Re: fun with d_invalidate() vs. d_splice_alias() was Re: [f2fs-dev] [PATCH v6 0/9] Support negative dentries on case-insensitive ext4 and f2fs

[Al Viro <viro@zeniv.linux.org.uk>]

On Mon, Nov 27, 2023 at 04:03:18PM +0000, Al Viro wrote:
> On Mon, Nov 27, 2023 at 09:47:47AM -0600, Eric W. Biederman wrote:
> 
> > There is a lot going on there.  I remember one of the relevant
> > restrictions was marking dentries dont_mount, and inodes S_DEAD
> > in unlink and rmdir.
> > 
> > But even without out that marking if d_invalidate is called
> > from d_revalidate the inode and all of it's dentries must be
> > dead because the inode is stale and most go.  There should
> > be no resurrecting it at that point.
> > 
> > I suspect the most fruitful way to think of the d_invalidate vs
> > d_splice_alias races is an unlink vs rename race.
> > 
> > I don't think the mechanism matters, but deeply and fundamentally
> > if we detect a directory inode is dead we need to stick with
> > that decision and not attempt to resurrect it with d_splice_alias.
> 
> Wrong.  Deeply and fundamentally we detect a dentry that does not
> match the directory contents according to the server.
> 
> For example, due to rename done on server.  With object in question
> perfectly alive there - fhandle still works, etc.
> 
> However, it's no longer where it used to be.  And we would bloody better
> not have lookups for the old name result in access to that object.
> We also should never allow the access to *new* name lead to two live
> dentries for the same directory inode.
> 
> Again, this is not about rmdir() or unlink() - invalidation can happen
> for object that is still open, still accessed and still very much alive.
> Does that all the time for any filesystem with ->d_revalidate().

Put another way, there used to be very odd song and dance in ->d_revalidate()
instances along the lines of "we can't possibly tell the caller to invalidate
a mountpoint"; it was racy in the best case and during the rewrite of
d_invalidate() to teach it how to evict submounts those attempts had been
dropped - ->d_revalidate() returning 0 does end up with mounts dissolved
by d_invalidate() from caller.

It always had been racy, starting with the checks that used to be in
->d_revalidate() instances way before all those changes.  So the switch
of d_invalidate() to dissolving submounts had been a step in the right
direction, but it's not being careful enough.

Again, it's about d_invalidate() caused by pathwalk running into a dentry that
doesn't match the reality vs. d_splice_alias() finding that it matches the
inode we had looked up elsewhere.