[PATCH v2 10/14] i386: Avoid lazy relocation of tlsdesc [BZ #27137]

[Szabolcs Nagy <szabolcs.nagy@arm.com>]

Lazy tlsdesc relocation is racy because the static tls optimization and
tlsdesc management operations are done without holding the dlopen lock.

This similar to the commit b7cf203b5c17dd6d9878537d41e0c7cc3d270a67
for aarch64, but it fixes a different race: bug 27137.

On i386 the code is a bit more complicated than on x86_64 because both
rel and rela relocs are supported.
---
 sysdeps/i386/dl-machine.h | 76 ++++++++++++++++++---------------------
 1 file changed, 34 insertions(+), 42 deletions(-)

diff --git a/sysdeps/i386/dl-machine.h b/sysdeps/i386/dl-machine.h
index 23e9cc3bfb..590b41d8d7 100644
--- a/sysdeps/i386/dl-machine.h
+++ b/sysdeps/i386/dl-machine.h
@@ -688,50 +688,32 @@ elf_machine_lazy_rel (struct link_map *map,
     }
   else if (__glibc_likely (r_type == R_386_TLS_DESC))
     {
-      struct tlsdesc volatile * __attribute__((__unused__)) td =
-	(struct tlsdesc volatile *)reloc_addr;
-
-      /* Handle relocations that reference the local *ABS* in a simple
-	 way, so as to preserve a potential addend.  */
-      if (ELF32_R_SYM (reloc->r_info) == 0)
-	td->entry = _dl_tlsdesc_resolve_abs_plus_addend;
-      /* Given a known-zero addend, we can store a pointer to the
-	 reloc in the arg position.  */
-      else if (td->arg == 0)
-	{
-	  td->arg = (void*)reloc;
-	  td->entry = _dl_tlsdesc_resolve_rel;
-	}
-      else
-	{
-	  /* We could handle non-*ABS* relocations with non-zero addends
-	     by allocating dynamically an arg to hold a pointer to the
-	     reloc, but that sounds pointless.  */
-	  const Elf32_Rel *const r = reloc;
-	  /* The code below was borrowed from elf_dynamic_do_rel().  */
-	  const ElfW(Sym) *const symtab =
-	    (const void *) D_PTR (map, l_info[DT_SYMTAB]);
+      const Elf32_Rel *const r = reloc;
+      /* The code below was borrowed from elf_dynamic_do_rel().  */
+      const ElfW(Sym) *const symtab =
+	(const void *) D_PTR (map, l_info[DT_SYMTAB]);
 
+      /* Always initialize TLS descriptors completely at load time, in
+	 case static TLS is allocated for it that requires locking.  */
 # ifdef RTLD_BOOTSTRAP
-	  /* The dynamic linker always uses versioning.  */
-	  assert (map->l_info[VERSYMIDX (DT_VERSYM)] != NULL);
+      /* The dynamic linker always uses versioning.  */
+      assert (map->l_info[VERSYMIDX (DT_VERSYM)] != NULL);
 # else
-	  if (map->l_info[VERSYMIDX (DT_VERSYM)])
+      if (map->l_info[VERSYMIDX (DT_VERSYM)])
 # endif
-	    {
-	      const ElfW(Half) *const version =
-		(const void *) D_PTR (map, l_info[VERSYMIDX (DT_VERSYM)]);
-	      ElfW(Half) ndx = version[ELFW(R_SYM) (r->r_info)] & 0x7fff;
-	      elf_machine_rel (map, r, &symtab[ELFW(R_SYM) (r->r_info)],
-			       &map->l_versions[ndx],
-			       (void *) (l_addr + r->r_offset), skip_ifunc);
-	    }
+	{
+	  const ElfW(Half) *const version =
+	    (const void *) D_PTR (map, l_info[VERSYMIDX (DT_VERSYM)]);
+	  ElfW(Half) ndx = version[ELFW(R_SYM) (r->r_info)] & 0x7fff;
+	  elf_machine_rel (map, r, &symtab[ELFW(R_SYM) (r->r_info)],
+			   &map->l_versions[ndx],
+			   (void *) (l_addr + r->r_offset), skip_ifunc);
+	}
 # ifndef RTLD_BOOTSTRAP
-	  else
-	    elf_machine_rel (map, r, &symtab[ELFW(R_SYM) (r->r_info)], NULL,
-			     (void *) (l_addr + r->r_offset), skip_ifunc);
+      else
+	elf_machine_rel (map, r, &symtab[ELFW(R_SYM) (r->r_info)], NULL,
+			 (void *) (l_addr + r->r_offset), skip_ifunc);
 # endif
-	}
     }
   else if (__glibc_unlikely (r_type == R_386_IRELATIVE))
     {
@@ -758,11 +740,21 @@ elf_machine_lazy_rela (struct link_map *map,
     ;
   else if (__glibc_likely (r_type == R_386_TLS_DESC))
     {
-      struct tlsdesc volatile * __attribute__((__unused__)) td =
-	(struct tlsdesc volatile *)reloc_addr;
+      const Elf_Symndx symndx = ELFW (R_SYM) (reloc->r_info);
+      const ElfW (Sym) *symtab = (const void *)D_PTR (map, l_info[DT_SYMTAB]);
+      const ElfW (Sym) *sym = &symtab[symndx];
+      const struct r_found_version *version = NULL;
+
+      if (map->l_info[VERSYMIDX (DT_VERSYM)] != NULL)
+	{
+	  const ElfW (Half) *vernum =
+	    (const void *)D_PTR (map, l_info[VERSYMIDX (DT_VERSYM)]);
+	  version = &map->l_versions[vernum[symndx] & 0x7fff];
+	}
 
-      td->arg = (void*)reloc;
-      td->entry = _dl_tlsdesc_resolve_rela;
+      /* Always initialize TLS descriptors completely at load time, in
+	 case static TLS is allocated for it that requires locking.  */
+      elf_machine_rela (map, reloc, sym, version, reloc_addr, skip_ifunc);
     }
   else if (__glibc_unlikely (r_type == R_386_IRELATIVE))
     {
-- 
2.17.1