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Message-ID: <CAEf4Bzb+p4fXkCL01MVrvCwPvboeMWXgu4uTSMhweO_MYL+tqg@mail.gmail.com>
Date: Tue, 13 Jan 2026 17:22:41 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Menglong Dong <menglong8.dong@...il.com>
Cc: ast@...nel.org, andrii@...nel.org, daniel@...earbox.net,
martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org,
yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org,
sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, davem@...emloft.net,
dsahern@...nel.org, tglx@...utronix.de, mingo@...hat.com,
jiang.biao@...ux.dev, bp@...en8.de, dave.hansen@...ux.intel.com,
x86@...nel.org, hpa@...or.com, bpf@...r.kernel.org, netdev@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v9 01/11] bpf: add fsession support
On Sat, Jan 10, 2026 at 6:11 AM Menglong Dong <menglong8.dong@...il.com> wrote:
>
> The fsession is something that similar to kprobe session. It allow to
> attach a single BPF program to both the entry and the exit of the target
> functions.
>
> Introduce the struct bpf_fsession_link, which allows to add the link to
> both the fentry and fexit progs_hlist of the trampoline.
>
> Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> Co-developed-by: Leon Hwang <leon.hwang@...ux.dev>
> Signed-off-by: Leon Hwang <leon.hwang@...ux.dev>
> ---
> v5:
> - unify the name to "fsession"
> - use more explicit way in __bpf_trampoline_link_prog()
>
> v4:
> - instead of adding a new hlist to progs_hlist in trampoline, add the bpf
> program to both the fentry hlist and the fexit hlist.
> ---
> include/linux/bpf.h | 19 +++++++++
> include/uapi/linux/bpf.h | 1 +
> kernel/bpf/btf.c | 2 +
> kernel/bpf/syscall.c | 18 ++++++++-
> kernel/bpf/trampoline.c | 40 ++++++++++++++++---
> kernel/bpf/verifier.c | 12 ++++--
> net/bpf/test_run.c | 1 +
> net/core/bpf_sk_storage.c | 1 +
> tools/include/uapi/linux/bpf.h | 1 +
> .../bpf/prog_tests/tracing_failure.c | 2 +-
> 10 files changed, 87 insertions(+), 10 deletions(-)
>
[...]
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index 539c9fdea41d..8b1dcd440356 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -6107,6 +6107,7 @@ static int btf_validate_prog_ctx_type(struct bpf_verifier_log *log, const struct
> case BPF_TRACE_FENTRY:
> case BPF_TRACE_FEXIT:
> case BPF_MODIFY_RETURN:
> + case BPF_TRACE_FSESSION:
> /* allow u64* as ctx */
> if (btf_is_int(t) && t->size == 8)
> return 0;
> @@ -6704,6 +6705,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
> fallthrough;
> case BPF_LSM_CGROUP:
> case BPF_TRACE_FEXIT:
> + case BPF_TRACE_FSESSION:
According to the comment below we make this exception due to LSM.
FSESSION won't be using FSESSION programs, no? So this is not
necessary?
> /* When LSM programs are attached to void LSM hooks
> * they use FEXIT trampolines and when attached to
> * int LSM hooks, they use MODIFY_RETURN trampolines.
[...]
> @@ -4350,6 +4365,7 @@ attach_type_to_prog_type(enum bpf_attach_type attach_type)
> case BPF_TRACE_RAW_TP:
> case BPF_TRACE_FENTRY:
> case BPF_TRACE_FEXIT:
> + case BPF_TRACE_FSESSION:
> case BPF_MODIFY_RETURN:
> return BPF_PROG_TYPE_TRACING;
> case BPF_LSM_MAC:
> diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> index 2a125d063e62..11e043049d68 100644
> --- a/kernel/bpf/trampoline.c
> +++ b/kernel/bpf/trampoline.c
> @@ -111,7 +111,7 @@ bool bpf_prog_has_trampoline(const struct bpf_prog *prog)
>
> return (ptype == BPF_PROG_TYPE_TRACING &&
> (eatype == BPF_TRACE_FENTRY || eatype == BPF_TRACE_FEXIT ||
> - eatype == BPF_MODIFY_RETURN)) ||
> + eatype == BPF_MODIFY_RETURN || eatype == BPF_TRACE_FSESSION)) ||
> (ptype == BPF_PROG_TYPE_LSM && eatype == BPF_LSM_MAC);
this is getting crazy, switch to the switch (lol) maybe?
