Messages in this thread | | | From | Florian Weimer <> | Subject | Re: [PATCH v5] x86: use builtins to read eflags | Date | Thu, 17 Mar 2022 21:13:05 +0100 |
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* Linus Torvalds:
> You can actually operate on EFLAGS at multiple granularities. > > - normal pushf/popf. Don't do it unless you are doing system software.
There's one exception: PUSHF/twiddle/POPF/PUSHF/compare is the recommended sequence to detect CPUID support on i386 (in userspace and elsewhere).
> - you can use lahf/sahc to load/store only the arithmetic flags > into/from AH. Deprecated, and going away, but historically supported.
And these instructions were missing from the original long mode, but they were added back.
> Yes, yes, that complete mental breakdown with pushf/popf did get > fixed, but it really makes me very wary of thinking that we should > ever use a built-in that compiler writers really fundamentally got so > wrong before. > > What would make me think that you'd get it right now? In user space, > you'll basically never actually see the whole system flags issues, so > your test-cases would never work or be very contrieved. You'd have to > really work at it to see the problems.
I think as the result of the nature of that kind of bug it does not matter whether you use a compiler builtin to access the flags (to put their combined value into a general-purpose register).
GCC doesn't have barriers in the built-ins (if we are talking about __builtin_ia32_readeflags_u64 and __builtin_ia32_writeeflags_u64). I expect they are actually pretty useless, and were merely added for completeness of the intrinsics headers.
It's not that you can write
unsigned a, b, c; // … c = a + b;
and examine __builtin_ia32_readeflags_u64() to see if there was an overflow. Neither GCC nor Clang model the EFLAGS register and arithmetic expression side effects to make this possible.
Thanks, Florian
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