Re: [musl] Getting access to section data during dynlink.c

[Farid Zakaria <fmzakari@ucsc.edu>]

Here is a link for posterity with the code:
https://github.com/fzakaria/musllibc/blob/ea4b030db9dfab2b6163883bf15e33f5b22d70f1/ldso/dynlink.c#L1997
For those that are curious how I achieved it.

On Tue, Oct 17, 2023 at 1:28 AM Szabolcs Nagy <nsz@port70.net> wrote:
>
> * Rich Felker <dalias@libc.org> [2023-10-16 18:04:11 -0400]:
> > On Mon, Oct 16, 2023 at 11:53:07PM +0200, Szabolcs Nagy wrote:
> > > note that (not too old) bfd ld and lld defines a hidden linker symbol
> > > __ehdr_start that at runtime resolves to where the ehdr is.
> > >
> > > example:
> > >
> > > #include <elf.h>
> > > #include <stdio.h>
> > >
> > > __attribute__((visibility("hidden"), weak)) extern char __ehdr_start[];
> > >
> > > int main()
> > > {
> > >     if (__ehdr_start) {
> > >             Elf64_Ehdr *ehdr = (void *)__ehdr_start;
> > >             printf("ehdr %p\n", ehdr);
> > >             Elf64_Phdr *phdr = (void *)(__ehdr_start + ehdr->e_phoff);
> > >             printf("phdr %p\n", phdr);
> > >     } else
> > >             printf("__ehdr_start is undefined\n");
> > >
> > >     // to compare against the actual mappings
> > >     char buf[9999];
> > >     FILE *f = fopen("/proc/self/maps","r");
> > >     size_t n = fread(buf, 1, sizeof buf, f);
> > >     fwrite(buf, 1, n, stdout);
> > > }
> > >
> > > this should work for 64bit elf exe if ehdr is mapped into memory.
> > >
> > > if you want link time error on an old linker instead of 0 __ehdr_start,
> > > then just drop "weak" and the runtime check. (the code as written assumes
> > > ehdr is not at exact 0 address, which is guaranteed by usual linux setups)
> >
> > Interesting -- perhaps we should find a way to use this in ldso to
> > find its own ehdr.
>
> for that use it is a bit target specific:
> the symbol address computation must be pc-relative with no dynamic reloc,
> e.g. 'weak' would create a got reloc so not usable before relocs are done.
>
> glibc switched using it (but can use auxv too), requires binutils >= 2.23.
> i think lld had issues with setting GOT[0] up with vaddr of _DYNAMIC
> which is what glibc was relying on previously on many targets.