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

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Date:   Mon, 23 Nov 2020 21:49:24 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Andrii Nakryiko <andrii@...nel.org>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        daniel@...earbox.net, kernel-team@...com
Subject: Re: [PATCH bpf-next 1/6] bpf: fix bpf_put_raw_tracepoint()'s use of
 __module_address()

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.