[PATCH bpf 1/1] bpf, riscv: Fix stack arguments of struct_ops trampolines

[KaFai Wan]

When a struct_ops trampoline takes more than 8 arguments (up to 12), the
9th and beyond are passed on the stack. store_args() copies them into the
trampoline frame using a hard-coded FP + 16 base:

	emit_ld(RV_REG_T1, 16 + (i - RV_MAX_REG_ARGS) * 8, RV_REG_FP, ctx);

That offset only holds for fentry trampolines, where the traced function's
T0/FP are saved at FP - 8/FP - 16 and its stack arguments start at FP + 16.
A struct_ops trampoline is called directly, so its stack arguments start at
FP + 0 and store_args() reads the wrong words. BPF programs then see garbage
for arg9 and beyond, which fails the selftest:

  struct_ops_multi_args/test_trampoline_stack_args:FAIL

Pass the stack argument base offset to store_args(): 0 for struct_ops
trampolines, 16 otherwise.

Fixes: 6801b0aef79d ("riscv, bpf: Add 12-argument support for RV64 bpf trampoline")
Signed-off-by: KaFai Wan <kafai.wan at linux.dev>
---
 arch/riscv/net/bpf_jit_comp64.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
index 01fe66774f02..4ae6674f58ed 100644
--- a/arch/riscv/net/bpf_jit_comp64.c
+++ b/arch/riscv/net/bpf_jit_comp64.c
@@ -875,7 +875,7 @@ int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type old_t,
 	return ret;
 }
 
-static void store_args(int nr_arg_slots, int args_off, struct rv_jit_context *ctx)
+static void store_args(int nr_arg_slots, int args_off, int stack_base, struct rv_jit_context *ctx)
 {
 	int i;
 
@@ -883,8 +883,7 @@ static void store_args(int nr_arg_slots, int args_off, struct rv_jit_context *ct
 		if (i < RV_MAX_REG_ARGS) {
 			emit_sd(RV_REG_FP, -args_off, RV_REG_A0 + i, ctx);
 		} else {
-			/* skip slots for T0 and FP of traced function */
-			emit_ld(RV_REG_T1, 16 + (i - RV_MAX_REG_ARGS) * 8, RV_REG_FP, ctx);
+			emit_ld(RV_REG_T1, stack_base + (i - RV_MAX_REG_ARGS) * 8, RV_REG_FP, ctx);
 			emit_sd(RV_REG_FP, -args_off, RV_REG_T1, ctx);
 		}
 		args_off -= 8;
@@ -1179,7 +1178,8 @@ static int __arch_prepare_bpf_trampoline(struct bpf_tramp_image *im,
 	func_meta = nr_arg_slots;
 	emit_store_stack_imm64(RV_REG_T1, -func_meta_off, func_meta, ctx);
 
-	store_args(nr_arg_slots, args_off, ctx);
+	/* skip slots for T0 and FP of traced function */
+	store_args(nr_arg_slots, args_off, is_struct_ops ? 0 : 16, ctx);
 
 	if (bpf_fsession_cnt(tnodes)) {
 		/* clear all session cookies' value */
-- 
2.43.0