[PATCH] x86/vdso: fix lsl operand order

[PATCH] x86/vdso: fix lsl operand order

[Samuel Neves]

In the __getcpu function, lsl was using the wrong target
and destination registers. Luckily, the compiler tends to
choose %eax for both variables, so it has been working
so far.

Cc: x86@kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Samuel Neves <sneves@dei.uc.pt>
---
 arch/x86/include/asm/vgtod.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index fb856c9f0449..53748541c487 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -93,7 +93,7 @@ static inline unsigned int __getcpu(void)
 	 *
 	 * If RDPID is available, use it.
 	 */
-	alternative_io ("lsl %[p],%[seg]",
+	alternative_io ("lsl %[seg],%[p]",
 			".byte 0xf3,0x0f,0xc7,0xf8", /* RDPID %eax/rax */
 			X86_FEATURE_RDPID,
 			[p] "=a" (p), [seg] "r" (__PER_CPU_SEG));
-- 
2.17.1

Re: [PATCH] x86/vdso: fix lsl operand order

[Andy Lutomirski]

On Sat, Sep 1, 2018 at 1:14 PM, Samuel Neves <sneves@dei.uc.pt> wrote:
> In the __getcpu function, lsl was using the wrong target
> and destination registers. Luckily, the compiler tends to
> choose %eax for both variables, so it has been working
> so far.
>
> Cc: x86@kernel.org
> Cc: stable@vger.kernel.org
> Signed-off-by: Samuel Neves <sneves@dei.uc.pt>

Acked-by: Andy Lutomirski <luto@kernel.org>
Fixes: a582c540ac1b ("x86/vdso: Use RDPID in preference to LSL when available")

Whoops!  I even wrote a selftest just for the offending commit, but,
of course, the selftest passes :(  I tested this by giving gcc some
gentle encouragement to allocate different registers, and the existing
code is indeed wrong and the fix indeed fixes it.

--Andy

[tip:x86/urgent] x86/vdso: Fix lsl operand order

[tip-bot for Samuel Neves]

Commit-ID:  e78e5a91456fcecaa2efbb3706572fe043766f4d
Gitweb:     https://git.kernel.org/tip/e78e5a91456fcecaa2efbb3706572fe043766f4d
Author:     Samuel Neves <sneves@dei.uc.pt>
AuthorDate: Sat, 1 Sep 2018 21:14:52 +0100
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sat, 1 Sep 2018 23:01:56 +0200

x86/vdso: Fix lsl operand order

In the __getcpu function, lsl is using the wrong target and destination
registers. Luckily, the compiler tends to choose %eax for both variables,
so it has been working so far.

Fixes: a582c540ac1b ("x86/vdso: Use RDPID in preference to LSL when available")
Signed-off-by: Samuel Neves <sneves@dei.uc.pt>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andy Lutomirski <luto@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180901201452.27828-1-sneves@dei.uc.pt

---
 arch/x86/include/asm/vgtod.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/vgtod.h b/arch/x86/include/asm/vgtod.h
index fb856c9f0449..53748541c487 100644
--- a/arch/x86/include/asm/vgtod.h
+++ b/arch/x86/include/asm/vgtod.h
@@ -93,7 +93,7 @@ static inline unsigned int __getcpu(void)
 	 *
 	 * If RDPID is available, use it.
 	 */
-	alternative_io ("lsl %[p],%[seg]",
+	alternative_io ("lsl %[seg],%[p]",
 			".byte 0xf3,0x0f,0xc7,0xf8", /* RDPID %eax/rax */
 			X86_FEATURE_RDPID,
 			[p] "=a" (p), [seg] "r" (__PER_CPU_SEG));