linux-kernel - Re: [PATCH v2 13/14] fs: xfs: Validate atomic writes

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Message-ID: <07d53a91-5451-43e2-ae8c-0c863cf0ec9f@oracle.com>
Date: Thu, 7 Mar 2024 10:19:24 +0000
From: John Garry <john.g.garry@...cle.com>
To: Dave Chinner <david@...morbit.com>
Cc: djwong@...nel.org, hch@....de, viro@...iv.linux.org.uk, brauner@...nel.org,
        jack@...e.cz, chandan.babu@...cle.com, axboe@...nel.dk,
        martin.petersen@...cle.com, linux-kernel@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        tytso@....edu, jbongio@...gle.com, ojaswin@...ux.ibm.com,
        ritesh.list@...il.com, linux-block@...r.kernel.org
Subject: Re: [PATCH v2 13/14] fs: xfs: Validate atomic writes

On 06/03/2024 21:22, Dave Chinner wrote:
> On Mon, Mar 04, 2024 at 01:04:27PM +0000, John Garry wrote:
>> Validate that an atomic write adheres to length/offset rules. Since we
>> require extent alignment for atomic writes, this effectively also enforces
>> that the BIO which iomap produces is aligned.
>>
>> Signed-off-by: John Garry<john.g.garry@...cle.com>
>> ---
>>   fs/xfs/xfs_file.c | 11 ++++++++++-
>>   1 file changed, 10 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
>> index d0bd9d5f596c..cecc5428fd7c 100644
>> --- a/fs/xfs/xfs_file.c
>> +++ b/fs/xfs/xfs_file.c
>> @@ -709,11 +709,20 @@ xfs_file_dio_write(
>>   	struct kiocb		*iocb,
>>   	struct iov_iter		*from)
>>   {
>> -	struct xfs_inode	*ip = XFS_I(file_inode(iocb->ki_filp));
>> +	struct inode		*inode = file_inode(iocb->ki_filp);
>> +	struct xfs_inode	*ip = XFS_I(inode);
>>   	struct xfs_mount	*mp = ip->i_mount;
>>   	struct xfs_buftarg      *target = xfs_inode_buftarg(ip);
>>   	size_t			count = iov_iter_count(from);
>>   
>> +	if (iocb->ki_flags & IOCB_ATOMIC) {
>> +		if (!generic_atomic_write_valid(iocb->ki_pos, from,
>> +			i_blocksize(inode),
> a.k.a. mp->m_bsize. If you use that here, then the need for the VFS
> inode goes away, too.

ok

> 
>> +			XFS_FSB_TO_B(mp, xfs_get_extsz(ip)))) {
>> +			return -EINVAL;
>> +		}
>> +	}
> Also, I think the checks are the wrong way around here. We can only
> do IOCB_ATOMIC IO on force aligned/atomic write inodes, so shouldn't
> we be checking that first,

We are checking that, but not here.

In 14/14, we only set FMODE_CAN_ATOMIC_WRITE for when 
xfs_inode_has_atomicwrites() is true, and only when 
FMODE_CAN_ATOMIC_WRITE is set can we get this far.

I don't see a point in duplicating this xfs_inode_has_atomicwrites() 
check, so I will make the commit message clearer on this - ok? Or add a 
comment.

> then basing the rest of the checks on the
> assumption that atomic writes are allowed and have been set up
> correctly on the inode? i.e.
> 
> 	if (iocb->ki_flags & IOCB_ATOMIC) {
> 		if (!xfs_inode_has_atomicwrites(ip))
> 			return -EINVAL;
> 		if (!generic_atomic_write_valid(iocb->ki_pos, from,
> 				mp->m_bsize, ip->i_extsize))
> 			return -EINVAL;
> 	}
> 
> because xfs_inode_has_atomicwrites() implies ip->i_extsize has been
> set to the required atomic IO size?

I was not too comfortable using ip->i_extsize, as this can be set 
without forcealign being set. I know that we would not get this far 
without forcealign (being set).

Having said that, I don't like all the xfs_get_extsz() calls, so 
something better is required. Let me know you you think.

Thanks,
John