On Tue, Jan 10, 2017 at 12:00 PM, Ard Biesheuvel <ard.biesheu...@linaro.org> wrote: > On 10 January 2017 at 19:22, Andy Lutomirski <l...@amacapital.net> wrote: >> 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? > > If it does what it says on the tin, it should fix the issue, but after > adding the attribute, I get the exact same object output, so there's > something dodgy going on here.
Ugh, that's annoying. Maybe it needs noinline too? --Andy
