linux-ext4 - btrfs may be broken too - Re: Is ext4_dio_read

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Date:   Thu, 12 Mar 2020 10:26:10 +0000
From:   David Howells <dhowells@...hat.com>
To:     clm@...com, josef@...icpanda.com, dsterba@...e.com
Cc:     dhowells@...hat.com, mbobrowski@...browski.org,
        darrick.wong@...cle.com, jack@...e.cz, hch@....de,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-btrfs@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: btrfs may be broken too - Re: Is ext4_dio_read_iter() broken?

David Howells <dhowells@...hat.com> wrote:

> > Is ext4_dio_read_iter() broken?  It calls:
> > 
> > 	file_accessed(iocb->ki_filp);
> > 
> > at the end of the function - but surely iocb should be expected to have been
> > freed when iocb->ki_complete() was called?
> 
> I think it's actually worse than that.  You also can't call
> inode_unlock_shared(inode) because you no longer own a ref on the inode since
> ->ki_complete() is expected to call fput() on iocb->ki_filp.
> 
> Yes, you own a shared lock on it, but unless somewhere along the
> fput-dput-iput chain the inode lock is taken exclusively, the inode can be
> freed whilst you're still holding the lock.
> 
> Oh - and ext4_dax_read_iter() is also similarly broken.
> 
> And xfs_file_dio_aio_read() appears to be broken as it touches the inode after
> calling iomap_dio_rw() to unlock it.

Seems btrfs_file_write_iter() is also broken:

	if (iocb->ki_flags & IOCB_DIRECT) {
		num_written = __btrfs_direct_write(iocb, from);
	} else {
		num_written = btrfs_buffered_write(iocb, from);
		if (num_written > 0)
			iocb->ki_pos = pos + num_written;
		if (clean_page)
			pagecache_isize_extended(inode, oldsize,
						i_size_read(inode));
	}

	inode_unlock(inode);

But if __btrfs_direct_write() returned -EIOCBQUEUED then inode may have been
deallocated by the point it's calling inode_unlock().  Holding the lock is not
a preventative measure that I can see.

David