Re: [PATCH bpf-next v9 04/11] bpf: support fsession for bpf_session_is_return
From: Andrii Nakryiko <hidden>
Date: 2026-01-14 01:23:27
Also in:
bpf, lkml
On Sat, Jan 10, 2026 at 6:12 AM Menglong Dong [off-list ref] wrote:
If fsession exists, we will use the bit (1 << BPF_TRAMP_M_IS_RETURN) in
ctx[-1] to store the "is_return" flag.
The logic of bpf_session_is_return() for fsession is implemented in the
verifier by inline following code:
bool bpf_session_is_return(void *ctx)
{
return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN));this look unnecessarily scary :) !! part is unnecessary because non-zero integer will be converted to proper true(1)/false(0) by compiler. But I'd just rewrite it in arguably slightly simpler form that lays itself to assembly more directly: return ((u64 *)ctx[-1] >> BPF_TRAMP_M_IS_RETURN) & 1;
quoted hunk ↗ jump to hunk
} Signed-off-by: Menglong Dong <redacted> Co-developed-by: Leon Hwang <redacted> Signed-off-by: Leon Hwang <redacted> --- v9: - remove the definition of bpf_fsession_is_return() v7: - reuse the kfunc bpf_session_is_return() instead of introduce new kfunc v4: - split out the bpf_fsession_cookie() to another patch v3: - merge the bpf_tracing_is_exit and bpf_fsession_cookie into a single patch v2: - store the session flags after return value, instead of before nr_args - inline the bpf_tracing_is_exit, as Jiri suggested --- include/linux/bpf.h | 3 +++ kernel/bpf/verifier.c | 15 ++++++++++++++- kernel/trace/bpf_trace.c | 28 +++++++++++++++++----------- 3 files changed, 34 insertions(+), 12 deletions(-)diff --git a/include/linux/bpf.h b/include/linux/bpf.h index 41228b0add52..2640ec2157e1 100644 --- a/include/linux/bpf.h +++ b/include/linux/bpf.h@@ -1229,6 +1229,9 @@ enum { #endif }; +#define BPF_TRAMP_M_NR_ARGS 0 +#define BPF_TRAMP_M_IS_RETURN 8
nit: What does "M" stand for? Macro? Mask? Menglong? ;) Some new convention, why?
quoted hunk ↗ jump to hunk
+ struct bpf_tramp_links { struct bpf_tramp_link *links[BPF_MAX_TRAMP_LINKS]; int nr_links;diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index bfff3f84fd91..1b0292a03186 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c@@ -12374,6 +12374,7 @@ enum special_kfunc_type { KF_bpf_arena_alloc_pages, KF_bpf_arena_free_pages, KF_bpf_arena_reserve_pages, + KF_bpf_session_is_return, }; BTF_ID_LIST(special_kfunc_list)@@ -12451,6 +12452,7 @@ BTF_ID(func, bpf_task_work_schedule_resume_impl) BTF_ID(func, bpf_arena_alloc_pages) BTF_ID(func, bpf_arena_free_pages) BTF_ID(func, bpf_arena_reserve_pages) +BTF_ID(func, bpf_session_is_return) static bool is_task_work_add_kfunc(u32 func_id) {@@ -12505,7 +12507,8 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env, struct bpf_reg_state *reg = ®s[regno]; bool arg_mem_size = false; - if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx]) + if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] || + meta->func_id == special_kfunc_list[KF_bpf_session_is_return]) return KF_ARG_PTR_TO_CTX; if (argno + 1 < nargs &&@@ -22558,6 +22561,16 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn, desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) { insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1); *cnt = 1; + } else if (desc->func_id == special_kfunc_list[KF_bpf_session_is_return] && + env->prog->expected_attach_type == BPF_TRACE_FSESSION) { + /* implement and inline the bpf_session_is_return() for
nit: comment style
+ * fsession, and the logic is: + * return !!(((u64 *)ctx)[-1] & (1 << BPF_TRAMP_M_IS_RETURN)) + */ + insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8); + insn_buf[1] = BPF_ALU64_IMM(BPF_RSH, BPF_REG_0, BPF_TRAMP_M_IS_RETURN); + insn_buf[2] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
lol, your assembly is simpler than that C expression above, let's keep C close to what you actually are doing in assembler
quoted hunk ↗ jump to hunk
+ *cnt = 3; } if (env->insn_aux_data[insn_idx].arg_prog) {diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index 297dcafb2c55..1fe508d451b7 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c@@ -3334,34 +3334,40 @@ __bpf_kfunc __u64 *bpf_session_cookie(void *ctx) __bpf_kfunc_end_defs(); -BTF_KFUNCS_START(kprobe_multi_kfunc_set_ids) +BTF_KFUNCS_START(session_kfunc_set_ids) BTF_ID_FLAGS(func, bpf_session_is_return) BTF_ID_FLAGS(func, bpf_session_cookie) -BTF_KFUNCS_END(kprobe_multi_kfunc_set_ids) +BTF_KFUNCS_END(session_kfunc_set_ids) -static int bpf_kprobe_multi_filter(const struct bpf_prog *prog, u32 kfunc_id) +static int bpf_session_filter(const struct bpf_prog *prog, u32 kfunc_id) { - if (!btf_id_set8_contains(&kprobe_multi_kfunc_set_ids, kfunc_id)) + if (!btf_id_set8_contains(&session_kfunc_set_ids, kfunc_id)) return 0; - if (!is_kprobe_session(prog) && !is_uprobe_session(prog)) + if (!is_kprobe_session(prog) && !is_uprobe_session(prog) && + prog->expected_attach_type != BPF_TRACE_FSESSION)
check both expected_attach_type *and* prog_type, please (and I think it would be good to check prog type for kprobe_session and uprobe_session as well, because now it's not guaranteed that program will be of BPF_PROG_TYPE_KPROBE
return -EACCES;
return 0;
}
-static const struct btf_kfunc_id_set bpf_kprobe_multi_kfunc_set = {
+static const struct btf_kfunc_id_set bpf_session_kfunc_set = {
.owner = THIS_MODULE,
- .set = &kprobe_multi_kfunc_set_ids,
- .filter = bpf_kprobe_multi_filter,
+ .set = &session_kfunc_set_ids,
+ .filter = bpf_session_filter,
};
-static int __init bpf_kprobe_multi_kfuncs_init(void)
+static int __init bpf_trace_kfuncs_init(void)
{
- return register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_kprobe_multi_kfunc_set);
+ int err = 0;
+
+ err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_session_kfunc_set);
+ err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_session_kfunc_set);
+
+ return err;
}
-late_initcall(bpf_kprobe_multi_kfuncs_init);
+late_initcall(bpf_trace_kfuncs_init);
typedef int (*copy_fn_t)(void *dst, const void *src, u32 size, struct task_struct *tsk);
--
2.52.0