[PATCH 2/2] x86: require exact match for "noxsave" command line option

[Dave Hansen <dave@sr71.net>]

From: Dave Hansen <dave.hansen@linux.intel.com>

We have some very similarly named command-line options:

arch/x86/kernel/cpu/common.c:__setup("noxsave", x86_xsave_setup);
arch/x86/kernel/cpu/common.c:__setup("noxsaveopt", x86_xsaveopt_setup);
arch/x86/kernel/cpu/common.c:__setup("noxsaves", x86_xsaves_setup);

__setup() is designed to match options that take arguments, like
"foo=bar" where you would have:

	__setup("foo", x86_foo_func...);

The problem is that "noxsave" actually _matches_ "noxsaves" in
the same way that "foo" matches "foo=bar".  If you boot an old
kernel that does not know about "noxsaves" with "noxsaves" on the
command line, it will interpret the argument as "noxsave", which
is not what you want at all.

This makes the "noxsave" handler only return success when it finds
an *exact* match.

We should try and get this patch in to as many old kernels as
possible.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: x86@kernel.org

---

 b/arch/x86/kernel/cpu/common.c |    2 ++
 1 file changed, 2 insertions(+)

diff -puN arch/x86/kernel/cpu/common.c~mpx-xsave-setup-exact arch/x86/kernel/cpu/common.c
--- a/arch/x86/kernel/cpu/common.c~mpx-xsave-setup-exact	2014-11-11 14:00:19.061376676 -0800
+++ b/arch/x86/kernel/cpu/common.c	2014-11-11 14:00:19.063376767 -0800
@@ -146,6 +146,8 @@ EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);
 
 static int __init x86_xsave_setup(char *s)
 {
+	if (strlen(s))
+		return 0;
 	setup_clear_cpu_cap(X86_FEATURE_XSAVE);
 	setup_clear_cpu_cap(X86_FEATURE_XSAVEOPT);
 	setup_clear_cpu_cap(X86_FEATURE_XSAVES);
_