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

[Jerry Shih]

On Tue, Dec 2, 2025 at 2:32 PM Eric Biggers <ebiggers at 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.

-Jerry