Date:   Fri, 17 Mar 2023 11:14:37 -0700
From:   John Fastabend <john.fastabend@gmail.com>
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        starmiku1207184332@gmail.com
Cc:     ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
        martin.lau@linux.dev, song@kernel.org, yhs@fb.com,
        john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com,
        haoluo@google.com, jolsa@kernel.org, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, baijiaju1990@outlook.com
Message-ID: <6414ae0db3f69_984f8208cb@john.notmuch>
In-Reply-To: <20230317171636.ftelyp6ty7mgo4rt@dhcp-172-26-102-232.dhcp.thefacebook.com>
References: <20230317035227.22293-1-starmiku1207184332@gmail.com>
 <20230317171636.ftelyp6ty7mgo4rt@dhcp-172-26-102-232.dhcp.thefacebook.com>
Subject: Re: [PATCH v2] kernel: bpf: stackmap: fix a possible sleep-in-atomic
 bug in bpf_mmap_unlock_get_irq_work()
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Alexei Starovoitov wrote:
> On Fri, Mar 17, 2023 at 03:52:27AM +0000, starmiku1207184332@gmail.com wrote:
> > context because of its possible sleep operation. However, mmap_read_unlock()
> > is unsafely called in a preempt disabled context when spin_lock() or
> > rcu_read_lock() has been called.
> 
> Why is that unsafe?
> See __up_read(). It's doing preempt_disable().

Yep I didn't see the issue either that is why I asked for the stack trace. If
its a bug we would want a reproducer as well seems like it should be trivially
tested in selftests.

> 
> 
> > -	if (irqs_disabled()) {
> > +	if (in_atomic() || irqs_disabled()) {
> 
> We cannot do this. It will significantly hurt stack traces with build_id.