x86: use early clobbers in usercopy*.c

Impact: fix rare (but currently harmless) miscompile with certain configs and gcc versions

Hugh Dickins noticed that strncpy_from_user() was miscompiled
in some circumstances with gcc 4.3.

Thanks to Hugh's excellent analysis it was easy to track down.

Hugh writes:

> Try building an x86_64 defconfig 2.6.29-rc1 kernel tree,
> except not quite defconfig, switch CONFIG_PREEMPT_NONE=y
> and CONFIG_PREEMPT_VOLUNTARY off (because it expands a
> might_fault() there, which hides the issue): using a
> gcc 4.3.2 (I've checked both openSUSE 11.1 and Fedora 10).
>
> It generates the following:
>
> 0000000000000000 <__strncpy_from_user>:
>    0:   48 89 d1                mov    %rdx,%rcx
>    3:   48 85 c9                test   %rcx,%rcx
>    6:   74 0e                   je     16 <__strncpy_from_user+0x16>
>    8:   ac                      lods   %ds:(%rsi),%al
>    9:   aa                      stos   %al,%es:(%rdi)
>    a:   84 c0                   test   %al,%al
>    c:   74 05                   je     13 <__strncpy_from_user+0x13>
>    e:   48 ff c9                dec    %rcx
>   11:   75 f5                   jne    8 <__strncpy_from_user+0x8>
>   13:   48 29 c9                sub    %rcx,%rcx
>   16:   48 89 c8                mov    %rcx,%rax
>   19:   c3                      retq
>
> Observe that "sub %rcx,%rcx; mov %rcx,%rax", whereas gcc 4.2.1
> (and many other configs) say "sub %rcx,%rdx; mov %rdx,%rax".
> Isn't it returning 0 when it ought to be returning strlen?

The asm constraints for the strncpy_from_user() result were missing an
early clobber, which tells gcc that the last output arguments
are written before all input arguments are read.

Also add more early clobbers in the rest of the file and fix 32-bit
usercopy.c in the same way.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
[ since this API is rarely used and no in-kernel user relies on a 'len'
  return value (they only rely on negative return values) this miscompile
  was never noticed in the field. But it's worth fixing it nevertheless. ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 2 insertions, 2 deletions

diff --git a/arch/x86/lib/usercopy_32.c b/arch/x86/lib/usercopy_32.c
index 4a20b2f..7c8ca91 100644
--- a/arch/x86/lib/usercopy_32.c
+++ b/arch/x86/lib/usercopy_32.c
@@ -56,7 +56,7 @@ do {									   \
 		"	jmp 2b\n"					   \
 		".previous\n"						   \
 		_ASM_EXTABLE(0b,3b)					   \
-		: "=d"(res), "=c"(count), "=&a" (__d0), "=&S" (__d1),	   \
+		: "=&d"(res), "=&c"(count), "=&a" (__d0), "=&S" (__d1),	   \
 		  "=&D" (__d2)						   \
 		: "i"(-EFAULT), "0"(count), "1"(count), "3"(src), "4"(dst) \
 		: "memory");						   \
@@ -218,7 +218,7 @@ long strnlen_user(const char __user *s, long n)
 		"	.align 4\n"
 		"	.long 0b,2b\n"
 		".previous"
-		:"=r" (n), "=D" (s), "=a" (res), "=c" (tmp)
+		:"=&r" (n), "=&D" (s), "=&a" (res), "=&c" (tmp)
 		:"0" (n), "1" (s), "2" (0), "3" (mask)
 		:"cc");
 	return res & mask;