Re: PATCH: Make STT_GNU_IFUNC symbol dynamic

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Sat, May 23, 2009 at 04:54:37PM -0700, H.J. Lu wrote:
> On Sat, May 23, 2009 at 2:44 PM, Ulrich Drepper <drepper@redhat.com> wrote:
> > -----BEGIN PGP SIGNED MESSAGE-----
> > Hash: SHA1
> >
> > H.J. Lu wrote:
> >> Which tool will generate the new relocation? Will it be internal to linker.
> >
> > The code the compiler generates contains ordinary calls. ?The assembler
> > will have to refrain from optimizing calls to static IFUNC functions.
> 
> Assembler will always generate PLT relocation for @PLT. I don't think
> we need to change the assembler.
> 
> > But the real logic is entirely internal to the static linker. ?For every
> > local IFUNC symbol create a PLT slot. ?This slot has no global symbol
> > attached and the relocation should be R_*_IRELATIVE (my proposed name).
> > ?This relocation works like a R_*_RELATIVE relocation but it uses the
> > return value as for SHT_GNU_IFUNC symbols. ?The reason we need such a
> > relocation is that we have no symbol at runtime. ?We cannot use a normal
> > PLT slot relocation since this would perform a global name lookup, which
> > is wrong. ?The R_*_IRELATIVE relocation is also more efficient.
> >
> >
> 
> I took a look. It isn't hard to implement. Should it be R_*_IJUMP_SLOT
> since it will be processed closer to R_*_JUMP_SLOT than R_*_RELATIVE?
> My linker only patch for x86-64 is ready for test. I just need to
> verify it works
> with glibc on x86-64.
> 

This patch works with the modified glibc. I am also enclosing
glibc patch for reference.


H.J.
---
bfd/

2009-05-23  H.J. Lu  <hongjiu.lu@intel.com>

	* elf64-x86-64.c (elf64_x86_64_finish_dynamic_symbol): Generate
	R_X86_64_IJUMP_SLOT relocation for local STT_GNU_IFUNC symbol.

	* elflink.c (_bfd_elf_symbol_refs_local_p): Return FALSE on
	STT_GNU_IFUNC symbol.
	(_bfd_elf_link_hash_hide_symbol): Don't clean plt on
	STT_GNU_IFUNC symbol.

include/elf/

2009-05-23  H.J. Lu  <hongjiu.lu@intel.com>

	* x86-64.h (R_X86_64_IJUMP_SLOT): New.

Index: bfd/elflink.c
===================================================================
--- bfd/elflink.c	(revision 5932)
+++ bfd/elflink.c	(working copy)
@@ -2845,6 +2845,10 @@ _bfd_elf_symbol_refs_local_p (struct elf
   if (h == NULL)
     return TRUE;
 
+  /* STT_GNU_IFUNC symbol must go through PLT.  */
+  if (h->type == STT_GNU_IFUNC)
+    return FALSE;
+
   /* STV_HIDDEN or STV_INTERNAL ones must be local.  */
   if (ELF_ST_VISIBILITY (h->other) == STV_HIDDEN
       || ELF_ST_VISIBILITY (h->other) == STV_INTERNAL)
@@ -6669,8 +6673,12 @@ _bfd_elf_link_hash_hide_symbol (struct b
 				struct elf_link_hash_entry *h,
 				bfd_boolean force_local)
 {
-  h->plt = elf_hash_table (info)->init_plt_offset;
-  h->needs_plt = 0;
+  /* STT_GNU_IFUNC symbol must go through PLT.  */
+  if (ELF_ST_TYPE (h->type) != STT_GNU_IFUNC)
+    {
+      h->plt = elf_hash_table (info)->init_plt_offset;
+      h->needs_plt = 0;
+    }
   if (force_local)
     {
       h->forced_local = 1;
Index: bfd/elf64-x86-64.c
===================================================================
--- bfd/elf64-x86-64.c	(revision 5933)
+++ bfd/elf64-x86-64.c	(working copy)
@@ -3328,7 +3328,10 @@ elf64_x86_64_finish_dynamic_symbol (bfd 
 
       /* This symbol has an entry in the procedure linkage table.  Set
 	 it up.	 */
-      if (h->dynindx == -1
+      if ((h->dynindx == -1
+	   && !(h->forced_local
+		&& h->def_regular
+		&& ELF_ST_TYPE (h->type) == STT_GNU_IFUNC))
 	  || htab->splt == NULL
 	  || htab->sgotplt == NULL
 	  || htab->srelplt == NULL)
@@ -3381,8 +3384,18 @@ elf64_x86_64_finish_dynamic_symbol (bfd 
       rela.r_offset = (htab->sgotplt->output_section->vma
 		       + htab->sgotplt->output_offset
 		       + got_offset);
-      rela.r_info = ELF64_R_INFO (h->dynindx, R_X86_64_JUMP_SLOT);
-      rela.r_addend = 0;
+      if (h->dynindx != -1)
+	{
+	  rela.r_info = ELF64_R_INFO (h->dynindx, R_X86_64_JUMP_SLOT);
+	  rela.r_addend = 0;
+	}
+      else
+	{
+	  rela.r_info = ELF64_R_INFO (0, R_X86_64_IJUMP_SLOT);
+	  rela.r_addend = (h->root.u.def.value
+			   + h->root.u.def.section->output_section->vma
+			   + h->root.u.def.section->output_offset);
+	}
       loc = htab->srelplt->contents + plt_index * sizeof (Elf64_External_Rela);
       bfd_elf64_swap_reloca_out (output_bfd, &rela, loc);
 
Index: include/elf/x86-64.h
===================================================================
--- include/elf/x86-64.h	(revision 5933)
+++ include/elf/x86-64.h	(working copy)
@@ -71,6 +71,7 @@ START_RELOC_NUMBERS (elf_x86_64_reloc_ty
      RELOC_NUMBER (R_X86_64_TLSDESC_CALL, 35) /* Relaxable call through TLS
 						 descriptor.  */
      RELOC_NUMBER (R_X86_64_TLSDESC, 36)      /* 2x64-bit TLS descriptor.  */
+     RELOC_NUMBER (R_X86_64_IJUMP_SLOT,37)      /* Create indirect PLT entry */
      RELOC_NUMBER (R_X86_64_GNU_VTINHERIT, 250)       /* GNU C++ hack  */
      RELOC_NUMBER (R_X86_64_GNU_VTENTRY, 251)         /* GNU C++ hack  */
 END_RELOC_NUMBERS (R_X86_64_max)

Attachment: libc-ifunc-3.patch
Description: Text document