[PATCH] IFUNC: Update IFUNC resolver check with DT_TEXTREL

[Alan Modra]

On Mon, Jun 08, 2020 at 08:49:06AM -0700, H.J. Lu wrote:
> @@ -264,17 +260,13 @@ _bfd_elf_allocate_ifunc_dyn_relocs (struct bfd_link_info *info,
>        bfd_size_type count = 0;
>        do
>  	{
> -	  if (!readonly_dynrelocs_against_ifunc)
> -	    {
> -	      asection *s = p->sec->output_section;
> -	      if (s != NULL && (s->flags & SEC_READONLY) != 0)
> -		readonly_dynrelocs_against_ifunc = TRUE;
> -	    }
>  	  count += p->count;
>  	  p = p->next;
>  	}
>        while (p != NULL);
>  
> +      htab->ifunc_resolvers = count != 0;
> +
>        /* Dynamic relocations are stored in
>  	 1. .rel[a].ifunc section in PIC object.
>  	 2. .rel[a].got section in dynamic executable.

For ppc64 I set flags when recording the dynamic relocation rather
than when allocating space.  That allows you to distinguish three
cases:
1) The dynamic ifunc relocation is in an executable and will always be
   to an ifunc resolver in the executable.
2) The dynamic ifunc relocation is in a shared library which provides
   an ifunc resolver, but that may be overridden at runtime to use a
   resolver in another binary.
3) The dynamic ifunc relocation is not to a locally defined ifunc
   resolver.

Case (3) won't cause a segfault trying to run resolver code that is
non-exec on older glibc.

I made case (1) an error for ppc64, but since newer glibc ld.so does
allow running ifunc resolvers when segments are writable I suppose I
should downgrade that to a warning like case (2).

	* elf64-ppc.c (struct ppc_link_hash_table): Delete
	maybe_local_ifunc_resolver field.
	(build_global_entry_stubs_and_plt): Set local_ifunc_resolver in
	cases where maybe_local_ifunc_resolver was set.
	(ppc64_elf_relocate_section): Likewise.
	(ppc64_elf_finish_dynamic_sections): Downgrade ifunc with textrel
	error to a warning.

diff --git a/bfd/elf64-ppc.c b/bfd/elf64-ppc.c
index e28546deb5..9868f6a755 100644
--- a/bfd/elf64-ppc.c
+++ b/bfd/elf64-ppc.c
@@ -3242,7 +3242,6 @@ struct ppc_link_hash_table
   /* Whether there exist local gnu indirect function resolvers,
      referenced by dynamic relocations.  */
   unsigned int local_ifunc_resolver:1;
-  unsigned int maybe_local_ifunc_resolver:1;
 
   /* Whether plt calls for ELFv2 localentry:0 funcs have been optimized.  */
   unsigned int has_plt_localentry0:1;
@@ -13935,7 +13934,7 @@ build_global_entry_stubs_and_plt (struct elf_link_hash_entry *h, void *inf)
 		   + ((ent->plt.offset - PLT_INITIAL_ENTRY_SIZE (htab))
 		      / PLT_ENTRY_SIZE (htab) * sizeof (Elf64_External_Rela)));
 	    if (h->type == STT_GNU_IFUNC && is_static_defined (h))
-	      htab->maybe_local_ifunc_resolver = 1;
+	      htab->local_ifunc_resolver = 1;
 	    bfd_elf64_swap_reloca_out (info->output_bfd, &rela, loc);
 	  }
       }
@@ -16103,10 +16102,8 @@ ppc64_elf_relocate_section (bfd *output_bfd,
 		if (ifunc)
 		  {
 		    relgot = htab->elf.irelplt;
-		    if (indx == 0)
+		    if (indx == 0 || is_static_defined (&h->elf))
 		      htab->local_ifunc_resolver = 1;
-		    else if (is_static_defined (&h->elf))
-		      htab->maybe_local_ifunc_resolver = 1;
 		  }
 		else if (indx != 0
 			 || (bfd_link_pic (info)
@@ -16635,10 +16632,8 @@ ppc64_elf_relocate_section (bfd *output_bfd,
 		  : ELF_ST_TYPE (sym->st_info) == STT_GNU_IFUNC)
 		{
 		  sreloc = htab->elf.irelplt;
-		  if (indx == 0)
+		  if (indx == 0 || is_static_defined (&h->elf))
 		    htab->local_ifunc_resolver = 1;
-		  else if (is_static_defined (&h->elf))
-		    htab->maybe_local_ifunc_resolver = 1;
 		}
 	      if (sreloc == NULL)
 		abort ();
@@ -17403,10 +17398,6 @@ ppc64_elf_finish_dynamic_sections (bfd *output_bfd,
 
 	    case DT_TEXTREL:
 	      if (htab->local_ifunc_resolver)
-		info->callbacks->einfo
-		  (_("%X%P: text relocations and GNU indirect "
-		     "functions will result in a segfault at runtime\n"));
-	      else if (htab->maybe_local_ifunc_resolver)
 		info->callbacks->einfo
 		  (_("%P: warning: text relocations and GNU indirect "
 		     "functions may result in a segfault at runtime\n"));

-- 
Alan Modra
Australia Development Lab, IBM