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Message-Id: <20201115103718.298186-4-christian.brauner@ubuntu.com>
Date: Sun, 15 Nov 2020 11:36:42 +0100
From: Christian Brauner <christian.brauner@...ntu.com>
To: Alexander Viro <viro@...iv.linux.org.uk>,
Christoph Hellwig <hch@...radead.org>,
linux-fsdevel@...r.kernel.org
Cc: John Johansen <john.johansen@...onical.com>,
James Morris <jmorris@...ei.org>,
Mimi Zohar <zohar@...ux.ibm.com>,
Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
Stephen Smalley <stephen.smalley.work@...il.com>,
Casey Schaufler <casey@...aufler-ca.com>,
Arnd Bergmann <arnd@...db.de>,
Andreas Dilger <adilger.kernel@...ger.ca>,
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
Geoffrey Thomas <geofft@...reload.com>,
Mrunal Patel <mpatel@...hat.com>,
Josh Triplett <josh@...htriplett.org>,
Andy Lutomirski <luto@...nel.org>,
Theodore Tso <tytso@....edu>, Alban Crequy <alban@...volk.io>,
Tycho Andersen <tycho@...ho.ws>,
David Howells <dhowells@...hat.com>,
James Bottomley <James.Bottomley@...senpartnership.com>,
Jann Horn <jannh@...gle.com>,
Seth Forshee <seth.forshee@...onical.com>,
Stéphane Graber <stgraber@...ntu.com>,
Aleksa Sarai <cyphar@...har.com>,
Lennart Poettering <lennart@...ttering.net>,
"Eric W. Biederman" <ebiederm@...ssion.com>, smbarber@...omium.org,
Phil Estes <estesp@...il.com>, Serge Hallyn <serge@...lyn.com>,
Kees Cook <keescook@...omium.org>,
Todd Kjos <tkjos@...gle.com>, Jonathan Corbet <corbet@....net>,
containers@...ts.linux-foundation.org,
linux-security-module@...r.kernel.org, linux-api@...r.kernel.org,
linux-ext4@...r.kernel.org, linux-audit@...hat.com,
linux-integrity@...r.kernel.org, selinux@...r.kernel.org,
Christian Brauner <christian.brauner@...ntu.com>,
Christoph Hellwig <hch@....de>
Subject: [PATCH v2 03/39] namespace: only take read lock in do_reconfigure_mnt()
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: Christoph Hellwig <hch@....de>
Cc: David Howells <dhowells@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>
Cc: linux-fsdevel@...r.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
---
/* v2 */
unchanged
---
fs/namespace.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index 798bbf4f48ad..8497d149ecaa 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2512,10 +2512,6 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
return 0;
}
-/*
- * 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;
@@ -2565,13 +2561,17 @@ 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);
--
2.29.2
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