From: Miklos Szeredi <mszeredi@xxxxxxx>
Currently remouting superblock read-only is racy in a major way.
With the per mount read-only infrastructure it is now possible to
prevent most races, which this patch attempts.
Before starting the remount read-only, set MNT_READONLY on all mounts
so that writes are prevented during the remount. Also set
MNT_WAS_WRITE on mounts that were not read-only.
If the remounting is unsuccessful, restore the mounts to read-write.
This can result in transient EROFS errors. Unfortunately hodling off
writes is difficult as remount itself may touch the filesystem
(e.g. through load_nls()) which would then deadlock.
Careful thought needs to be given to places where mnt_flags are set
such as do_add_mount() and clone_mnt() to ensure that a read-write
mount is not added to a read-only superblock.
Small races still remain because delayed writes due to nlink going to
zero but inode still having a reference are not dealt with by this
patch.
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxx>
---
fs/internal.h | 3 ++
fs/namespace.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++--
fs/super.c | 25 ++++++++++++++---
include/linux/mount.h | 1
4 files changed, 96 insertions(+), 6 deletions(-)
Index: linux-2.6/fs/internal.h
===================================================================
--- linux-2.6.orig/fs/internal.h 2010-10-21 19:38:36.000000000 +0200
+++ linux-2.6/fs/internal.h 2010-10-22 17:48:16.000000000 +0200
@@ -69,6 +69,9 @@ extern void mnt_set_mountpoint(struct vf
extern void release_mounts(struct list_head *);
extern void umount_tree(struct vfsmount *, int, struct list_head *);
extern struct vfsmount *copy_tree(struct vfsmount *, struct dentry *, int);
+extern int sb_prepare_remount_readonly(struct super_block *);
+extern void sb_cancel_remount_readonly(struct super_block *);
+extern void sb_finish_remount_readonly(struct super_block *);
extern void __init mnt_init(void);
Index: linux-2.6/fs/namespace.c
===================================================================
--- linux-2.6.orig/fs/namespace.c 2010-10-22 16:02:14.000000000 +0200
+++ linux-2.6/fs/namespace.c 2010-10-22 17:59:30.000000000 +0200
@@ -419,6 +419,62 @@ void sb_force_remount_readonly(struct su
}
EXPORT_SYMBOL(sb_force_remount_readonly);
+int sb_prepare_remount_readonly(struct super_block *sb)
+{
+ struct vfsmount *mnt;
+ int err = 0;
+
+ br_write_lock(vfsmount_lock);
+ list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+ if (!(mnt->mnt_flags & MNT_READONLY)) {
+ mnt->mnt_flags |= MNT_WRITE_HOLD;
+ smp_mb();
+ if (count_mnt_writers(mnt) > 0) {
+ err = -EBUSY;
+ break;
+ }
+ }
+ }
+ list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+ if (mnt->mnt_flags & MNT_WRITE_HOLD) {
+ if (!err) {
+ mnt->mnt_flags |= MNT_READONLY | MNT_WAS_WRITE;
+ smp_wmb();
+ }
+ mnt->mnt_flags &= ~MNT_WRITE_HOLD;
+ }
+ }
+ br_write_unlock(vfsmount_lock);
+
+ return err;
+}
+
+void sb_cancel_remount_readonly(struct super_block *sb)
+{
+ struct vfsmount *mnt;
+
+ br_write_lock(vfsmount_lock);
+ list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+ if (mnt->mnt_flags & MNT_WAS_WRITE)
+ mnt->mnt_flags &= ~(MNT_WAS_WRITE | MNT_READONLY);
+ }
+
+ br_write_unlock(vfsmount_lock);
+}
+
+void sb_finish_remount_readonly(struct super_block *sb)
+{
+ struct vfsmount *mnt;
+
+ br_write_lock(vfsmount_lock);
+ list_for_each_entry(mnt, &sb->s_mounts, mnt_instance) {
+ if (mnt->mnt_flags & MNT_WAS_WRITE)
+ mnt->mnt_flags &= ~MNT_WAS_WRITE;
+ }
+ sb->s_flags |= MS_RDONLY;
+ br_write_unlock(vfsmount_lock);
+}
+
void simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
mnt->mnt_sb = sb;
@@ -608,7 +664,7 @@ static struct vfsmount *clone_mnt(struct
goto out_free;
}
- mnt->mnt_flags = old->mnt_flags & ~MNT_WRITE_HOLD;
+ mnt->mnt_flags = old->mnt_flags & MNT_PROPAGATION_MASK;
atomic_inc(&sb->s_active);
mnt->mnt_sb = sb;
mnt->mnt_root = dget(root);
@@ -636,7 +692,13 @@ static struct vfsmount *clone_mnt(struct
list_add(&mnt->mnt_expire, &old->mnt_expire);
}
+ /*
+ * Add new mount to s_mounts. Do this after fiddling
+ * with propagation flags to prevent races with
+ * remount ro/rw.
