From: Eric Biggers <ebiggers@kernel.org>
To: Gabriel Krisman Bertazi <krisman@suse.de>
Cc: brauner@kernel.org, tytso@mit.edu,
linux-f2fs-devel@lists.sourceforge.net, viro@zeniv.linux.org.uk,
linux-fsdevel@vger.kernel.org, jaegeuk@kernel.org,
linux-ext4@vger.kernel.org,
Gabriel Krisman Bertazi <krisman@collabora.com>
Subject: Re: [PATCH v3 3/7] libfs: Validate negative dentries in case-insensitive directories
Date: Wed, 19 Jul 2023 23:41:03 -0700 [thread overview]
Message-ID: <20230720064103.GC2607@sol.localdomain> (raw)
In-Reply-To: <20230720060657.GB2607@sol.localdomain>
On Wed, Jul 19, 2023 at 11:06:57PM -0700, Eric Biggers wrote:
>
> I'm also having trouble understanding exactly when ->d_name is stable here.
> AFAICS, unfortunately the VFS has an edge case where a dentry can be moved
> without its parent's ->i_rwsem being held. It happens when a subdirectory is
> "found" under multiple names. The VFS doesn't support directory hard links, so
> if it finds a second link to a directory, it just moves the whole dentry tree to
> the new location. This can happen if a filesystem image is corrupted and
> contains directory hard links. Coincidentally, it can also happen in an
> encrypted directory due to the no-key name => normal name transition...
Sorry, I think I got this slightly wrong. The move does happen with the
parent's ->i_rwsem held, but it's for read, not for write. First, before
->lookup is called, the ->i_rwsem of the parent directory is taken for read.
->lookup() calls d_splice_alias() which can call __d_unalias() which does the
__d_move(). If the old alias is in a different directory (which cannot happen
in that fscrypt case, but can happen in the general "directory hard links"
case), __d_unalias() takes that directory's ->i_rwsem for read too.
So it looks like the parent's ->i_rwsem does indeed exclude moves of child
dentries, but only if it's taken for *write*. So I guess you can rely on that;
it's just a bit more subtle than it first appears. Though, some of your
explanation seems to assume that a read lock is sufficient ("In __lookup_slow,
either the parent inode is locked by the caller (lookup_slow) ..."), so maybe
there is still a problem.
- Eric
next prev parent reply other threads:[~2023-07-20 6:42 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-19 22:19 [PATCH v3 0/7] Support negative dentries on case-insensitive ext4 and f2fs Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 1/7] fs: Expose name under lookup to d_revalidate hook Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 2/7] fs: Add DCACHE_CASEFOLDED_NAME flag Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 3/7] libfs: Validate negative dentries in case-insensitive directories Gabriel Krisman Bertazi
2023-07-20 6:06 ` Eric Biggers
2023-07-20 6:41 ` Eric Biggers [this message]
2023-07-21 20:16 ` Gabriel Krisman Bertazi
2023-07-22 4:29 ` Eric Biggers
2023-07-24 21:33 ` Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 4/7] libfs: Chain encryption checks after case-insensitive revalidation Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 5/7] libfs: Merge encrypted_ci_dentry_ops and ci_dentry_ops Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 6/7] ext4: Enable negative dentries on case-insensitive lookup Gabriel Krisman Bertazi
2023-07-19 22:19 ` [PATCH v3 7/7] f2fs: " Gabriel Krisman Bertazi
2023-07-20 7:43 ` [PATCH v3 0/7] Support negative dentries on case-insensitive ext4 and f2fs Eric Biggers
2023-07-20 17:35 ` Gabriel Krisman Bertazi
2023-07-21 3:12 ` Eric Biggers
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