linux-kernel - Re: [PATCH bpf v2] ftrace: fix incorrect hash size in register_ftrace

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Message-ID: <CADxym3a2sr-CB=G-ryew+LV3tcbZ1EZ6kzbAPDyQmV=xbaF4bQ@mail.gmail.com>
Date: Wed, 16 Apr 2025 09:06:14 +0800
From: Menglong Dong <menglong8.dong@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Masami Hiramatsu <mhiramat@...nel.org>, rostedt@...dmis.org, mark.rutland@....com, 
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, 
	Menglong Dong <dongml2@...natelecom.cn>
Subject: Re: [PATCH bpf v2] ftrace: fix incorrect hash size in register_ftrace_direct()

On Wed, Apr 16, 2025 at 7:14 AM Andrii Nakryiko
<andrii.nakryiko@...il.com> wrote:
>
> On Mon, Apr 14, 2025 at 12:05 AM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > On Sun, 13 Apr 2025 09:44:44 +0800
> > Menglong Dong <menglong8.dong@...il.com> wrote:
> >
> > > The maximum of the ftrace hash bits is made fls(32) in
> > > register_ftrace_direct(), which seems illogical. So, we fix it by making
> > > the max hash bits FTRACE_HASH_MAX_BITS instead.
> > >
> >
> > Loogs good to me.
> >
> > Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
> >
> > Thanks!
> >
>
> I'm a bit confused by the "[PATCH bpf]" prefix... This fix doesn't
> seem to be BPF-related, so I'm not sure why it would go through the
> bpf tree. I presume Masami or Steven will route it through their tree,
> is that right?
>

Sorry about the confusing......I throught the register_ftrace_direct()
is mainly used by BPF, and it should go to the ftrace tree :/

Thanks!
Menglong Dong

>
> > > Fixes: d05cb470663a ("ftrace: Fix modification of direct_function hash while in use")
> > > Signed-off-by: Menglong Dong <dongml2@...natelecom.cn>
> > > ---
> > > v2:
> > > - thanks for Steven's advice, we fix the problem by making the max hash
> > >   bits FTRACE_HASH_MAX_BITS instead.
> > > ---
> > >  kernel/trace/ftrace.c | 7 ++++---
> > >  1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > > index 1a48aedb5255..d153ad13e0e0 100644
> > > --- a/kernel/trace/ftrace.c
> > > +++ b/kernel/trace/ftrace.c
> > > @@ -5914,9 +5914,10 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
> > >
> > >       /* Make a copy hash to place the new and the old entries in */
> > >       size = hash->count + direct_functions->count;
> > > -     if (size > 32)
> > > -             size = 32;
> > > -     new_hash = alloc_ftrace_hash(fls(size));
> > > +     size = fls(size);
> > > +     if (size > FTRACE_HASH_MAX_BITS)
> > > +             size = FTRACE_HASH_MAX_BITS;
> > > +     new_hash = alloc_ftrace_hash(size);
> > >       if (!new_hash)
> > >               goto out_unlock;
> > >
> > > --
> > > 2.39.5
> > >
> > >
> >
> >
> > --
> > Masami Hiramatsu (Google) <mhiramat@...nel.org>
> >