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Mon, 23 Nov 2020 21:49:36 -0800 (PST) Date: Mon, 23 Nov 2020 21:49:24 -0800 From: Alexei Starovoitov To: Andrii Nakryiko Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, ast@fb.com, daniel@iogearbox.net, kernel-team@fb.com Subject: Re: [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of __module_address() Message-ID: <20201124054924.i7zq7vig4xqmddyr@ast-mbp> References: <20201119232244.2776720-1-andrii@kernel.org> <20201119232244.2776720-2-andrii@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201119232244.2776720-2-andrii@kernel.org> Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org 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 > --- > 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.