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Subject[PATCH 29/30] r/o bind mounts: honor mount writer counts at remount
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Originally from: Herbert Poetzl <herbert@13thfloor.at>

This is the core of the read-only bind mount patch set.

Note that this does _not_ add a "ro" option directly to the bind mount
operation. If you require such a mount, you must first do the bind, then
follow it up with a 'mount -o remount,ro' operation:

If you wish to have a r/o bind mount of /foo on bar:

mount --bind /foo /bar
mount -o remount,ro /bar

Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

linux-2.6.git-dave/fs/namespace.c | 50 ++++++++++++++++++++++++++-----
linux-2.6.git-dave/include/linux/mount.h | 1
2 files changed, 44 insertions(+), 7 deletions(-)

diff -puN fs/namespace.c~r-o-bind-mounts-honor-r-w-changes-at-do_remount-time fs/namespace.c
--- linux-2.6.git/fs/namespace.c~r-o-bind-mounts-honor-r-w-changes-at-do_remount-time 2008-02-15 13:25:59.000000000 -0800
+++ linux-2.6.git-dave/fs/namespace.c 2008-02-15 13:25:59.000000000 -0800
@@ -105,7 +105,11 @@ struct vfsmount *alloc_vfsmnt(const char
*/
int __mnt_is_readonly(struct vfsmount *mnt)
{
- return (mnt->mnt_sb->s_flags & MS_RDONLY);
+ if (mnt->mnt_flags & MNT_READONLY)
+ return 1;
+ if (mnt->mnt_sb->s_flags & MS_RDONLY)
+ return 1;
+ return 0;
}
EXPORT_SYMBOL_GPL(__mnt_is_readonly);

@@ -299,7 +303,7 @@ void mnt_drop_write(struct vfsmount *mnt
}
EXPORT_SYMBOL_GPL(mnt_drop_write);

-int mnt_make_readonly(struct vfsmount *mnt)
+static int mnt_make_readonly(struct vfsmount *mnt)
{
int ret = 0;

@@ -312,15 +316,25 @@ int mnt_make_readonly(struct vfsmount *m
goto out;
}
/*
- * actually set mount's r/o flag here to make
- * __mnt_is_readonly() true, which keeps anyone
- * from doing a successful mnt_want_write().
+ * nobody can do a successful mnt_want_write() with all
+ * of the counts in MNT_DENIED_WRITE and the locks held.
*/
+ spin_lock(&vfsmount_lock);
+ if (!ret)
+ mnt->mnt_flags |= MNT_READONLY;
+ spin_unlock(&vfsmount_lock);
out:
unlock_mnt_writers();
return ret;
}

+static void __mnt_unmake_readonly(struct vfsmount *mnt)
+{
+ spin_lock(&vfsmount_lock);
+ mnt->mnt_flags &= ~MNT_READONLY;
+ spin_unlock(&vfsmount_lock);
+}
+
int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
mnt->mnt_sb = sb;
@@ -683,7 +697,7 @@ static int show_vfsmnt(struct seq_file *
seq_putc(m, '.');
mangle(m, mnt->mnt_sb->s_subtype);
}
- seq_puts(m, mnt->mnt_sb->s_flags & MS_RDONLY ? " ro" : " rw");
+ seq_puts(m, __mnt_is_readonly(mnt) ? " ro" : " rw");
for (fs_infop = fs_info; fs_infop->flag; fs_infop++) {
if (mnt->mnt_sb->s_flags & fs_infop->flag)
seq_puts(m, fs_infop->str);
@@ -1277,6 +1291,23 @@ out:
return err;
}

+static int change_mount_flags(struct vfsmount *mnt, int ms_flags)
+{
+ int error = 0;
+ int readonly_request = 0;
+
+ if (ms_flags & MS_RDONLY)
+ readonly_request = 1;
+ if (readonly_request == __mnt_is_readonly(mnt))
+ return 0;
+
+ if (readonly_request)
+ error = mnt_make_readonly(mnt);
+ else
+ __mnt_unmake_readonly(mnt);
+ return error;
+}
+
/*
* change filesystem flags. dir should be a physical root of filesystem.
* If you've mounted a non-root directory somewhere and want to do remount
@@ -1299,7 +1330,10 @@ static noinline int do_remount(struct na
return -EINVAL;

down_write(&sb->s_umount);
- err = do_remount_sb(sb, flags, data, 0);
+ if (flags & MS_BIND)
+ err = change_mount_flags(nd->path.mnt, flags);
+ else
+ err = do_remount_sb(sb, flags, data, 0);
if (!err)
nd->path.mnt->mnt_flags = mnt_flags;
up_write(&sb->s_umount);
@@ -1752,6 +1786,8 @@ long do_mount(char *dev_name, char *dir_
mnt_flags |= MNT_NODIRATIME;
if (flags & MS_RELATIME)
mnt_flags |= MNT_RELATIME;
+ if (flags & MS_RDONLY)
+ mnt_flags |= MNT_READONLY;

flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT);
diff -puN include/linux/mount.h~r-o-bind-mounts-honor-r-w-changes-at-do_remount-time include/linux/mount.h
--- linux-2.6.git/include/linux/mount.h~r-o-bind-mounts-honor-r-w-changes-at-do_remount-time 2008-02-15 13:25:59.000000000 -0800
+++ linux-2.6.git-dave/include/linux/mount.h 2008-02-15 13:25:59.000000000 -0800
@@ -29,6 +29,7 @@ struct mnt_namespace;
#define MNT_NOATIME 0x08
#define MNT_NODIRATIME 0x10
#define MNT_RELATIME 0x20
+#define MNT_READONLY 0x40 /* does the user want this to be r/o? */

#define MNT_SHRINKABLE 0x100
#define MNT_IMBALANCED_WRITE_COUNT 0x200 /* just for debugging */
_

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