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Date: Sun, 28 Feb 2010 19:36:07 -0500 From: Bill Davidsen <davidsen@....com> To: linux-kernel@...r.kernel.org Cc: linux-crypto@...r.kernel.org Subject: Re: [PATCH v1] compiler: prevent dead store elimination Andi Kleen wrote: >> Every byte in the [p,p+n[ range must be used. If you only use the >> first byte, via e.g. asm("" :: "m"(*(char*)p)), then the compiler >> _will_ skip scrubbing bytes beyond the first. This works with >> gcc-3.2.3 up to gcc-4.4.3. > > You forgot to credit Mikael who did all the hard work figuring > this out? > >> /* >> + * Dead store elimination (DSE) is an optimization that may remove a write to >> + * a buffer that is not used anymore. Use ARRAY_PREVENT_DSE after a write when >> + * the scrub is required for security reasons. >> + */ >> +#define ARRAY_PREVENT_DSE(p, n) \ > > Maybe it's just me, but the name is ugly. > >> + do { \ >> + struct __scrub { char c[n]; }; \ > > > Better typeof(*p)[n] > >> +++ b/include/linux/compiler-intel.h >> @@ -14,9 +14,11 @@ >> * It uses intrinsics to do the equivalent things. >> */ >> #undef barrier >> +#undef ARRAY_PREVENT_DSE >> #undef RELOC_HIDE >> >> #define barrier() __memory_barrier() >> +#define ARRAY_PREVENT_DSE(p, n) > > Who says the Intel compiler doesn't need this? > > I'm sure it does dead store elimination too and it understands > gcc asm syntax. > According to the Intel forum, it not only doesn't, but a request for this as a feature was rejected, so it won't. Or am I misreading this? http://software.intel.com/en-us/forums/showthread.php?t=46770 -- Bill Davidsen <davidsen@....com> "We have more to fear from the bungling of the incompetent than from the machinations of the wicked." - from Slashdot -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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