linux-kernel - Re: [PATCH] lib/crypto: riscv/chacha: Maintain a frame pointer

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Message-ID: <20251130182914.GA1395@sol>
Date: Sun, 30 Nov 2025 10:29:14 -0800
From: Eric Biggers <ebiggers@...nel.org>
To: Vivian Wang <wangruikang@...as.ac.cn>
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] lib/crypto: riscv/chacha: Maintain a frame pointer

On Sun, Nov 30, 2025 at 06:23:50PM +0800, Vivian Wang wrote:
> crypto_zvkb doesn't maintain a frame pointer and also uses s0, which
> means that if it crashes we don't get a stack trace. Modify prologue and
> epilogue to maintain a frame pointer as -fno-omit-frame-pointer would.
> Also reallocate registers to match.
> 
> Signed-off-by: Vivian Wang <wangruikang@...as.ac.cn>
> ---
> Found while diagnosing a crypto_zvkb "load address misaligned" crash [1]
> 
> [1]: https://lore.kernel.org/r/b3cfcdac-0337-4db0-a611-258f2868855f@iscas.ac.cn/
> ---
>  lib/crypto/riscv/chacha-riscv64-zvkb.S | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)

Do I understand correctly that the problem isn't so much that
crypto_zvkb() doesn't set up its own frame pointer, but rather it reuses
the frame pointer register (s0 i.e. fp) for other data?

That's what we've seen on other architectures, like x86_64 with %rbp.
Assembly functions need to set their own frame pointer only if they call
other functions.  Otherwise, they can just run with their parent's frame
pointer.  However, in either case, they must not store other data in the
frame pointer register.

Is that the case on RISC-V too?  If so, the appropriate fix is to just
stop using s0 for other data; we don't actually need to set up a frame
pointer.  (Note that none of the RISC-V crypto assembly code sets up
frame pointers.  So if that was an issue, it would affect every file.)

- Eric