> }
>
> @@ -559,6 +559,8 @@ static enum bpf_tramp_prog_type bpf_attach_type_to_tramp(struct bpf_prog *prog)
> return BPF_TRAMP_MODIFY_RETURN;
> case BPF_TRACE_FEXIT:
> return BPF_TRAMP_FEXIT;
> + case BPF_TRACE_FSESSION:
> + return BPF_TRAMP_FSESSION;
> case BPF_LSM_MAC:
> if (!prog->aux->attach_func_proto->type)
> /* The function returns void, we cannot modify its
> @@ -596,6 +598,8 @@ static int __bpf_trampoline_link_prog(struct bpf_tramp_link *link,
> {
> enum bpf_tramp_prog_type kind;
> struct bpf_tramp_link *link_exiting;
> + struct bpf_fsession_link *fslink;
initialize to NULL to avoid compiler (falsely, but still) complaining
about potentially using uninitialized value
> + struct hlist_head *prog_list;
> int err = 0;
> int cnt = 0, i;
>
[...]
> - hlist_add_head(&link->tramp_hlist, &tr->progs_hlist[kind]);
> - tr->progs_cnt[kind]++;
> + hlist_add_head(&link->tramp_hlist, prog_list);
> + if (kind == BPF_TRAMP_FSESSION) {
> + tr->progs_cnt[BPF_TRAMP_FENTRY]++;
> + fslink = container_of(link, struct bpf_fsession_link, link.link);
> + hlist_add_head(&fslink->fexit.tramp_hlist,
> + &tr->progs_hlist[BPF_TRAMP_FEXIT]);
fits under 100 characters? keep on a single line then
> + tr->progs_cnt[BPF_TRAMP_FEXIT]++;
> + } else {
> + tr->progs_cnt[kind]++;
> + }
> err = bpf_trampoline_update(tr, true /* lock_direct_mutex */);
> if (err) {
> hlist_del_init(&link->tramp_hlist);
> - tr->progs_cnt[kind]--;
> + if (kind == BPF_TRAMP_FSESSION) {
> + tr->progs_cnt[BPF_TRAMP_FENTRY]--;
> + hlist_del_init(&fslink->fexit.tramp_hlist);
> + tr->progs_cnt[BPF_TRAMP_FEXIT]--;
> + } else {
> + tr->progs_cnt[kind]--;
> + }
> }
> return err;
> }
> @@ -659,6 +683,7 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
> struct bpf_trampoline *tr,
> struct bpf_prog *tgt_prog)
> {
> + struct bpf_fsession_link *fslink;
used in only one branch, move declaration there?
> enum bpf_tramp_prog_type kind;
> int err;
>
> @@ -672,6 +697,11 @@ static int __bpf_trampoline_unlink_prog(struct bpf_tramp_link *link,
> guard(mutex)(&tgt_prog->aux->ext_mutex);
> tgt_prog->aux->is_extended = false;
> return err;
> + } else if (kind == BPF_TRAMP_FSESSION) {
> + fslink = container_of(link, struct bpf_fsession_link, link.link);
> + hlist_del_init(&fslink->fexit.tramp_hlist);
> + tr->progs_cnt[BPF_TRAMP_FEXIT]--;
> + kind = BPF_TRAMP_FENTRY;
> }
> hlist_del_init(&link->tramp_hlist);
> tr->progs_cnt[kind]--;
[...]
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