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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>
Subject: Re: [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of __module_address()
Date: Mon, 23 Nov 2020 23:49:54 -0800	[thread overview]
Message-ID: <CAEf4BzbjgSv2u+rZs-97PkeXwtKdcvkAdTG=nrX9DNus0ufOPg@mail.gmail.com> (raw)
In-Reply-To: <20201124054924.i7zq7vig4xqmddyr@ast-mbp>

On Mon, Nov 23, 2020 at 9:49 PM Alexei Starovoitov
<alexei.starovoitov@gmail.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@kernel.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.

  reply	other threads:[~2020-11-24  7:50 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-19 23:22 [PATCH bpf-next 0/6] libbpf: add support for kernel module BTF CO-RE relocations Andrii Nakryiko
2020-11-19 23:22 ` [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of __module_address() Andrii Nakryiko
2020-11-20 17:55   ` Martin KaFai Lau
2020-11-24  5:49   ` Alexei Starovoitov
2020-11-24  7:49     ` Andrii Nakryiko [this message]
2020-11-19 23:22 ` [PATCH bpf-next 2/6] libbpf: add internal helper to load BTF data by FD Andrii Nakryiko
2020-11-20 18:20   ` Martin KaFai Lau
2020-11-20 18:25     ` Andrii Nakryiko
2020-11-19 23:22 ` [PATCH bpf-next 3/6] libbpf: refactor CO-RE relocs to not assume a single BTF object Andrii Nakryiko
2020-11-20 22:11   ` Martin KaFai Lau
2020-11-19 23:22 ` [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for CO-RE relocations Andrii Nakryiko
2020-11-20  0:46   ` Maciej Fijalkowski
2020-11-20  1:24     ` Andrii Nakryiko
2020-11-20  2:05       ` Maciej Fijalkowski
2020-11-20  3:31         ` Andrii Nakryiko
2020-11-20 23:05   ` Martin KaFai Lau
2020-11-20 23:12     ` Andrii Nakryiko
2020-11-19 23:22 ` [PATCH bpf-next 5/6] selftests/bpf: add bpf_sidecar kernel module for testing Andrii Nakryiko
2020-11-20 23:21   ` Martin KaFai Lau
2020-11-19 23:22 ` [PATCH bpf-next 6/6] selftests/bpf: add CO-RE relocs selftest relying on kernel module BTF Andrii Nakryiko
2020-11-20 23:27   ` Martin KaFai Lau

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