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

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Message-ID: <80cb6553-af8f-4fce-a010-dff3a33c3779@iscas.ac.cn>
Date: Tue, 2 Dec 2025 14:24:46 +0800
From: Vivian Wang <wangruikang@...as.ac.cn>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Jerry Shih <jerry.shih@...ive.com>, "Jason A. Donenfeld"
 <Jason@...c4.com>, Ard Biesheuvel <ardb@...nel.org>,
 Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
 Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
 linux-crypto@...r.kernel.org, linux-riscv@...ts.infradead.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] lib/crypto: riscv/chacha: Avoid s0/fp register

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.