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Subject[PATCH v2 03/14] x86,smap: Fix smap_{save,restore}() alternatives
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) <peterz@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");
}


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