Re: Is ext4_dio_read_iter() broken? - and xfs_file_dio_aio_read()

Re: Is ext4_dio_read_iter() broken? - and xfs_file_dio_aio_read()

[David Howells]

David Howells <dhowells@redhat.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.

David

btrfs may be broken too - Re: Is ext4_dio_read_iter() broken?

[David Howells]

David Howells <dhowells@redhat.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

Re: Is ext4_dio_read_iter() broken? - and xfs_file_dio_aio_read()

[Christoph Hellwig]

On Thu, Mar 12, 2020 at 10:07:57AM +0000, David Howells wrote:
> David Howells <dhowells@redhat.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?

The iocb is refcounted and only completed when the refcount hits zero,
and an extra reference is held until the submission has completed.
Take a look at iocb_put().

> 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.

the file reference also hold an inode reference.

> 
> 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.

In addition to that DAX never executes asynchronously.

> And xfs_file_dio_aio_read() appears to be broken as it touches the inode after
> calling iomap_dio_rw() to unlock it.

Same as above.

Re: Is ext4_dio_read_iter() broken? - and xfs_file_dio_aio_read()

[David Howells]

Christoph Hellwig <hch@lst.de> wrote:

> > > at the end of the function - but surely iocb should be expected to have
> > > been freed when iocb->ki_complete() was called?
> 
> The iocb is refcounted and only completed when the refcount hits zero,
> and an extra reference is held until the submission has completed.
> Take a look at iocb_put().

Ah...  This is in struct aio_kiocb and not struct kiocb - that's why I missed
it.  Thanks.

David

Re: Is ext4_dio_read_iter() broken? - and xfs_file_dio_aio_read()

[Christoph Hellwig]

On Thu, Mar 12, 2020 at 10:49:40AM +0000, David Howells wrote:
> Christoph Hellwig <hch@lst.de> wrote:
> 
> > > > at the end of the function - but surely iocb should be expected to have
> > > > been freed when iocb->ki_complete() was called?
> > 
> > The iocb is refcounted and only completed when the refcount hits zero,
> > and an extra reference is held until the submission has completed.
> > Take a look at iocb_put().
> 
> Ah...  This is in struct aio_kiocb and not struct kiocb - that's why I missed
> it.  Thanks.

That being said we have a few other spots using ->ki_complete for
asynchronous execution, which might not be as careful.  As someone
having written one or two of those I have my doubts I got everthing
right and will audit those.

> 
> David
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