[PATCH v2 03/14] x86,smap: Fix smap_{save,restore}() alternatives

[Peter Zijlstra]

As reported by objtool:

  lib/ubsan.o: warning: objtool: .altinstr_replacement+0x0: alternative 
modifies stack
  lib/ubsan.o: warning: objtool: .altinstr_replacement+0x7: alternative 
modifies stack

the smap_{save,restore}() alternatives violate (the newly enforced)
rule on stack invariance. That is, due to there only being a single
ORC table it must be valid to any alternative. These alternatives
violate this with the direct result that unwinds will not be correct
in between these calls.

[ In specific, since we force SMAP on for objtool, running on !SMAP
hardware will observe a different stack-layout and the ORC unwinder
will stumble. ]

So rewrite the functions to unconditionally save/restore the flags,
which gives an invariant stack layout irrespective of the SMAP state.

Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
 arch/x86/include/asm/smap.h |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -57,16 +57,19 @@ static __always_inline unsigned long sma
 {
        unsigned long flags;
 
-       asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
-                                 X86_FEATURE_SMAP)
-                     : "=rm" (flags) : : "memory", "cc");
+       asm volatile ("# smap_save\n\t"
+                     "pushf; pop %0"
+                     : "=rm" (flags) : : "memory");
+
+       clac();
 
        return flags;
 }
 
 static __always_inline void smap_restore(unsigned long flags)
 {
-       asm volatile (ALTERNATIVE("", "push %0; popf", X86_FEATURE_SMAP)
+       asm volatile ("# smap_restore\n\t"
+                     "push %0; popf"
                      : : "g" (flags) : "memory", "cc");
 }