[PATCH v3] LoongArch: BPF: Fix stack arguments for indirect trampolines

[Tiezhu Yang]

From: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>

LoongArch passes arguments beyond a0-a7 at the caller stack pointer. The
trampoline store_args() helper always reads those arguments at FP + 16,
which is correct for a regular fentry trampoline: its prologue leaves FP
16 bytes below the stack pointer at trampoline entry to hold the saved
RA/FP frame record of the parent function.

A struct_ops indirect trampoline is entered through a function pointer and
only saves its own RA and FP before setting FP to the entry stack pointer.
Its stack arguments therefore start at FP, not FP + 16. As a result, every
stack-passed struct_ops argument is currently read two slots late.

Select the source offset based on whether the trampoline is indirect. This
also prepares the stack-passed arena argument path to consume the actual
pointer slot.

With this patch, the BPF selftest "./test_progs -t struct_ops_multi_args"
passed on LoongArch.

Fixes: c9ebe2016de9 ("LoongArch: BPF: Support up to 12 function arguments for trampoline")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
v3:
  - Reword the log and comment to clarify that the 16-byte frame record
    is saved for the parent function, not the traced function. (bot+bpf-ci)
  - Remove a stray blank line after the opening brace of store_args(). (bot+bpf-ci)
  - Reorder the comment sentences to match the true/false branches of
    is_struct_ops for better readability.

Here is the original link:
https://lore.kernel.org/bpf/20260821233516.3426127-8-memxor@xxxxxxxxx/

This version is based on the series:
https://lore.kernel.org/loongarch/20261016081006.26069-1-yangtiezhu@xxxxxxxxxxx/

 arch/loongarch/net/bpf_jit.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 1a7c8effc37b..c6784602f029 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -1725,17 +1725,22 @@ int bpf_arch_text_invalidate(void *dst, size_t len)
 	return ret;
 }
 
-static void store_args(struct jit_ctx *ctx, int nr_arg_slots, int args_off)
+static void store_args(struct jit_ctx *ctx, int nr_arg_slots, int args_off, bool is_struct_ops)
 {
+	/*
+	 * Indirect struct_ops trampolines place incoming stack arguments directly at FP.
+	 * Regular fentry trampolines need to skip the 16-byte RA/FP frame record saved
+	 * for the parent function.
+	 */
+	int stack_args_off = is_struct_ops ? 0 : 16;
 	int i;
 
 	for (i = 0; i < nr_arg_slots; i++) {
 		if (i < LOONGARCH_MAX_REG_ARGS)
 			emit_insn(ctx, std, LOONGARCH_GPR_A0 + i, LOONGARCH_GPR_FP, -args_off);
 		else {
-			/* Skip slots for T0 and FP of traced function */
 			emit_insn(ctx, ldd, LOONGARCH_GPR_T1, LOONGARCH_GPR_FP,
-				  16 + (i - LOONGARCH_MAX_REG_ARGS) * 8);
+				  stack_args_off + (i - LOONGARCH_MAX_REG_ARGS) * 8);
 			emit_insn(ctx, std, LOONGARCH_GPR_T1, LOONGARCH_GPR_FP, -args_off);
 		}
 		args_off -= 8;
@@ -2068,7 +2073,7 @@ static int __arch_prepare_bpf_trampoline(struct jit_ctx *ctx, struct bpf_tramp_i
 	func_meta = nr_arg_slots;
 	emit_store_stack_imm64(ctx, LOONGARCH_GPR_T1, -func_meta_off, func_meta);
 
-	store_args(ctx, nr_arg_slots, args_off);
+	store_args(ctx, nr_arg_slots, args_off, is_struct_ops);
 
 	if (bpf_fsession_cnt(tnodes)) {
 		/* clear all session cookies' value */
-- 
2.42.0