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[PATCH 04/14] Open-code the memcpy() at static TLS initialization time.

From: Nick Alcock <>

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 cannot use the multiarch implementations, even if we
move the irel initialization up above TLS initialization, because ifunc
resolvers may not work this early (among other things, they might be
compiled with stack-protection!).  We cannot use posix/memcpy.c
unmodified 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.
 csu/libc-tls.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/csu/libc-tls.c b/csu/libc-tls.c
index 3d67a64..20f478d 100644
--- a/csu/libc-tls.c
+++ b/csu/libc-tls.c
@@ -176,8 +176,17 @@ __libc_setup_tls (size_t tcbsize, size_t tcbalign)
 # error "Either TLS_TCB_AT_TP or TLS_DTV_AT_TP must be defined"
   _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.  */

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