linux-kernel - Re: [syzbot] [io-uring?] WARNING in __io_uring

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Message-ID: <bc40bd75-7eac-4635-8c91-ccd42c2f1aa6@kernel.dk>
Date: Fri, 29 Nov 2024 07:17:55 -0700
From: Jens Axboe <axboe@...nel.dk>
To: Matthew Wilcox <willy@...radead.org>, Jann Horn <jannh@...gle.com>
Cc: syzbot <syzbot+cc36d44ec9f368e443d3@...kaller.appspotmail.com>,
 asml.silence@...il.com, io-uring@...r.kernel.org,
 linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
 linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [syzbot] [io-uring?] WARNING in __io_uring_free

On 11/28/24 4:57 PM, Matthew Wilcox wrote:
> On Fri, Nov 29, 2024 at 12:30:35AM +0100, Jann Horn wrote:
>>> ------------[ cut here ]------------
>>> WARNING: CPU: 0 PID: 16 at io_uring/tctx.c:51 __io_uring_free+0xfa/0x140 io_uring/tctx.c:51
>>
>> This warning is a check for WARN_ON_ONCE(!xa_empty(&tctx->xa)); and as
>> Jens pointed out, this was triggered after error injection caused a
>> memory allocation inside xa_store() to fail.
>>
>> Is there maybe an issue where xa_store() can fail midway through while
>> allocating memory for the xarray, so that xa_empty() is no longer true
>> even though there is nothing in the xarray? (And if yes, is that
>> working as intended?)

Heh, I had the exact same thought when I originally looked at this
issue. I did code inspection on the io_uring side and tried with error
injection, but could not trigger it. Hence the io_uring side looks fine,
so must be lower down.

> Yes, that's a known possibility.  We have similar problems when people
> use error injection with mapping->i_pages.  The effort to fix it seems
> disproportionate to the severity of the problem.

Doesn't seem like a big deal, particularly when you essentially need
fault injection to trigger it. As long as the xa_empty() is the only
false positive. I wonder if I should just change the io_uring side to do
something ala:

xa_for_each(&tctx->xa, index, node) {
	WARN_ON_ONCE(1);
	break;
}

rather than the xa_empty() warn on. That should get rid of it on my side
at least.

-- 
Jens Axboe