x86-64 prelink support for TLS dialect gnu2

[Steven Newbury <steven dot newbury at googlemail dot com>]

I'm working on gettting TLS gnu2 dialect support working for x86-64
with prelink.  Yes, it's still maintained, see:
https://git.yoctoproject.org/cgit/cgit.cgi/prelink-cross/

I've written the attached patch which enables prelink to recognise
R_X86_64_TLSDESC relocations and do the same as is done currently for
ARM.  Unfortunately, while this seems to work, when a prelinked binary
is loaded with a shared library with such a relocation it fails with:

  "error while loading shared libraries: unexpected reloc type 0x24".

reloc type 0x24 is indeed R_X86_64_TLSDESC.

The strange thing is the code in glibc which generates this error
(sysdeps/x86_64/dl-machine.h) specifically handles R_X86_64_TLSDESC.
I can't understand how it is falling through to _dl_reloc_bad_type ()
with reloc 0x24 since in both instances the error case shouldn't be
reached!

Can anybody help?

From 963c95558ac1167121393d584bf00641a26eda4d Mon Sep 17 00:00:00 2001
From: Steven Newbury <steve@snewbury.org.uk>
Date: Mon, 5 Nov 2018 09:29:05 +0000
Subject: [PATCH] Add support for R_X86_64_TLSDESC

This is based on the R_ARM_TLS_DESC support code as
integrated in commit 874b3d3f6413c16597ec92ed92c57d6bfe2bbb68

Signed-off-by: Steven Newbury <steve@snewbury.org.uk>
---
 src/arch-x86_64.c | 30 +++++++++++++++++++++++++++++-
 1 file changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/arch-x86_64.c b/src/arch-x86_64.c
index 2f6c551..9b1b7fb 100644
--- a/src/arch-x86_64.c
+++ b/src/arch-x86_64.c
@@ -133,6 +133,7 @@ x86_64_prelink_rela (struct prelink_info *info, GElf_Rela *rela,
 {
   DSO *dso;
   GElf_Addr value;
+  Elf64_Sword val;
 
   dso = info->dso;
   if (GELF_R_TYPE (rela->r_info) == R_X86_64_NONE
@@ -184,6 +185,24 @@ x86_64_prelink_rela (struct prelink_info *info, GElf_Rela *rela,
 	return 0;
       error (0, 0, "%s: R_X86_64_COPY reloc in shared library?", dso->filename);
       return 1;
+    case R_X86_64_TLSDESC:
+      if (!dso->info_DT_TLSDESC_PLT)
+        {
+          error (0, 0,
+		 "%s: Unsupported R_X86_64_TLSDESC relocation in non-lazily bound object.",
+                 dso->filename);
+          return 1;
+        }
+      val = read_ule64 (dso, rela->r_offset + 8);
+      if (val != 0 && !dynamic_info_is_set (dso, DT_GNU_PRELINKED_BIT))
+        {
+          error (0, 0,
+		 "%s: Unexpected non-zero value (0x%x) in R_X86_64_TLSDESC?",
+                 dso->filename, val);
+          return 1;
+        }
+      write_le64 (dso, rela->r_offset + 8, dso->info_DT_TLSDESC_PLT);
+      break;
     default:
       error (0, 0, "%s: Unknown X86-64 relocation type %d", dso->filename,
 	     (int) GELF_R_TYPE (rela->r_info));
@@ -303,6 +322,9 @@ x86_64_prelink_conflict_rela (DSO *dso, struct prelink_info *info,
 	/* Similarly IRELATIVE relocations always need conflicts.  */
 	case R_X86_64_IRELATIVE:
 	  break;
+	/* Likewise TLSDESC.  */
+	case R_X86_64_TLSDESC:
+	  break;
 	default:
 	  return 0;
 	}
@@ -372,7 +394,9 @@ x86_64_prelink_conflict_rela (DSO *dso, struct prelink_info *info,
 	  break;
 	}
       break;
-
+    case R_X86_64_TLSDESC:
+      /* Nothing to do.  */
+      break;
     default:
       error (0, 0, "%s: Unknown X86-64 relocation type %d", dso->filename,
 	     (int) GELF_R_TYPE (rela->r_info));
@@ -498,6 +522,9 @@ x86_64_undo_prelink_rela (DSO *dso, GElf_Rela *rela, GElf_Addr relaaddr)
     case R_X86_64_TPOFF64:
       write_le64 (dso, rela->r_offset, 0);
       break;
+    case R_X86_64_TLSDESC:
+      write_le64 (dso, rela->r_offset + 8, 0);
+      break;
     case R_X86_64_32:
     case R_X86_64_PC32:
       write_le32 (dso, rela->r_offset, 0);
@@ -556,6 +583,7 @@ x86_64_reloc_class (int reloc_type)
     case R_X86_64_DTPMOD64:
     case R_X86_64_DTPOFF64:
     case R_X86_64_TPOFF64:
+    case R_X86_64_TLSDESC:
       return RTYPE_CLASS_TLS;
     default: return RTYPE_CLASS_VALID;
     }
-- 
2.19.1