On Wed, Mar 18, 2015, at 13:42, Daniel Borkmann wrote: > On 03/18/2015 01:20 PM, Stephan Mueller wrote: > > Am Mittwoch, 18. März 2015, 13:19:07 schrieb Hannes Frederic Sowa: > >>>> My proposal would be to add a > >>>> > >>>> #define OPTIMIZER_HIDE_MEM(ptr, len) __asm__ __volatile__ ("" : : > >>>> "m"( > >>>> ({ struct { u8 b[len]; } *p = (void *)ptr ; *p; }) ) > >>>> > >>>> and use this in the code function. > >>>> > >>>> This is documented in gcc manual 6.43.2.5. > >>> > >>> That one adds the zeroization instructuctions. But now there are much > >>> more than with the barrier. > >>> > >>> 400469: 48 c7 04 24 00 00 00 movq $0x0,(%rsp) > >>> 400470: 00 > >>> 400471: 48 c7 44 24 08 00 00 movq $0x0,0x8(%rsp) > >>> 400478: 00 00 > >>> 40047a: c7 44 24 10 00 00 00 movl $0x0,0x10(%rsp) > >>> 400481: 00 > >>> 400482: 48 c7 44 24 20 00 00 movq $0x0,0x20(%rsp) > >>> 400489: 00 00 > >>> 40048b: 48 c7 44 24 28 00 00 movq $0x0,0x28(%rsp) > >>> 400492: 00 00 > >>> 400494: c7 44 24 30 00 00 00 movl $0x0,0x30(%rsp) > >>> 40049b: 00 > >>> > >>> Any ideas? > >> > >> Hmm, correct definition of u8? > > > > I use unsigned char > >> > >> Which version of gcc do you use? I can't see any difference if I > >> compile your example at -O2. > > > > gcc-Version 4.9.2 20150212 (Red Hat 4.9.2-6) (GCC) Well, was an error on my side, I see the same behavior. > > I can see the same with the gcc version I previously posted. So > it clears the 20 bytes from your example (movq, movq, movl) at > two locations, presumably buf[] and b[]. Yes, it looks like that. The reservation on the stack changes, too. Seems like just using barrier() is the best and easiest option. Thanks, Hannes -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html