[PATCH bpf-next v11 02/12] bpf: use the least significant byte for the nr_args in trampoline
From: Menglong Dong <hidden>
Date: 2026-01-23 07:36:01
Also in:
bpf, lkml
Subsystem:
bpf [core], bpf [general] (safe dynamic programs and tools), bpf [security & lsm] (security audit and enforcement using bpf), bpf [tracing], the rest, tracing · Maintainers:
Alexei Starovoitov, Daniel Borkmann, Andrii Nakryiko, Eduard Zingerman, Kumar Kartikeya Dwivedi, KP Singh, Matt Bobrowski, Song Liu, Linus Torvalds, Steven Rostedt, Masami Hiramatsu
For now, ((u64 *)ctx)[-1] is used to store the nr_args in the trampoline. However, 1 byte is enough to store such information. Therefore, we use only the least significant byte of ((u64 *)ctx)[-1] to store the nr_args, and reserve the rest for other usages. Signed-off-by: Menglong Dong <redacted> --- v11: - fix the rebase conflict v10: - some adjustment to the subject and commit log to make the description more precise. v8: - fix the missed get_func_arg_cnt --- kernel/bpf/verifier.c | 38 ++++++++++++++++++++++---------------- kernel/trace/bpf_trace.c | 6 +++--- 2 files changed, 25 insertions(+), 19 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 41bbed6418b5..b3f912ad18ba 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c@@ -23747,19 +23747,21 @@ static int do_misc_fixups(struct bpf_verifier_env *env) /* skip 'void *__data' in btf_trace_##name() and save to reg0 */ insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args - 1); + cnt = 1; } else { /* 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); + cnt = 2; } - 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[cnt++] = BPF_JMP32_REG(BPF_JGE, BPF_REG_2, BPF_REG_0, 6); + insn_buf[cnt++] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_2, 3); + insn_buf[cnt++] = BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1); + insn_buf[cnt++] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_2, 0); + insn_buf[cnt++] = BPF_STX_MEM(BPF_DW, BPF_REG_3, BPF_REG_0, 0); + insn_buf[cnt++] = BPF_MOV64_IMM(BPF_REG_0, 0); + insn_buf[cnt++] = BPF_JMP_A(1); + insn_buf[cnt++] = BPF_MOV64_IMM(BPF_REG_0, -EINVAL); new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt); if (!new_prog)
@@ -23779,12 +23781,13 @@ 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;
@@ -23808,12 +23811,15 @@ static int do_misc_fixups(struct bpf_verifier_env *env) /* skip 'void *__data' in btf_trace_##name() and save to reg0 */ insn_buf[0] = BPF_MOV64_IMM(BPF_REG_0, nr_args - 1); + cnt = 1; } else { /* 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); + cnt = 2; } - new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, 1); + new_prog = bpf_patch_insn_data(env, i + delta, insn_buf, cnt); if (!new_prog) return -ENOMEM;
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index 0e9635bcd783..d466a1503da3 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;
@@ -1231,7 +1231,7 @@ static const struct bpf_func_proto bpf_get_func_ret_proto = { BPF_CALL_1(get_func_arg_cnt, void *, ctx) { /* This helper call is inlined by verifier. */ - return ((u64 *)ctx)[-1]; + return ((u64 *)ctx)[-1] & 0xFF; } static const struct bpf_func_proto bpf_get_func_arg_cnt_proto = {
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2.52.0