Re: How can I wrap ld-linux or execve into it?

[Farid Zakaria <fmzakari@ucsc.edu>]

Thank you -- I need to parse your solution more in depth.
I will also take a look more at rtld -- however I was hoping to be
compatible with musl as well.

I was told by a colleague that execve solution may work if the static
binary has "no relocations".
How can I control that through the compilation of a program beyond
limiting myself to a single object-file?

On Tue, Feb 1, 2022 at 4:43 AM Adhemerval Zanella
<adhemerval.zanella@linaro.org> wrote:
>
>
>
> On 31/01/2022 17:10, Farid Zakaria via Libc-help wrote:
> > Hi,
> >
> > I am looking to perform some functionality before the dynamic linker
> > (linux-ld/ld.so) is invoked.
> >
> > My naive assessment was that I would be able to set in the PT_INTERP
> > section of a binary, my *static binary*, which will then execve into
> > the dynamic linker after doing some precanned actions.
> >
> > Unfortunately, trying this has resulted in some SIGSEGV...
> >
> > I came across https://github.com/Mic92/nix-ld which seems to do
> > something similar, but I was curious why it has to do a lot more to
> > achieve the same effect with a jump.
> >
> > I have also been pointed to LD_AUDIT however I am also interested in
> > having it agnostic to libc (glibc vs. musl)
> >
> > Thank you for any tips, guidance or links you can provide.
> > FZ
>
> The issue is for static linking _dl_aux_init will setup the _dl_phdr
> to the loaded binary (since it was done by the kernel) passed on auxiliary
> vectors:
>
> elf/dl-support.c:
>
> 246 void
> 247 _dl_aux_init (ElfW(auxv_t) *av)
> 248 {
> [...]
> 269       case AT_PHDR:
> 270         GL(dl_phdr) = (const void *) av->a_un.a_val;
> 271         break;
> [...]
>
> And this is later used to setup the TCB:
>
> csu/libc-tls.c
>
> 104 void
> 105 __libc_setup_tls (void)
> 106 {
> [...]
> 120   /* Look through the TLS segment if there is any.  */
> 121   if (_dl_phdr != NULL)
> 122     for (phdr = _dl_phdr; phdr < &_dl_phdr[_dl_phnum]; ++phdr)
> 123       if (phdr->p_type == PT_TLS)
> 124         {
> 125           /* Remember the values we need.  */
> 126           memsz = phdr->p_memsz;
> 127           filesz = phdr->p_filesz;
> 128           initimage = (void *) phdr->p_vaddr + main_map->l_addr;
> 129           align = phdr->p_align;
> 130           if (phdr->p_align > max_align)
> 131             max_align = phdr->p_align;
> 132           break;
> 133         }
>
> The problem is seice _dl_phdr is not pointing to the static programs acting
> as loader, the PT_TLS is not considered and thus not initialized correctly.
> That's why once __ctype_init tries to access TLS variables it triggers an
> invalid memory reference:
>
> (gdb) c
> Continuing.
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x000000000045f464 in __ctype_init () at ctype-info.c:31
> 31        *bp = (const uint16_t *) _NL_CURRENT (LC_CTYPE, _NL_CTYPE_CLASS) + 128;
>
>
> And I don't think this would be easy to support without changing a *lot*
> on static linking organization.  If you check the loader code, it avoids
> to use TLS exactly to avoid this initialization issue.
>
> I think the best option to work by checking elt/rtld.c and see if you can
> hack a way link you code after its initialization.