Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 6 Feb 2018 12:42:24 -0800 | Subject | Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation |
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On Tue, Feb 6, 2018 at 12:37 PM, Dan Williams <dan.j.williams@intel.com> wrote: > > Are there any compilers that would miscompile: > > mask = 0 - (index < size); > > That might be a way to improve the assembly.
Sadly, that is *very* easy to miscompile. In fact, I'd be very surprised indeed if any compiler worth its name wouldn't combine the comparison with the conditional branch it accompanies, and just turn that into a constant. IOW, you'd get
mask = 0 - (index < size); if (index <= size) { ... use mask ..
and the compiler would just turn that into
if (index <= size) { mask = -1;
and be done with it.
Linus
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