Re: [PATCH 05/18] Open-code the memcpy() at static TLS initialization time.

[Nix <nix at esperi dot org dot uk>]

On 9 Mar 2016, Mike Frysinger uttered the following:

> On 08 Mar 2016 13:50, Nix wrote:
>> 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.
>
> can we utilize _HAVE_STRING_ARCH_memcpy here ?  the string includes will
> sometimes provide inlined asm versions ...

I think so, *if* we can be sure that arches that define
_HAVE_STRING_ARCH_memcpy will never expand memcpy() to macros that
sometimes call functions, but only to pure inline asm stuff. Since there
is only one definer of that (x86), I'm not sure that's guaranteed...

(This does explain why my original implementation, which didn't do any
of this, also didn't have any problems on x86!)

How's this instead? (Seems to work just as well on x86, which is after
all the only arch this change will affect at all.)

>From 3427487ee08586f76f711758795dba31b62c4238 Mon Sep 17 00:00:00 2001
From: Nick Alcock <nick.alcock@oracle.com>
Date: Tue, 23 Feb 2016 11:08:38 +0000
Subject: [PATCH] Open-code the memcpy() at static TLS initialization time.

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.  There is one exception: if
the arch provides an inline assembler memcpy() implementation, we can
use that in preference.

(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.
v6: Use the inline assembler ARCH_memcpy if available.

	* csu/libc-tls.c (__libc_setup_tls): Add inhibit_loop_to_libcall
	to avoid calls to potentially ifunced or stack-protected memcpy.
	Open-code the TLS-initialization memcpy.
---
 csu/libc-tls.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/csu/libc-tls.c b/csu/libc-tls.c
index 3d67a64..af0d3e6 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 +177,22 @@ __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.  */
+
+  /* sbrk gives us zero'd memory, so we don't need to clear the remainder.
+
+     Use inlined asm implementation if available: otherwise, copy by hand,
+     because memcpy() is stack-protected and is often multiarch too.  */
+
+#if defined _HAVE_STRING_ARCH_memcpy
   memcpy (_dl_static_dtv[2].pointer.val, initimage, filesz);
+#else
+  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;
+#endif
 
   /* Install the pointer to the dtv.  */
 
-- 
2.7.0.198.g6dd47b6