On Fri, Nov 08, 2019 at 02:46:06PM +0100, Jan Kara wrote:

On Fri 08-11-19 14:12:38, Jan Kara wrote:

quoted

On Mon 21-10-19 15:49:31, Ira Weiny wrote:

quoted

On Mon, Oct 21, 2019 at 11:45:36AM +1100, Dave Chinner wrote:

quoted

On Sun, Oct 20, 2019 at 08:59:35AM -0700, ira.weiny@intel.com wrote:
That, fundamentally, is the issue here - it's not setting/clearing
the DAX flag that is the issue, it's doing a swap of the
mapping->a_ops while there may be other code using that ops
structure.

IOWs, if there is any code anywhere in the kernel that
calls an address space op without holding one of the three locks we
hold here (i_rwsem, MMAPLOCK, ILOCK) then it can race with the swap
of the address space operations.

By limiting the address space swap to file sizes of zero, we rule
out the page fault path (mmap of a zero length file segv's with an
access beyond EOF on the first read/write page fault, right?).

Yes I checked that and thought we were safe here...

quoted

However, other aops callers that might run unlocked and do the wrong
thing if the aops pointer is swapped between check of the aop method
existing and actually calling it even if the file size is zero?

A quick look shows that FIBMAP (ioctl_fibmap())) looks susceptible
to such a race condition with the current definitions of the XFS DAX
aops. I'm guessing there will be others, but I haven't looked
further than this...

I'll check for others and think on what to do about this.  ext4 will have the
same problem I think.  :-(

Just as a datapoint, ext4 is bold and sets inode->i_mapping->a_ops on
existing inodes when switching journal data flag and so far it has not
blown up. What we did to deal with issues Dave describes is that we
introduced percpu rw-semaphore guarding switching of aops and then inside
problematic functions redirect callbacks in the right direction under this
semaphore. Somewhat ugly but it seems to work.

Ah I am glad you brought this up.  I had not seen this before.

Is that s_journal_flag_rwsem?

In the general case I don't think that correctly protects against:

	if (a_ops->call)
		a_ops->call();

Because not all operations are defined in both ext4_aops and
ext4_journalled_aops.  Specifically migratepage.

move_to_new_page() specifically follows the pattern above with migratepage.  So
is there a bug here?

Thinking about this some more, perhaps this scheme could be actually
transformed in something workable. We could have a global (or maybe per-sb
but I'm not sure it's worth it) percpu rwsem and we could transform aops
calls into:

percpu_down_read(aops_rwsem);
do_call();
percpu_up_read(aops_rwsem);

With some macro magic it needn't be even that ugly.

I think this is safer.  And what I have been investigating/coding up.  Because
that also would protect against the above with:

percpu_down_read(aops_rwsem);
	if (a_ops->call)
		a_ops->call();
percpu_up_read(aops_rwsem);

However I have been looking at SRCU because we also have patterns like:


	generic_file_buffered_read
		if (a_ops->is_partially_uptodate)
			a_ops->is_partially_uptodate()
		page_cache_sync_readahead
			force_page_cache_readahead
				if (!a_ops->readpage && !a_ops->readpages)
					return;
				__do_page_cache_readahead
					read_pages
						if (a_ops->readpages)
							a_ops->readpages()
						a_ops->readpage


So we would have to pass the a_ops through to use a rwsem.  Where SRCU I think
would be fine to just take the SRCU read lock multiple times.  Am I wrong?


We also have a 3rd (2nd?) issue.  There are callers who check for the presence
of an operation to be used later.  For example do_dentry_open():

do_dentry_open()
{
...
	if (<flags> & O_DIRECT)
		if (!<a_ops> || !<a_ops>->direct_IO)
			return -EINVAL;
...
}

After this open direct_IO better be there AFAICT so changing the a_ops later
would not be good.  For ext4 direct_IO is defined for all the a_ops...  so I
guess that is not a big deal.  However, is the user really getting the behavior
they expect in this case?

I'm afraid of requiring FSs to have to follow rules in defining their a_ops.
Because I'm afraid maintaining those rules would be hard and would eventually
lead to crashes when someone did it wrong.

:-/

So for this 3rd (2nd) case I think we should simply take a reference to the
a_ops and fail changing the mode.  For the DAX case that means the user is best
served by taking a write lease on the file to ensure there are no other opens
which could cause issues.

Would that work for changing the journaling mode?

And I _think_ this is the only issue we have with this right now. But if other
callers of a_ops needed the pattern of using the a_ops at a time across context
changes they would need to ensure this reference was taken.

What I have come up with thus far is an interface like:

/*
 * as_get_a_ops() -- safely get the a_ops from the address_space specified
 *
 * @as: address space to get a_ops from
 * @ref: used to indicate if a reference is required on this a_ops
 * @tok: srcu token to be returned in as_put_a_ops()
 *
 * The a_ops returned is protected from changing until as_put_a_ops().
 *
 * If ref is specified then ref must also be specified in as_put_a_ops() to
 * release this reference.  In this case a reference is taken on the a_ops
 * which will prevent it from changing until the reference is released.
 *
 * References should _ONLY_ be taken when the a_ops needs to be constant
 * across a user context switch because doing so will block changing the a_ops
 * until that reference is released.
 *
 * Examples of using a reference are checks for specific a_ops pointers which
 * are expected to support functionality at a later date (example direct_IO)
 */
static inline const struct address_space_operations *
as_get_a_ops(struct address_space *as, bool ref, int *tok)
{
	...
}

static inline void
as_assign_a_ops(struct address_space *as,
                const struct address_space_operations *a_ops)
{
	...
}

static inline void as_put_a_ops(struct address_space *as, int tok, bool ref)
{
	...
}


I'm still working out the details of using SRCU and a ref count.  I have made
at least 1 complete pass of all the a_ops users and I think this would cover
them all.

Thoughts?
Ira

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR