On 10 January 2017 at 19:00, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Tue, Jan 10, 2017 at 9:30 AM, Ard Biesheuvel > <ard.biesheuvel@xxxxxxxxxx> wrote: >> On 10 January 2017 at 14:33, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> 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. -- 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
