[PATCH v3 4/8] riscv: ftrace: always preserve s0 in dynamic ftrace register frame
From: Wang Han <hidden>
Date: 2026-06-09 06:30:13
Also in:
linux-kselftest, linux-perf-users, linux-riscv, live-patching, lkml
Subsystem:
risc-v architecture, the rest · Maintainers:
Paul Walmsley, Palmer Dabbelt, Albert Ou, Linus Torvalds
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> Signed-off-by: Wang Han <redacted> --- 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 082fe0b0e3c0..26c55fba8fec 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)
--
2.43.0