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Date: Mon, 27 Nov 2023 16:03:18 +0000
From: Al Viro <viro@zeniv.linux.org.uk>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Gabriel Krisman Bertazi <gabriel@krisman.be>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christian Brauner <brauner@kernel.org>, tytso@mit.edu,
	linux-f2fs-devel@lists.sourceforge.net, ebiggers@kernel.org,
	linux-fsdevel@vger.kernel.org, jaegeuk@kernel.org,
	linux-ext4@vger.kernel.org, Miklos Szeredi <miklos@szeredi.hu>
Subject: 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
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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().