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From: Valerie Aurora on 8 Aug 2010 12:00
While we can check if a file system is currently read-only, we can't
guarantee that it will stay read-only. The file system can be
remounted read-write at any time; it's also conceivable that a file
system can be mounted a second time and converted to read-write if the
underlying fs allows it. This is a problem for union mounts, which
require the underlying file system be read-only. Add a read-only
users count and don't allow remounts to change the file system to
read-write or read-write mounts if there are any read-only users.

Signed-off-by: Valerie Aurora <vaurora(a)redhat.com>
Cc: Alexander Viro <viro(a)zeniv.linux.org.uk>
---
fs/namespace.c | 13 +++++++++++++
fs/super.c | 23 +++++++++++++++++++++++
include/linux/fs.h | 8 ++++++++
3 files changed, 44 insertions(+), 0 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index b8a66db..984c331 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -200,6 +200,19 @@ int __mnt_is_readonly(struct vfsmount *mnt)
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);

+static void inc_hard_readonly_users(struct vfsmount *mnt)
+{
+ BUG_ON(!__mnt_is_readonly(mnt));
+ mnt->mnt_sb->s_hard_readonly_users++;
+}
+
+static void dec_hard_readonly_users(struct vfsmount *mnt)
+{
+ BUG_ON(!__mnt_is_readonly(mnt));
+ BUG_ON(mnt->mnt_sb->s_hard_readonly_users == 0);
+ mnt->mnt_sb->s_hard_readonly_users--;
+}
+
static inline void inc_mnt_writers(struct vfsmount *mnt)
{
#ifdef CONFIG_SMP
diff --git a/fs/super.c b/fs/super.c
index 938119a..86bdf1f 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -108,6 +108,7 @@ out:
*/
static inline void destroy_super(struct super_block *s)
{
+ BUG_ON(s->s_hard_readonly_users);
security_sb_free(s);
kfree(s->s_subtype);
kfree(s->s_options);
@@ -512,6 +513,21 @@ rescan:
return NULL;
}

+/*
+ * Some uses of file systems require that they never be mounted
+ * read-write anywhere (e.g., the lower layers of union mounts must
+ * always be read-only). If there are any of these "hard" read-only
+ * mounts, don't permit a transition to read-write.
+ *
+ * Must be called while holding the namespace lock.
+ */
+
+int sb_is_hard_readonly(struct super_block *sb)
+{
+ return sb->s_hard_readonly_users ? 1 : 0;
+}
+EXPORT_SYMBOL(sb_is_hard_readonly);
+
/**
* do_remount_sb - asks filesystem to change mount options.
* @sb: superblock in question
@@ -550,6 +566,9 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force)
return -EBUSY;
}

+ if (!(flags & MS_RDONLY) && sb_is_hard_readonly(sb))
+ return -EROFS;
+
if (sb->s_op->remount_fs) {
retval = sb->s_op->remount_fs(sb, &flags, data);
if (retval)
@@ -924,6 +943,10 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
WARN((mnt->mnt_sb->s_maxbytes < 0), "%s set sb->s_maxbytes to "
"negative value (%lld)\n", type->name, mnt->mnt_sb->s_maxbytes);

+ error = -EROFS;
+ if (!(flags & MS_RDONLY) && sb_is_hard_readonly(mnt->mnt_sb))
+ goto out_sb;
+
mnt->mnt_mountpoint = mnt->mnt_root;
mnt->mnt_parent = mnt;
up_write(&mnt->mnt_sb->s_umount);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 68ca1b0..eeb49d7 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1382,6 +1382,13 @@ struct super_block {
* generic_show_options()
*/
char *s_options;
+
+ /*
+ * Some mounts require that the underlying file system never
+ * transition to read-write. They mark the sb itself as
+ * read-only.
+ */
+ int s_hard_readonly_users;
};

extern struct timespec current_fs_time(struct super_block *sb);
@@ -1768,6 +1775,7 @@ extern int get_sb_nodev(struct file_system_type *fs_type,
int (*fill_super)(struct super_block *, void *, int),
struct vfsmount *mnt);
void generic_shutdown_super(struct super_block *sb);
+int sb_is_hard_readonly(struct super_block *sb);
void kill_block_super(struct super_block *sb);
void kill_anon_super(struct super_block *sb);
void kill_litter_super(struct super_block *sb);
--
1.6.3.3

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Prev: [PATCH 39/39] union-mount: Implement union-aware lsetxattr()
Next: [PATCH 25/39] fallthru: tmpfs fallthru support