Re: [PATCH v3 25/28] bpf: Use RCU in all users of __module_text_address().

[Alexei Starovoitov]

On Wed, Jan 8, 2025 at 1:05 AM Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
>
> __module_address() can be invoked within a RCU section, there is no
> requirement to have preemption disabled.
>
> Replace the preempt_disable() section around __module_address() with
> RCU.
>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Cc: Hao Luo <haoluo@xxxxxxxxxx>
> Cc: Jiri Olsa <jolsa@xxxxxxxxxx>
> Cc: John Fastabend <john.fastabend@xxxxxxxxx>
> Cc: KP Singh <kpsingh@xxxxxxxxxx>
> Cc: Martin KaFai Lau <martin.lau@xxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Matt Bobrowski <mattbobrowski@xxxxxxxxxx>
> Cc: Song Liu <song@xxxxxxxxxx>
> Cc: Stanislav Fomichev <sdf@xxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Yonghong Song <yonghong.song@xxxxxxxxx>
> Cc: bpf@xxxxxxxxxxxxxxx
> Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> ---
>  kernel/trace/bpf_trace.c | 19 ++++++++-----------
>  1 file changed, 8 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 1b8db5aee9d38..020df7b6ff90c 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2336,10 +2336,9 @@ void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp)
>  {
>         struct module *mod;
>
> -       preempt_disable();
> +       guard(rcu)();
>         mod = __module_address((unsigned long)btp);
>         module_put(mod);
> -       preempt_enable();
>  }
>
>  static __always_inline
> @@ -2907,16 +2906,14 @@ static int get_modules_for_addrs(struct module ***mods, unsigned long *addrs, u3
>         for (i = 0; i < addrs_cnt; i++) {
>                 struct module *mod;
>
> -               preempt_disable();
> -               mod = __module_address(addrs[i]);
> -               /* Either no module or we it's already stored  */
> -               if (!mod || has_module(&arr, mod)) {
> -                       preempt_enable();
> -                       continue;
> +               scoped_guard(rcu) {
> +                       mod = __module_address(addrs[i]);
> +                       /* Either no module or we it's already stored  */
> +                       if (!mod || has_module(&arr, mod))
> +                               continue;
> +                       if (!try_module_get(mod))
> +                               err = -EINVAL;

lgtm.
Should we take into bpf-next or the whole set is handled together
somewhere?