Re: [PATCH v5 18/22] elf: Issue la_symbind() for bind-now (BZ #23734)

[Florian Weimer <fweimer@redhat.com>]

* Adhemerval Zanella:

> The audit callback is not called for binaries built with -Wl,-z,now or
> when LD_BIND_NOW=1 is used.  The PLT tracking callbacks are still not
> issue for such case, since this will would change the expected program
> semantic (where no PTL is expected) and also yield performance
> implications (such as for BZ#15533).
>
> To handle powerpc64 ELFv1 function descriptor, _dl_audit_symbind()
> requires to know whether bind-now is used so the symbol value is
> updated to function text segment instead of the ODP (for lazy binding
> this is done by PPC64_LOAD_FUNCPTR on _dl_runtime_resolve).

Typo: OPD (I think)

This needs updates to sysdeps/hppa/dl-lookupcfg.h and
sysdeps/ia64/dl-lookupcfg.h as well.  Presently these two targets fail
to build.

> diff --git a/elf/dl-audit.c b/elf/dl-audit.c
> index c3569cb357..4540d93913 100644
> --- a/elf/dl-audit.c
> +++ b/elf/dl-audit.c
> @@ -190,7 +190,7 @@ rtld_hidden_def (_dl_audit_symbind_alt)
>  void
>  _dl_audit_symbind (struct link_map *l, struct reloc_result *reloc_result,
>  		   const ElfW(Sym) *defsym, DL_FIXUP_VALUE_TYPE *value,
> -		   lookup_t result)
> +		   lookup_t result, bool bindnow)
>  {
>    reloc_result->bound = result;
>    /* Compute index of the symbol entry in the symbol table of the DSO with the
> @@ -238,7 +238,8 @@ _dl_audit_symbind (struct link_map *l, struct reloc_result *reloc_result,
>  	      if (new_value != (uintptr_t) sym.st_value)
>  		{
>  		  flags |= LA_SYMB_ALTVALUE;
> -		  sym.st_value = new_value;
> +		  sym.st_value = bindnow
> +		    ? DL_FIXUP_BINDNOW_ADDR_VALUE (new_value) : new_value;
>  		}
>  	    }

I think if PLT enter/exit hooks are not available, we should set
LA_SYMB_NOPLTENTER | LA_SYMB_NOPLTEXIT in flags before calling the
symbind callback.

I find DL_FIXUP_BINDNOW_ADDR_VALUE very confusing.  Why is it safe to
extract just the code address from the function descriptor?

In the future, I want to call symbind for all symbol bindings.
Basically, I want to remove

> +	      if (ELFW(R_TYPE) (r->r_info) == ELF_MACHINE_JMP_SLOT

eventually, and add symbind callbacks for dlsym lookups.  Hopefully we
can make these changes compatible.

Thanks,
Florian