linux-kernel - Re: [PATCH] fs: nfs: fix a possible sleep-in-atomic-context bug in _pnfs_grab_empty

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Date:   Wed, 18 Dec 2019 09:54:31 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Trond Myklebust <trondmy@...merspace.com>,
        "anna.schumaker@...app.com" <anna.schumaker@...app.com>
Cc:     "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] fs: nfs: fix a possible sleep-in-atomic-context bug in
 _pnfs_grab_empty_layout()



On 2019/12/17 22:37, Trond Myklebust wrote:
> On Tue, 2019-12-17 at 21:33 +0800, Jia-Ju Bai wrote:
>> The filesystem may sleep while holding a spinlock.
>> The function call path (from bottom to top) in Linux 4.19 is:
>>
>> fs/nfs/pnfs.c, 2052:
>> 	pnfs_find_alloc_layout(GFP_KERNEL) in _pnfs_grab_empty_layout
>> fs/nfs/pnfs.c, 2051:
>> 	spin_lock in _pnfs_grab_empty_layout
>>
>> pnfs_find_alloc_layout(GFP_KERNEL) can sleep at runtime.
>>
>> To fix this possible bug, GFP_KERNEL is replaced with GFP_ATOMIC for
>> pnfs_find_alloc_layout().
>>
>> This bug is found by a static analysis tool STCheck written by
>> myself.
>>
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
>> ---
>>   fs/nfs/pnfs.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/fs/nfs/pnfs.c b/fs/nfs/pnfs.c
>> index cec3070ab577..cfbe170f0651 100644
>> --- a/fs/nfs/pnfs.c
>> +++ b/fs/nfs/pnfs.c
>> @@ -2138,7 +2138,7 @@ _pnfs_grab_empty_layout(struct inode *ino,
>> struct nfs_open_context *ctx)
>>   	struct pnfs_layout_hdr *lo;
>>   
>>   	spin_lock(&ino->i_lock);
>> -	lo = pnfs_find_alloc_layout(ino, ctx, GFP_KERNEL);
>> +	lo = pnfs_find_alloc_layout(ino, ctx, GFP_ATOMIC);
>>   	if (!lo)
>>   		goto out_unlock;
>>   	if (!test_bit(NFS_LAYOUT_INVALID_STID, &lo->plh_flags))
> I'm not seeing why this is necessary. As far as I can see,
> pnfs_find_alloc_layout() will release the ino->i_lock before sleeping.
>
> False positive?
>

Thanks for the reply.
You are right, my report is false...
I did not check the definition of pnfs_find_alloc_layout(), sorry...


Best wishes,
Jia-Ju Bai