables with no program interpreter (which is the case for program interpreter itself). In that case, does system/os itself loads the shared library? If so, how does it resolves the path of library?

You can have ELF executables without a program interpreter, but they are not dynamically linked, at least not in the ELF sense. There are no shared libraries to load, and certainly the system does not load any.

Is it possible to generate executable with no program interpreter using gcc? My gcc version is 'gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)'.

If you have static versions of all needed libraries available then you should be able to achieve that by including the -static option on the command line when you link the program. It is entirely possible, however, that you do not have the needed static libraries, even if libc is the only library you need.