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

[Ard Biesheuvel]

On 10 January 2017 at 19:22, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Tue, Jan 10, 2017 at 11:16 AM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> 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.
>
> 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.