linux-kernel - [PATCH bpf-next v4 2/9] bpf: use last 8-bits for the nr

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Message-ID: <20251217095445.218428-3-dongml2@chinatelecom.cn>
Date: Wed, 17 Dec 2025 17:54:38 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: ast@...nel.org,
	andrii@...nel.org
Cc: davem@...emloft.net,
	dsahern@...nel.org,
	daniel@...earbox.net,
	martin.lau@...ux.dev,
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	dave.hansen@...ux.intel.com,
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Subject: [PATCH bpf-next v4 2/9] bpf: use last 8-bits for the nr_args in trampoline

For now, ctx[-1] is used to store the nr_args in the trampoline. However,
1-byte is enough to store such information. Therefor, we use only the last
byts of ctx[-1] to store the nr_args, and reverve the rest for other
usages.

Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
---
 kernel/bpf/verifier.c    | 35 +++++++++++++++++++----------------
 kernel/trace/bpf_trace.c |  4 ++--
 2 files changed, 21 insertions(+), 18 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d399bfd2413f..96753833c090 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -23275,15 +23275,16 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 		    insn->imm == BPF_FUNC_get_func_arg) {
 			/* Load nr_args from ctx - 8 */
 			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
-			insn_buf[1] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6);
-			insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3);
-			insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
-			insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
-			insn_buf[5] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
-			insn_buf[6] = BPF_MOV64_IMM(BPF_REG_0, 0);
-			insn_buf[7] = BPF_JMP_A(1);
-			insn_buf[8] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
-			cnt = 9;
+			insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
+			insn_buf[2] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6);
+			insn_buf[3] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3);
+			insn_buf[4] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1);
+			insn_buf[5] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0);
+			insn_buf[6] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
+			insn_buf[7] = BPF_MOV64_IMM(BPF_REG_0, 0);
+			insn_buf[8] = BPF_JMP_A(1);
+			insn_buf[9] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL);
+			cnt = 10;
 
 			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt);
 			if (!new_prog)
@@ -23303,12 +23304,13 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 			    eatype == BPF_MODIFY_RETURN) {
 				/* Load nr_args from ctx - 8 */
 				insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
-				insn_buf[1] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
-				insn_buf[2] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
-				insn_buf[3] = BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
-				insn_buf[4] = BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_3, 0);
-				insn_buf[5] = BPF_MOV64_IMM(BPF_REG_0, 0);
-				cnt = 6;
+				insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
+				insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
+				insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
+				insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0);
+				insn_buf[5] = BPF_STX_MEM(BPF_DW, BPF_REG_2, BPF_REG_3, 0);
+				insn_buf[6] = BPF_MOV64_IMM(BPF_REG_0, 0);
+				cnt = 7;
 			} else {
 				insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, -EOPNOTSUPP);
 				cnt = 1;
@@ -23329,8 +23331,9 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
 		    insn->imm == BPF_FUNC_get_func_arg_cnt) {
 			/* Load nr_args from ctx - 8 */
 			insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
+			insn_buf[1] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 0xFF);
 
-			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1);
+			new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 2);
 			if (!new_prog)
 				return -ENOMEM;
 
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index d57727abaade..10c9992d2745 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -1194,7 +1194,7 @@ const struct bpf_func_proto bpf_get_branch_snapshot_proto = {
 BPF_CALL_3(get_func_arg, void *, ctx, u32, n, u64 *, value)
 {
 	/* This helper call is inlined by verifier. */
-	u64 nr_args = ((u64 *)ctx)[-1];
+	u64 nr_args = ((u64 *)ctx)[-1] & 0xFF;
 
 	if ((u64) n >= nr_args)
 		return -EINVAL;
@@ -1214,7 +1214,7 @@ static const struct bpf_func_proto bpf_get_func_arg_proto = {
 BPF_CALL_2(get_func_ret, void *, ctx, u64 *, value)
 {
 	/* This helper call is inlined by verifier. */
-	u64 nr_args = ((u64 *)ctx)[-1];
+	u64 nr_args = ((u64 *)ctx)[-1] & 0xFF;
 
 	*value = ((u64 *)ctx)[nr_args];
 	return 0;
-- 
2.52.0