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

[Menglong Dong <menglong.dong@linux.dev>]

On 2026/2/5 21:45, Jiri Olsa wrote:
> 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.

Ah, I didn't notice this part before. Thanks for the explain ;)

> 
> > 
> > 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;
> > > +};
> > > +
> > [...]
> > > 
> > > 
> > > 
> > 
> > 
> > 
> > 
>