+ */
br_write_lock(vfsmount_lock);
+ mnt->mnt_flags |= old->mnt_flags & ~MNT_PROPAGATION_MASK;
list_add_tail(&mnt->mnt_instance, &mnt->mnt_sb->s_mounts);
br_write_unlock(vfsmount_lock);
}
@@ -1782,6 +1844,14 @@ int do_add_mount(struct vfsmount *newmnt
mnt_flags &= ~(MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL);
+ /* Locking is necessary to prevent racing with remount r/o */
+ down_read(&newmnt->mnt_sb->s_umount);
+ if (newmnt->mnt_sb->s_flags & MS_RDONLY)
+ mnt_flags |= MNT_READONLY;
+
+ newmnt->mnt_flags = mnt_flags;
+ up_read(&newmnt->mnt_sb->s_umount);
+
down_write(&namespace_sem);
/* Something was mounted here while we slept */
while (d_mountpoint(path->dentry) &&
@@ -1801,7 +1871,6 @@ int do_add_mount(struct vfsmount *newmnt
if (S_ISLNK(newmnt->mnt_root->d_inode->i_mode))
goto unlock;
- newmnt->mnt_flags = mnt_flags;
if ((err = graft_tree(newmnt, path)))
goto unlock;
Index: linux-2.6/fs/super.c
===================================================================
--- linux-2.6.orig/fs/super.c 2010-10-22 16:02:14.000000000 +0200
+++ linux-2.6/fs/super.c 2010-10-22 17:48:16.000000000 +0200
@@ -573,18 +573,31 @@ int do_remount_sb(struct super_block *sb
/* If we are remounting RDONLY and current sb is read/write,
make sure there are no rw files opened */
if (remount_ro) {
- if (force)
+ if (force) {
mark_files_ro(sb);
- else if (!fs_may_remount_ro(sb))
- return -EBUSY;
+ } else {
+ retval = sb_prepare_remount_readonly(sb);
+ if (retval)
+ return retval;
+
+ retval = -EBUSY;
+ if (!fs_may_remount_ro(sb))
+ goto cancel_readonly;
+ }
}
if (sb->s_op->remount_fs) {
retval = sb->s_op->remount_fs(sb, &flags, data);
/* If forced remount, go ahead despite any errors */
- if (retval && !force)
+ if (retval && !force) {
+ if (remount_ro)
+ goto cancel_readonly;
return retval;
+ }
}
+ if (remount_ro && !force)
+ sb_finish_remount_readonly(sb);
+
sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);
/*
@@ -598,6 +611,10 @@ int do_remount_sb(struct super_block *sb
if (remount_ro && sb->s_bdev)
invalidate_bdev(sb->s_bdev);
return 0;
+
+cancel_readonly:
+ sb_cancel_remount_readonly(sb);
+ return retval;
}
static void do_emergency_remount(struct work_struct *work)
Index: linux-2.6/include/linux/mount.h
===================================================================
--- linux-2.6.orig/include/linux/mount.h 2010-10-22 16:02:14.000000000 +0200
+++ linux-2.6/include/linux/mount.h 2010-10-22 17:59:30.000000000 +0200
@@ -30,6 +30,7 @@ struct mnt_namespace;
#define MNT_SHRINKABLE 0x100
#define MNT_WRITE_HOLD 0x200
+#define MNT_WAS_WRITE 0x400 /* used by remount to remember write mounts */
#define MNT_SHARED 0x1000 /* if the vfsmount is a shared mount */
#define MNT_UNBINDABLE 0x2000 /* if the vfsmount is a unbindable mount */
--
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