netdev - Re: [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of __module

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Message-ID: <CAEf4BzbjgSv2u+rZs-97PkeXwtKdcvkAdTG=nrX9DNus0ufOPg@mail.gmail.com>
Date:   Mon, 23 Nov 2020 23:49:54 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of __module_address()

On Mon, Nov 23, 2020 at 9:49 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Thu, Nov 19, 2020 at 03:22:39PM -0800, Andrii Nakryiko wrote:
> > __module_address() needs to be called with preemption disabled or with
> > module_mutex taken. preempt_disable() is enough for read-only uses, which is
> > what this fix does.
> >
> > Fixes: a38d1107f937 ("bpf: support raw tracepoints in modules")
> > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > ---
> >  kernel/trace/bpf_trace.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index d255bc9b2bfa..bb98a377050a 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -2060,7 +2060,11 @@ struct bpf_raw_event_map *bpf_get_raw_tracepoint(const char *name)
> >
> >  void bpf_put_raw_tracepoint(struct bpf_raw_event_map *btp)
> >  {
> > -     struct module *mod = __module_address((unsigned long)btp);
> > +     struct module *mod;
> > +
> > +     preempt_disable();
> > +     mod = __module_address((unsigned long)btp);
> > +     preempt_enable();
> >
> >       if (mod)
> >               module_put(mod);
>
> I don't understand why 'mod' cannot become dangling pointer after preempt_enable().
> Either it needs a comment explaining why it's ok or module_put() should
> be in preempt disabled section.

Yeah, I think it can, assuming the kernel module can be unloaded
despite non-zero refcnt (probably happens with force unload?). I'll
drop the `if (mod)` part (module_put() checks that internally) and
will move module_put(mod) inside the preempt disable/enable region.