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From:   Christian Brauner <christian.brauner@ubuntu.com>
To:     David Howells <dhowells@redhat.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Christian Brauner <christian.brauner@ubuntu.com>
Subject: [PATCH 2/4] namespace: only take read lock in do_reconfigure_mnt()
Date:   Tue, 14 Jul 2020 18:14:14 +0200
Message-Id: <20200714161415.3886463-4-christian.brauner@ubuntu.com>
In-Reply-To: <20200714161415.3886463-1-christian.brauner@ubuntu.com>
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do_reconfigure_mnt() used to take the down_write(&sb->s_umount) lock
which seems unnecessary since we're not changing the superblock. We're
only checking whether it is already read-only. Setting other mount
attributes is protected by lock_mount_hash() afaict and not by s_umount.

So I think the history of down_write(&sb->s_umount) lock being taken
when setting mount attributes dates back to the introduction of
MNT_READONLY in [2]. Afaict, this introduced the concept of having
read-only mounts in contrast to just having a read-only superblock. When
it got introduced it was simply plumbed into do_remount() which already
took down_write(&sb->s_umount) because it was only used to actually
change the superblock before [2]. Afaict, it would've already been
possible back then to only use down_read(&sb->s_umount) for
MS_BIND | MS_REMOUNT since actual mount options were protected by
the vfsmount lock already. But that would've meant special casing the
locking for MS_BIND | MS_REMOUNT in do_remount() which people might not
have considered worth it.
Then in [1] MS_BIND | MS_REMOUNT mount option changes were split out of
do_remount() into do_reconfigure_mnt() but the down_write(&sb->s_umount)
lock was simply copied over.
Now that we have this be a separate helper only take
the down_read(&sb->s_umount) lock since we're only interested in
checking whether the super block is currently read-only and blocking any
writers from changing it. Essentially, checking that the super block is
read-only has the advantage that we can avoid having to go into the
slowpath and through MNT_WRITE_HOLD and can simply set the read-only
flag on the mount in set_mount_attributes().

[1]: commit 43f5e655eff7 ("vfs: Separate changing mount flags full remount")
[2]: commit 2e4b7fcd9260 ("[PATCH] r/o bind mounts: honor mount writer counts at remount")
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: David Howells <dhowells@redhat.com>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 fs/namespace.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 395b9c912edf..ab025dd3be04 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2507,10 +2507,6 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
 	return __mnt_unmake_readonly(mnt);
 }
 
-/*
- * Update the user-settable attributes on a mount.  The caller must hold
- * sb->s_umount for writing.
- */
 static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 {
 	mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
@@ -2560,13 +2556,17 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
 	if (!can_change_locked_flags(mnt, mnt_flags))
 		return -EPERM;
 
-	down_write(&sb->s_umount);
+	/*
+	 * We're only checking whether the superblock is read-only not changing
+	 * it, so only take down_read(&sb->s_umount).
+	 */
+	down_read(&sb->s_umount);
 	lock_mount_hash();
 	ret = change_mount_ro_state(mnt, mnt_flags);
 	if (ret == 0)
 		set_mount_attributes(mnt, mnt_flags);
 	unlock_mount_hash();
-	up_write(&sb->s_umount);
+	up_read(&sb->s_umount);
 
 	mnt_warn_timestamp_expiry(path, &mnt->mnt);
 
-- 
2.27.0