Re: [PATCH RFC bpf-next 7/7] bpf: implement "jmp" mode for trampoline

Menglong Dong
2025-11-15T02:39:50Z

On Sat, Nov 15, 2025 at 2:50 AM Alexei Starovoitov
<alexei.starovoitov@gmail.com> wrote:
>
> On Fri, Nov 14, 2025 at 1:25 AM Menglong Dong <menglong8.dong@gmail.com> wrote:
> >
> > Implement the "jmp" mode for the bpf trampoline. For the ftrace_managed
> > case, we need only to set the FTRACE_OPS_FL_JMP on the tr->fops if "jmp"
> > is needed.
> >
> > For the bpf poke case, the new flag BPF_TRAMP_F_JMPED is introduced to
> > store and check if the trampoline is in the "jmp" mode.
> >
> > Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> > ---
> > include/linux/bpf.h | 6 +++++
> > kernel/bpf/trampoline.c | 53 ++++++++++++++++++++++++++++++++++-------
> > 2 files changed, 50 insertions(+), 9 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index aec7c65539f5..3598785ac8d1 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1201,6 +1201,12 @@ struct btf_func_model {
> > */
> > #define BPF_TRAMP_F_INDIRECT BIT(8)
> >
> > +/*
> > + * Indicate that the trampoline is using "jmp" instead of "call". This flag
> > + * is only used in the !ftrace_managed case.
> > + */
> > +#define BPF_TRAMP_F_JMPED BIT(9)
> > +
> > /* Each call __bpf_prog_enter + call bpf_func + call __bpf_prog_exit is ~50
> > * bytes on x86.
> > */
> > diff --git a/kernel/bpf/trampoline.c b/kernel/bpf/trampoline.c
> > index 5949095e51c3..02a9f33d8f6c 100644
> > --- a/kernel/bpf/trampoline.c
> > +++ b/kernel/bpf/trampoline.c
> > @@ -175,15 +175,37 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key)
> > return tr;
> > }
> >
> > -static int unregister_fentry(struct bpf_trampoline *tr, void *old_addr)
> > +static int bpf_text_poke(struct bpf_trampoline *tr, void *old_addr,
> > + void *new_addr)
>
> The bpf_text_poke is a generic name. It really doesn't fit here.
> Use bpf_trampoline_update_fentry() or something along those lines.

ACK.

>
> > {
> > + enum bpf_text_poke_type new_t = BPF_MOD_CALL, old_t = BPF_MOD_CALL;
> > void *ip = tr->func.addr;
> > int ret;
> >
> > + if (bpf_trampoline_need_jmp(tr->flags))
> > + new_t = BPF_MOD_JUMP;
> > + if (tr->flags & BPF_TRAMP_F_JMPED)
> > + old_t = BPF_MOD_JUMP;
>
> Now I see why you picked _need_jmp().. to alternate with F_JMPED ?
> _uses_jmp() suggestions isn't quite right.

Ah, some kind. The flags BPF_TRAMP_F_CALL_ORIG and
BPF_TRAMP_F_SKIP_FRAME are reset during the trampoline update,
and they are not stored. So the "_need_jmp" means that use "jmp"
for the new trampoline that we are going to update.

The BPF_TRAMP_F_JMPED is used to store if the current trampoline
is in "jmp" mode.

>
> How about bpf_trampoline_must_jmp() ?
> and drop if (!ret) fallback and BPF_TRAMP_F_JMPED bit.
> It doesn't look to be necessary.

I think you are right. We can check if current trampoline is in "jmp"
mode with the "orig_flags" instead, and remove the
BPF_TRAMP_F_JMPED. That means that I need to pass the
"orig_flags" to
modify_fentry -> bpf_trampoline_update_fentry(bpf_text_poke).

Thanks!
Menglong Dong