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. --Andy -- 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
