x86: Specify a size for the cmp in the NMI handler

Linus noticed that the cmp used to check if the code segment is
__KERNEL_CS or not did not specify a size. Perhaps it does not matter
as H. Peter Anvin noted that user space can not set the bottom two
bits of the %cs register. But it's best not to let the assembly choose
and change things between different versions of gas, but instead just
pick the size.

Four bytes are used to compare the saved code segment against
__KERNEL_CS. Perhaps this might mess up Xen, but we can fix that when
the time comes.

Also I noticed that there was another non-specified cmp that checks
the special stack variable if it is 1 or 0. This too probably doesn't
matter what cmp is used, but this patch uses cmpl just to make it non
ambiguous.

Link: http://lkml.kernel.org/r/CA+55aFxfAn9MWRgS3O5k2tqN5ys1XrhSFVO5_9ZAoZKDVgNfGA@mail.gmail.com
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
index debd851..1333d98 100644 (file)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -1535,14 +1535,14 @@ ENTRY(nmi)
         * If %cs was not the kernel segment, then the NMI triggered in user
         * space, which means it is definitely not nested.
         */
-       cmp $__KERNEL_CS, 16(%rsp)
+       cmpl $__KERNEL_CS, 16(%rsp)
        jne first_nmi
 
        /*
         * Check the special variable on the stack to see if NMIs are
         * executing.
         */
-       cmp $1, -8(%rsp)
+       cmpl $1, -8(%rsp)
        je nested_nmi
 
        /*