Re: [PATCH 1/2] arm64: ftrace: enable single ftrace_ops for direct calls

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Jul 30, 2026 at 11:05:42AM +0800, Leon Hwang wrote:
> On 29/7/26 19:56, Jiri Olsa wrote:
> > On Mon, Jul 27, 2026 at 10:28:43PM +0800, Leon Hwang wrote:
> >> The BPF tracing multi link updates several direct-call sites through one
> >> ftrace_ops. Its implementation is therefore gated by
> >> HAVE_SINGLE_FTRACE_DIRECT_OPS in addition to
> >> DYNAMIC_FTRACE_WITH_DIRECT_CALLS.
> >>
> >> Select HAVE_SINGLE_FTRACE_DIRECT_OPS whenever arm64 enables dynamic ftrace
> >> direct calls. This enables BPF tracing multi links on arm64. Also
> >> generalize the unreachable-trampoline comment because the single-ops path
> >> does not use ops->direct_call.
> > 
> > hi,
> > iirc arm needs ops->direct_call for trampolines to work properly,
> > that's the reason we enabled single ftrace_ops on x86 only:
> > 
> >   424f6a361096 bpf,x86: Use single ftrace_ops for direct calls
> > 
> > I don't understand the arm usage of op->direct_call, but it looks like
> > (codex thinks) that the fast path won't always work without op->direct_call
> > being set
> 
> 
> In theory, op->direct_call is the fallback for the case:
> HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS=y, out-of-range target, and non-BPF
> direct ops. This is what Codex thinks about.
> 
> However, after searching the call sites of
> register_ftrace_direct()/__modify_ftrace_direct() in kernel, which would
> set ops->direct_call, the actual callers are in bpf/trampoline.c, which
> have been gated with HAVE_SINGLE_FTRACE_DIRECT_OPS. That said, for
> normal usage of HAVE_SINGLE_FTRACE_DIRECT_OPS=y, op->direct_call is unused.
> 
> If someone uses register_ftrace_direct() or
> modify_ftrace_direct/_nolock() outside BPF in the future,
> op->direct_call will be used.

so any 2nd user on the same function triggers the slow path,
and that's fine, the single_ops handles slow path correctly

but IIUC the out-of-range target will now always go through slow path,
which in current code it doesn't, right?

jirka