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

[Szabolcs Nagy <nsz@port70.net>]

* 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.