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

[Vivian Wang]

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.

Signed-off-by: Vivian Wang <wangruikang at iscas.ac.cn>
---
Changes in v2:
- Remove frame pointer maintenance, and simply avoid touching s0. Since
  this is a leaf function, this also allows unwinding to work.
- Link to v1: https://lore.kernel.org/r/20251130-riscv-chacha_zvkb-fp-v1-1-68ef7a6d477a@iscas.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 | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/lib/crypto/riscv/chacha-riscv64-zvkb.S b/lib/crypto/riscv/chacha-riscv64-zvkb.S
index b777d0b4e379..3d183ec818f5 100644
--- a/lib/crypto/riscv/chacha-riscv64-zvkb.S
+++ b/lib/crypto/riscv/chacha-riscv64-zvkb.S
@@ -60,7 +60,8 @@
 #define VL		t2
 #define STRIDE		t3
 #define ROUND_CTR	t4
-#define KEY0		s0
+#define KEY0		t5
+// Avoid s0/fp to allow for unwinding
 #define KEY1		s1
 #define KEY2		s2
 #define KEY3		s3
@@ -143,7 +144,6 @@
 // The updated 32-bit counter is written back to state->x[12] before returning.
 SYM_FUNC_START(chacha_zvkb)
 	addi		sp, sp, -96
-	sd		s0, 0(sp)
 	sd		s1, 8(sp)
 	sd		s2, 16(sp)
 	sd		s3, 24(sp)
@@ -280,7 +280,6 @@ SYM_FUNC_START(chacha_zvkb)
 	bnez		NBLOCKS, .Lblock_loop
 
 	sw		COUNTER, 48(STATEP)
-	ld		s0, 0(sp)
 	ld		s1, 8(sp)
 	ld		s2, 16(sp)
 	ld		s3, 24(sp)

---
base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
change-id: 20251130-riscv-chacha_zvkb-fp-5644ed88b1a2

Best regards,
-- 
Vivian "dramforever" Wang