On Tue, Jan 10, 2017 at 11:16 AM, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: > On 10 January 2017 at 19:00, Andy Lutomirski <l...@amacapital.net> wrote: >> On Tue, Jan 10, 2017 at 9:30 AM, Ard Biesheuvel >> <ard.biesheu...@linaro.org> wrote: >>> On 10 January 2017 at 14:33, Herbert Xu <herb...@gondor.apana.org.au> wrote: >>>> I recently applied the patch >>>> >>>> https://patchwork.kernel.org/patch/9468391/ >>>> >>>> and ended up with a boot crash when it tried to run the x86 chacha20 >>>> code. It turned out that the patch changed a manually aligned >>>> stack buffer to one that is aligned by gcc. What was happening was >>>> that gcc can stack align to any value on x86-64 except 16. The >>>> reason is that gcc assumes that the stack is always 16-byte aligned, >>>> which is not actually the case in the kernel. >>>> >>> >>> Apologies for introducing this breakage. It seemed like an obvious and >>> simple cleanup, so I didn't even bother to mention it in the commit >>> log, but if the kernel does not guarantee 16 byte alignment, I guess >>> we should revert to the old method. If SSE instructions are the only >>> ones that require this alignment, then I suppose not having a ABI >>> conforming stack pointer should not be an issue in general. >> >> Here's what I think is really going on. This is partially from >> memory, so I could be off base. The kernel is up against >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383, which means that, >> on some GCC versions (like the bad one and maybe even current ones), >> things compiled without -mno-sse can't have the stack alignment set >> properly. IMO we should fix this in the affected code, not the entry >> code. In fact, I think that fixing it in the entry code won't even >> fully fix it because modern GCC will compile the rest of the kernel >> with 8-byte alignment and the stack will get randomly unaligned (GCC >> 4.8 and newer). >> >> Can we just add __attribute__((force_align_arg_pointer)) to the >> affected functions? Maybe have: >> >> #define __USES_SSE __attribute__((force_align_arg_pointer)) >> >> on affected gcc versions? >> >> ***HOWEVER*** >> >> I think this is missing the tree for the supposed forest. The actual >> affected code appears to be: >> >> static int chacha20_simd(struct blkcipher_desc *desc, struct scatterlist >> *dst, >> struct scatterlist *src, unsigned int nbytes) >> { >> u32 *state, state_buf[16 + (CHACHA20_STATE_ALIGN / sizeof(u32)) - 1]; >> >> ... >> >> state = (u32 *)roundup((uintptr_t)state_buf, CHACHA20_STATE_ALIGN); >> >> gcc presumably infers (incorrectly) that state_buf is 16-byte aligned >> and optimizes out the roundup. How about just declaring an actual >> __aligned(16) buffer, marking the function >> __attribute__((force_align_arg_pointer)), and being done with it? >> After all, we need that forcible alignment on *all* gcc versions. >> > > Actually, the breakage is introduced by the patch Herbert refers to > > https://patchwork.kernel.org/patch/9468391/ > > where the state is replaced by a simple > > u32 state[16] __aligned(CHACHA20_STATE_ALIGN); > > which seemed harmless enough to me. So the code above works fine.
So how about just the one-line patch of adding the force_align_arg_pointer? Would that solve the problem?
