Re: Missing clobber on alternative use on Linux UM 32-bit

[Anton Ivanov]

On 04/11/2023 09:25, Nadav Amit wrote:
> I was reading (again) the x86 C macro of “alternative()” and I was a bit
> surprised it does clobber the flags (“cc”) as a precaution.
>
>    #define alternative(oldinstr, newinstr, ft_flags) \
> 	asm_inline volatile (ALTERNATIVE(oldinstr, newinstr, ft_flags) : : : "memory")
>
> Actually there seems to be only one instance of problematic cases - in um/32-bit:
>
>    #define mb() alternative("lock; addl $0,0(%%esp)", "mfence", X86_FEATURE_XMM2)
>    #define rmb() alternative("lock; addl $0,0(%%esp)", "lfence", X86_FEATURE_XMM2)
>    #define wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM)
>
> Presumably, if XMM or XMM2 are not supported, there would be instances where addl
> would be able to change eflags arithmetic flags without the compiler being aware
> of it.
>
> As it only affects 32-bit Linux UM - I don’t easily have an environment to test
> the fix. An alternative (word-pun unintended) is to add “cc” as a precaution
> to the alternative macro.
>
>
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Application alternatives in um is presently a NOP. It always uses the 
"blunt and heavy instrument" - the most conservative option.

It is on the TODO list.

--
Anton R. Ivanov
https://www.kot-begemot.co.uk/