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[v2,09/14] x86_64: Avoid lazy relocation of tlsdesc [BZ #27137]

Message ID b37a0e40db013871109af6cba09f79ba92b9594d.1618301209.git.szabolcs.nagy@arm.com
State Committed
Commit 8f7e09f4dbdb5c815a18b8285fbc5d5d7bc17d86
Delegated to: Adhemerval Zanella Netto
Headers show
Series Dynamic TLS related data race fixes | expand

Commit Message

Szabolcs Nagy April 13, 2021, 8:20 a.m. UTC
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.

Another issue is that ld auditing ignores DT_BIND_NOW and thus tries to
relocate tlsdesc lazily, but that does not work in a BIND_NOW module
due to missing DT_TLSDESC_PLT. Unconditionally relocating tlsdesc at
load time fixes this bug 27721 too.

--
v2:
- mention the ldaudit issue with bindnow and tlsdesc.
---
 sysdeps/x86_64/dl-machine.h | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

Comments

H.J. Lu April 13, 2021, 2:02 p.m. UTC | #1
On Tue, Apr 13, 2021 at 2:31 AM Szabolcs Nagy via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> 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.
>
> Another issue is that ld auditing ignores DT_BIND_NOW and thus tries to
> relocate tlsdesc lazily, but that does not work in a BIND_NOW module
> due to missing DT_TLSDESC_PLT. Unconditionally relocating tlsdesc at
> load time fixes this bug 27721 too.
>
> --
> v2:
> - mention the ldaudit issue with bindnow and tlsdesc.
> ---
>  sysdeps/x86_64/dl-machine.h | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/sysdeps/x86_64/dl-machine.h b/sysdeps/x86_64/dl-machine.h
> index 103eee6c3f..9a876a371e 100644
> --- a/sysdeps/x86_64/dl-machine.h
> +++ b/sysdeps/x86_64/dl-machine.h
> @@ -570,12 +570,21 @@ elf_machine_lazy_rel (struct link_map *map,
>      }
>    else if (__glibc_likely (r_type == R_X86_64_TLSDESC))
>      {
> -      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;
>
> -      td->arg = (void*)reloc;
> -      td->entry = (void*)(D_PTR (map, l_info[ADDRIDX (DT_TLSDESC_PLT)])
> -                         + map->l_addr);
> +      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];
> +       }
> +
> +      /* 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_X86_64_IRELATIVE))
>      {
> --
> 2.17.1
>

LGTM.

Thanks.
diff mbox series

Patch

diff --git a/sysdeps/x86_64/dl-machine.h b/sysdeps/x86_64/dl-machine.h
index 103eee6c3f..9a876a371e 100644
--- a/sysdeps/x86_64/dl-machine.h
+++ b/sysdeps/x86_64/dl-machine.h
@@ -570,12 +570,21 @@  elf_machine_lazy_rel (struct link_map *map,
     }
   else if (__glibc_likely (r_type == R_X86_64_TLSDESC))
     {
-      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;
 
-      td->arg = (void*)reloc;
-      td->entry = (void*)(D_PTR (map, l_info[ADDRIDX (DT_TLSDESC_PLT)])
-			  + map->l_addr);
+      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];
+	}
+
+      /* 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_X86_64_IRELATIVE))
     {