I will be analyzing x86_64, don't know which glibc version (latest as of publishing this). Things should be faily similar for other architectures and versions though. If anyway you're reading this in the future, just be aware of that
anyway lets get started
As usually, code starts at _start.
It does some things, mainly setting up the stack and calling __libc_start_main with the arguments passed to the program. (auxiliary vector, argc, argv, envp, inits + finis, etc). Code is at start.S in glibc source code, under sysdeps/x86_64/start.S iirc.
It then calls __libc_start_main. Let's analyze it:
_start():
__libc_start_main():
- gets environment variables from stack
- stores the the stack_end
- finds the auxillary vector
- calls _dl_aux_init():
- calls _dl_parse_auxv(): to set up the parsed auxv values
no much to elaborate here, just parsing the values from the auxv vector so it'll be easier to access them later
- initlisizes rest of values from auxv (_dl_phdr, _dl_phnum, etc)
- calls tunables_init():
- sets up the tunables (see comment at the end for more info)
- calls ARCH_INIT_CPU_FEATURES() -
this is a relatively new feature in glibc (as of today, that is) which allows the user to modify some of glibc's behaviour. (For example: ELF hwcap, stack size, and many more. its really worth checking it out, it has some neat options I didn't know about) as I understood, it was made to have a unified way to modify glibc's behaviour at runtime, since up until now you had to use various differnt environment variables to do that, which was a bit messy.