Patchwork [05/16] Open-code the memcpy() at static TLS initialization time.

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Submitter Nix
Date Feb. 28, 2016, 4:41 p.m.
Message ID <1456677695-29778-6-git-send-email-nix@esperi.org.uk>
Download mbox | patch
Permalink /patch/11134/
State New
Headers show

Comments

Nix - Feb. 28, 2016, 4:41 p.m.
From: Nick Alcock <nick.alcock@oracle.com>

This one is a bit nasty.  Now that we are initializing TLS earlier for
the stack canary's sake, existing memcpy() implementations become
problematic.  We can use the multiarch implementations, but they might
not always be present, and even if they are present they might not always
be in assembler, so might be compiled with stack-protection.  We cannot
use posix/memcpy.c without marking both it and */wordcopy.c as non-stack-
protected, which for memcpy() of all things seems like a seriously bad
idea: if any function in glibc should be stack-protected, it's memcpy()
(though stack-protecting the many optimized assembly versions is not done
in this patch series).

So we have two real options: hack up the guts of posix/memcpy.c and
*/wordcopy.c so that they can be #included (renamed and declared static)
inside libc-tls.c, or simply open-code the memcpy().  For simplicity's
sake, this patch open-codes it, on the grounds that static binaries are
relatively rare and quasi-deprecated anyway, and static binaries with
large TLS sections are yet rarer, and not worth the complexity of hacking
up all the arch-dependent wordcopy files.

(This was not revealed when testing on x86 because on that platform
GCC was open-coding the memcpy() for us.)

v2: New, lets us remove the memcpy() -fno-stack-protection, which wasn't
    enough in any case.
v4: Add an inhibit_loop_to_libcall to prevent GCC from turning the loop
    back into a memcpy() again.  Wrap long lines.
---
 csu/libc-tls.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

Patch

diff --git a/csu/libc-tls.c b/csu/libc-tls.c
index 3d67a64..ef5d76e 100644
--- a/csu/libc-tls.c
+++ b/csu/libc-tls.c
@@ -102,6 +102,7 @@  init_static_tls (size_t memsz, size_t align)
 }
 
 void
+inhibit_loop_to_libcall
 __libc_setup_tls (size_t tcbsize, size_t tcbalign)
 {
   void *tlsblock;
@@ -176,8 +177,18 @@  __libc_setup_tls (size_t tcbsize, size_t tcbalign)
 # error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
 #endif
   _dl_static_dtv[2].pointer.is_static = true;
-  /* sbrk gives us zero'd memory, so we don't need to clear the remainder.  */
-  memcpy (_dl_static_dtv[2].pointer.val, initimage, filesz);
+
+  /* sbrk gives us zero'd memory, so we don't need to clear the remainder.
+
+     Copy by hand, because memcpy() is stack-protected and is often multiarch
+     too.  */
+
+  char *dst = (char *) _dl_static_dtv[2].pointer.val;
+  char *src = (char *) initimage;
+  size_t i;
+
+  for (i = 0; i < filesz; dst++, src++, i++)
+    *dst = *src;
 
   /* Install the pointer to the dtv.  */