[PATCH v6 3/7] riscv: ftrace: always preserve s0 in dynamic ftrace register frame

[Shuai Xue <xueshuai@linux.alibaba.com>]

From: Wang Han <wanghan@linux.alibaba.com>

struct __arch_ftrace_regs declares s0 unconditionally, and both
ftrace_regs_get_frame_pointer() and ftrace_partial_regs() read it
unconditionally. But the SAVE_ABI_REGS / RESTORE_ABI_REGS macros in
mcount-dyn.S only stored s0 under HAVE_FUNCTION_GRAPH_FP_TEST
(CONFIG_FUNCTION_GRAPH_TRACER && CONFIG_FRAME_POINTER). With
CONFIG_FRAME_POINTER=n the slot held whatever was on the stack before,
so any callback going through ftrace_partial_regs() saw a garbage
regs->s0. RISC-V kernels default to FRAME_POINTER=y, which is why this
has not bitten in practice.

Save and restore s0 unconditionally in the dynamic ftrace ABI register
frame. This fixes the latent garbage-s0 case, brings the dynamic ftrace
path in line with the static _mcount path (mcount.S SAVE_ABI_STATE
already saves s0 unconditionally), and matches the frame layout already
documented in the comment above SAVE_ABI_REGS. It is also a prerequisite
for the upcoming reliable unwinder, which reads
ftrace_regs_get_frame_pointer(fregs) directly.

The cost is one extra REG_S/REG_L pair per traced call, negligible
compared to the overall ftrace cost; the existing FREGS_SIZE_ON_STACK
already reserved the slot, so no extra stack space is used.

Reviewed-by: Shuai Xue <xueshuai@linux.alibaba.com>
Assisted-by: Qoder:Qwen-3.8-MAX-Preview
Signed-off-by: Wang Han <wanghan@linux.alibaba.com>
Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
---
 arch/riscv/kernel/mcount-dyn.S | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/riscv/kernel/mcount-dyn.S b/arch/riscv/kernel/mcount-dyn.S
index 082fe0b0e3c08..26c55fba8fec7 100644
--- a/arch/riscv/kernel/mcount-dyn.S
+++ b/arch/riscv/kernel/mcount-dyn.S
@@ -85,9 +85,7 @@
 	addi	sp, sp, -FREGS_SIZE_ON_STACK
 	REG_S	t0,  FREGS_EPC(sp)
 	REG_S	x1,  FREGS_RA(sp)
-#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
 	REG_S	x8,  FREGS_S0(sp)
-#endif
 	REG_S	x6,  FREGS_T1(sp)
 #ifdef CONFIG_CC_IS_CLANG
 	REG_S	x7,  FREGS_T2(sp)
@@ -113,9 +111,7 @@
 	.macro RESTORE_ABI_REGS
 	REG_L	t0, FREGS_EPC(sp)
 	REG_L	x1, FREGS_RA(sp)
-#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
 	REG_L	x8, FREGS_S0(sp)
-#endif
 	REG_L	x6,  FREGS_T1(sp)
 #ifdef CONFIG_CC_IS_CLANG
 	REG_L	x7,  FREGS_T2(sp)
-- 
2.39.3