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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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