Re: [PATCH v2] lib/crypto: riscv/chacha: Avoid s0/fp register

[Samuel Holland <samuel.holland@sifive.com>]

On 2025-12-09 12:58 PM, Jerry Shih wrote:
> On Tue, Dec 2, 2025 at 2:32 PM Eric Biggers <ebiggers@kernel.org> wrote:
>>
>> On Tue, Dec 02, 2025 at 02:24:46PM +0800, Vivian Wang wrote:
>>> On 12/2/25 13:31, Eric Biggers wrote:
>>>> On Tue, Dec 02, 2025 at 01:25:07PM +0800, Vivian Wang wrote:
>>>>> In chacha_zvkb, avoid using the s0 register, which is the frame pointer,
>>>>> by reallocating KEY0 to t5. This makes stack traces available if e.g. a
>>>>> crash happens in chacha_zvkb.
>>>>>
>>>>> No frame pointer maintenence is otherwise required since this is a leaf
>>>>> function.
>>>> maintenence => maintenance
>>>>
>>> Ouch... I swear I specifically checked this before sending, but
>>> apparently didn't see this. Thanks for the catch.
>>>
>>>>>  SYM_FUNC_START(chacha_zvkb)
>>>>>    addi            sp, sp, -96
>>>>> -  sd              s0, 0(sp)
>>>> I know it's annoying, but would you mind also changing the 96 to 88, and
>>>> decreasing all the offsets by 8, so that we don't leave a hole in the
>>>> stack where s0 used to be?  Likewise at the end of the function.
>>>
>>> No can do. Stack alignment on RISC-V is 16 bytes, and 80 won't fit.
>>>
>>
>> Hmm, interesting.  It shouldn't actually matter, since this doesn't call
>> any other function, but we might as well leave it at 96 then.  I don't
>> think this was considered when any of the RISC-V crypto code was
>> written, but fortunately this is the only one that uses the stack.
>>
>> Anyway, I guess I'll apply this as-is then.
>>
>> - Eric
> 
> The 16-byte stack alignment is in RISC-V calling convention:
> https://riscv.org/wp-content/uploads/2024/12/riscv-calling.pdf
> It says:
> In the standard RISC-V calling convention, the stack grows downward
> and the stack pointer is always kept 16-byte aligned.

Indeed, and this does matter if the code runs with IRQs enabled, as the RISC-V
entry assembly assumes the kernel stack is already properly aligned.

Regards,
Samuel