[PATCH v2 01/39] namespace: take lock_mount_hash() directly when changing flags

From: Christian Brauner <christian.brauner@ubuntu.com>
To: Alexander Viro <viro@zeniv.linux.org.uk>,
	Christoph Hellwig <hch@infradead.org>,
	linux-fsdevel@vger.kernel.org
Cc: Lennart Poettering <lennart@poettering.net>,
	Mimi Zohar <zohar@linux.ibm.com>,
	James Bottomley <James.Bottomley@hansenpartnership.com>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	containers@lists.linux-foundation.org,
	Christoph Hellwig <hch@lst.de>, Tycho Andersen <tycho@tycho.ws>,
	Jonathan Corbet <corbet@lwn.net>,
	smbarber@chromium.org, linux-ext4@vger.kernel.org,
	Mrunal Patel <mpatel@redhat.com>,
	Kees Cook <keescook@chromium.org>, Arnd Bergmann <arnd@arndb.de>,
	Jann Horn <jannh@google.com>,
	selinux@vger.kernel.org, Josh Triplett <josh@joshtriplett.org>,
	Seth Forshee <seth.forshee@canonical.com>,
	Aleksa Sarai <cyphar@cyphar.com>,
	Andy Lutomirski <luto@kernel.org>,
	OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>,
	Geoffrey Thomas <geofft@ldpreload.com>,
	David Howells <dhowells@redhat.com>,
	John Johansen <john.johansen@canonical.com>,
	Theodore Tso <tytso@mit.edu>,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	linux-security-module@vger.kernel.org, linux-audit@redhat.com,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	linux-api@vger.kernel.org,
	Casey Schaufler <casey@schaufler-ca.com>,
	Alban Crequy <alban@kinvolk.io>,
	linux-integrity@vger.kernel.org, Todd Kjos <tkjos@google.com>
Subject: [PATCH v2 01/39] namespace: take lock_mount_hash() directly when changing flags
Date: Sun, 15 Nov 2020 11:36:40 +0100	[thread overview]
Message-ID: <20201115103718.298186-2-christian.brauner@ubuntu.com> (raw)
In-Reply-To: <20201115103718.298186-1-christian.brauner@ubuntu.com>

Changing mount options always ends up taking lock_mount_hash() but when
MNT_READONLY is requested and neither the mount nor the superblock are
not already MNT_READONLY we end up taking the lock, dropping it, and
retaking it to change the other mount attributes. Instead of this,
acquire the lock once when changing mount properties. This simplifies
the locking in these codepath, makes them easier to reason about and
avoids having to reacquire the lock right after dropping it.

Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
/* v2 */
- Christoph Hellwig:
  - Remove pointless __mnt_unmake_readonly() helper.
  - Even though Christoph suggested to lockdep_assert_held() into places that
    require {lock,unlock}_mount_hash() it seems that seqlock's don't support
    it.
---
 fs/namespace.c | 22 ++++++++--------------
 1 file changed, 8 insertions(+), 14 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index cebaa3e81794..f183161833ad 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -463,7 +463,6 @@ static int mnt_make_readonly(struct mount *mnt)
 {
 	int ret = 0;
 
-	lock_mount_hash();
 	mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
 	/*
 	 * After storing MNT_WRITE_HOLD, we'll read the counters. This store
@@ -497,18 +496,9 @@ static int mnt_make_readonly(struct mount *mnt)
 	 */
 	smp_wmb();
 	mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
-	unlock_mount_hash();
 	return ret;
 }
 
-static int __mnt_unmake_readonly(struct mount *mnt)
-{
-	lock_mount_hash();
-	mnt->mnt.mnt_flags &= ~MNT_READONLY;
-	unlock_mount_hash();
-	return 0;
-}
-
 int sb_prepare_remount_readonly(struct super_block *sb)
 {
 	struct mount *mnt;
@@ -2508,7 +2498,8 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
 	if (readonly_request)
 		return mnt_make_readonly(mnt);
 
-	return __mnt_unmake_readonly(mnt);
+	mnt->mnt.mnt_flags &= ~MNT_READONLY;
+	return 0;
 }
 
 /*
@@ -2517,11 +2508,9 @@ static int change_mount_ro_state(struct mount *mnt, unsigned int mnt_flags)
  */
 static void set_mount_attributes(struct mount *mnt, unsigned int mnt_flags)
 {
-	lock_mount_hash();
 	mnt_flags |= mnt->mnt.mnt_flags & ~MNT_USER_SETTABLE_MASK;
 	mnt->mnt.mnt_flags = mnt_flags;
 	touch_mnt_namespace(mnt->mnt_ns);
-	unlock_mount_hash();
 }
 
 static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *mnt)
@@ -2567,9 +2556,11 @@ static int do_reconfigure_mnt(struct path *path, unsigned int mnt_flags)
 		return -EPERM;
 
 	down_write(&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);
 
 	mnt_warn_timestamp_expiry(path, &mnt->mnt);
@@ -2610,8 +2601,11 @@ static int do_remount(struct path *path, int ms_flags, int sb_flags,
 		err = -EPERM;
 		if (ns_capable(sb->s_user_ns, CAP_SYS_ADMIN)) {
 			err = reconfigure_super(fc);
-			if (!err)
+			if (!err) {
+				lock_mount_hash();
 				set_mount_attributes(mnt, mnt_flags);
+				unlock_mount_hash();
+			}
 		}
 		up_write(&sb->s_umount);
 	}
-- 
2.29.2

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