Re: x86-64: Maintain 16-byte stack alignment

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jan 10, 2017 at 12:00 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> On 10 January 2017 at 19:22, Andy Lutomirski <luto@amacapital.net> wrote:
>> On Tue, Jan 10, 2017 at 11:16 AM, Ard Biesheuvel
>> <ard.biesheuvel@linaro.org> wrote:
>>> On 10 January 2017 at 19:00, Andy Lutomirski <luto@amacapital.net> wrote:
>>>> On Tue, Jan 10, 2017 at 9:30 AM, Ard Biesheuvel
>>>> <ard.biesheuvel@linaro.org> wrote:
>>>>> On 10 January 2017 at 14:33, Herbert Xu <herbert@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