[External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args

[梦龙董]

On Fri, Mar 29, 2024 at 7:17 AM Alexei Starovoitov
<alexei.starovoitov at gmail.com> wrote:
>
> On Thu, Mar 28, 2024 at 8:10 AM Steven Rostedt <rostedt at goodmis.org> wrote:
> >
> > On Thu, 28 Mar 2024 22:43:46 +0800
> > 梦龙董 <dongmenglong.8 at bytedance.com> wrote:
> >
> > > I have done a simple benchmark on creating 1000
> > > trampolines. It is slow, quite slow, which consume up to
> > > 60s. We can't do it this way.
> > >
> > > Now, I have a bad idea. How about we introduce
> > > a "dynamic trampoline"? The basic logic of it can be:
> > >
> > > """
> > > save regs
> > > bpfs = trampoline_lookup_ip(ip)
> > > fentry = bpfs->fentries
> > > while fentry:
> > >   fentry(ctx)
> > >   fentry = fentry->next
> > >
> > > call origin
> > > save return value
> > >
> > > fexit = bpfs->fexits
> > > while fexit:
> > >   fexit(ctx)
> > >   fexit = fexit->next
> > >
> > > xxxxxx
> > > """
> > >
> > > And we lookup the "bpfs" by the function ip in a hash map
> > > in trampoline_lookup_ip. The type of "bpfs" is:
> > >
> > > struct bpf_array {
> > >   struct bpf_prog *fentries;
> > >  struct bpf_prog *fexits;
> > >   struct bpf_prog *modify_returns;
> > > }
> > >
> > > When we need to attach the bpf progA to function A/B/C,
> > > we only need to create the bpf_arrayA, bpf_arrayB, bpf_arrayC
> > > and add the progA to them, and insert them to the hash map
> > > "direct_call_bpfs", and attach the "dynamic trampoline" to
> > > A/B/C. If bpf_arrayA exist, just add progA to the tail of
> > > bpf_arrayA->fentries. When we need to attach progB to
> > > B/C, just add progB to bpf_arrayB->fentries and
> > > bpf_arrayB->fentries.
> > >
> > > Compared to the trampoline, extra overhead is introduced
> > > by the hash lookuping.
> > >
> > > I have not begun to code yet, and I am not sure the overhead is
> > > acceptable. Considering that we also need to do hash lookup
> > > by the function in kprobe_multi, maybe the overhead is
> > > acceptable?
> >
> > Sounds like you are just recreating the function management that ftrace
> > has. It also can add thousands of trampolines very quickly, because it does
> > it in batches. It takes special synchronization steps to attach to fentry.
> > ftrace (and I believe multi-kprobes) updates all the attachments for each
> > step, so the synchronization needed is only done once.
> >
> > If you really want to have thousands of functions, why not just register it
> > with ftrace itself. It will give you the arguments via the ftrace_regs
> > structure. Can't you just register a program as the callback?
> >
> > It will probably make your accounting much easier, and just let ftrace
> > handle the fentry logic. That's what it was made to do.
>
> Absolutely agree.
> There is no point re-inventing this logic.
>
> Menlong,
> before you hook up into ftrace check whether
> it's going to be any different from kprobe-multi,
> since it's the same ftrace underneath.
> I suspect it will look exactly the same.

Yeah, I dig it a little. I think it is different. For multi-kprobe,
it registers a ftrace_ops to ftrace_ops_list for every bpf
program. This means that we can register 2 or more
multi-kprobe in the same function. The bpf is called in
the following step:

ftrace_regs_caller
|
__ftrace_ops_list_func -> fprobe_handler -> kprobe_multi_link_handler -> run BPF

And for trampoline, it needs to be called directly,
so it can't be registered as a callback to ftrace_ops_list.
It need to be called in the following step:

ftrace_regs_caller
|
__ftrace_ops_list_func -> call_direct_funcs -> save trampoline to
pt_regs->origin_ax
|
call pt_regs->origin_ax if not NULL

> So it sounds like multi-fentry idea will be shelved once again.

Enn...this is the best solution that I can think of. If it
doesn't work, I suspect it will be shelved again.

Thanks!
Menglong Dong