The difference between “LSB executable” (ET_EXEC) and “LSB shared object” (ET_DYN)

elfgccld

With two files, one compiled and linked with gcc and the other manually with nasm and ld I get

ELF 32-bit LSB shared object …
ELF 32-bit LSB executable …

What's the difference between these two things? I can see with readelf -h that one is

Type: DYN (Shared object file)
Type: EXEC (Executable file)

I can see these documented on Wikipedia as ET_DYN and ET_EXEC. What are the practical differences between these two?

Best Answer

It seems this has something to do with Position Independent Executable (PIE). When GCC compiles executable by defaults it makes them PIE which changes the output flag on the ELF Header to ET_DYN.

You can disable the generation of PIE executables with

gcc -no-pie

If you're seeing this check the default options gcc is configured with gcc -v, you should see something like --enable-default-pie.

Answer inspired by this submission on StackOverflow. I intend to play more with it and explain more here.

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Linux – Different formats of object files in Linux

Core dumps are also object files, of a sort, and usually in ELF format, too. Running this program will probably produce a file named "core":

int
main(int ac, char **av)
{
        char *p = 0;
        *p = 'a';
        return 0;
}

My file command says:

core: ELF 32-bit LSB core file Intel 80386, version 1 (SYSV), SVR4-style, from './dump'

Linux – Difference Between Shared Object File and Relocatable File

See the System V ABI, which contains the specifications of the ELF format. It says

Relocation entries for different object files have slightly different interpretations for the r_offset member.

In relocatable files, r_offset holds a section offset. That is, the relocation section itself describes how to modify another section in the file; relocation offsets designate a storage unit within the second section.

In executable and shared object files, r_offset holds a virtual address. To make these files’ relocation entries more useful for the dynamic linker, the section offset (file interpretation) gives way to a virtual address (memory interpretation).

Relocatable files are still fully relocatable, whereas shared objects are one step further along the linking process and have been largely relocated. Shared objects are only relocatable if their code is position-independent (e.g. it was built with GCC’s -fPIC option).

Kernel modules need to be relocatable without being position-independent, so they are shipped as relocatable files.

Best Answer

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Linux – Difference Between Shared Object File and Relocatable File

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