Re: [RFC bpf-next 06/12] bpf: Add bpf_trampoline_multi_attach/detach functions

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Feb 05, 2026 at 05:16:49PM +0800, Menglong Dong wrote:
> On 2026/2/3 17:38 Jiri Olsa <jolsa@kernel.org> write:
> > Adding bpf_trampoline_multi_attach/detach functions that allows
> > to attach/detach multi tracing trampoline.
> > 
> > The attachment is defined with bpf_program and array of BTF ids
> > of functions to attach the bpf program to.
> > 
> [...]
> > @@ -367,7 +367,11 @@ static struct bpf_trampoline *bpf_trampoline_lookup(u64 key, unsigned long ip)
> >  	head = &trampoline_ip_table[hash_64(tr->ip, TRAMPOLINE_HASH_BITS)];
> >  	hlist_add_head(&tr->hlist_ip, head);
> >  	refcount_set(&tr->refcnt, 1);
> > +#ifdef CONFIG_LOCKDEP
> > +	mutex_init_with_key(&tr->mutex, &__lockdep_no_track__);
> > +#else
> >  	mutex_init(&tr->mutex);
> > +#endif
> >  	for (i = 0; i < BPF_TRAMP_MAX; i++)
> >  		INIT_HLIST_HEAD(&tr->progs_hlist[i]);
> >  out:
> > @@ -1400,6 +1404,188 @@ int __weak arch_bpf_trampoline_size(const struct btf_func_model *m, u32 flags,
> >  	return -ENOTSUPP;
> >  }
> >  
> > +#if defined(CONFIG_DYNAMIC_FTRACE_WITH_DIRECT_CALLS) && defined(CONFIG_HAVE_SINGLE_FTRACE_DIRECT_OPS)
> 
> Hi, Jiri. It's great to see your tracing_multi link finally. It looks great ;)

heya, thanks ;-)

> 
> After analyzing a little deeper on the SINGLE_FTRACE_DIRECT_OPS, I
> understand why it is only supported on x86_64 for now. It seems that
> it's a little hard to implement it in the other arch, as we need to
> restructure the implement of ftrace direct call.
> 
> So do we need some more ftrace API here to make the tracing multi-link
> independent from SINGLE_FTRACE_DIRECT_OPS? Otherwise, we can only
> use it on x86_64.

I tried to describe it in commit [2] changelog:

    At the moment we can enable this only on x86 arch, because arm relies
    on ftrace_ops object representing just single trampoline image (stored
    in ftrace_ops::direct_call). Archs that do not support this will continue
    to use *_ftrace_direct api.

> 
> Have you ever tried to implement the SINGLE_FTRACE_DIRECT_OPS on arm64?
> The direct call on arm64 is so complex, and I didn't work it out :/

yes, it seems to be difficult atm, Mark commented on that in [1],
I don't know arm that good to be of much help in here, cc-ing Mark

jirka


[1] https://lore.kernel.org/bpf/aIyNOd18TRLu8EpY@J2N7QTR9R3/
[2] 424f6a361096 ("bpf,x86: Use single ftrace_ops for direct calls")

> 
> Thanks!
> Menglong Dong
> 
> > +
> > +struct fentry_multi_data {
> > +	struct ftrace_hash *unreg;
> > +	struct ftrace_hash *modify;
> > +	struct ftrace_hash *reg;
> > +};
> > +
> [...]
> > 
> > 
> > 
> 
> 
> 
>