linux-kernel - Re: [PATCH v2] kernel: bpf: stackmap: fix a possible sleep-in-atomic bug in bpf_mmap_unlock_get_irq

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Message-ID: <6414ae0db3f69_984f8208cb@john.notmuch>
Date:   Fri, 17 Mar 2023 11:14:37 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        starmiku1207184332@...il.com
Cc:     ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        martin.lau@...ux.dev, song@...nel.org, yhs@...com,
        john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com,
        haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, baijiaju1990@...look.com
Subject: Re: [PATCH v2] kernel: bpf: stackmap: fix a possible sleep-in-atomic
 bug in bpf_mmap_unlock_get_irq_work()

Alexei Starovoitov wrote:
> On Fri, Mar 17, 2023 at 03:52:27AM +0000, starmiku1207184332@...il.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.