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Subject[34-longterm 011/209] Fix sget() race with failing mount
Date
From: Al Viro <viro@zeniv.linux.org.uk>

=====================================================================
| This is a commit scheduled for the next v2.6.34 longterm release. |
| If you see a problem with using this for longterm, please comment.|
=====================================================================

commit 7a4dec53897ecd3367efb1e12fe8a4edc47dc0e9 upstream.

If sget() finds a matching superblock being set up, it'll
grab an active reference to it and grab s_umount. That's
fine - we'll wait for completion of foofs_get_sb() that way.
However, if said foofs_get_sb() fails we'll end up holding
the halfway-created superblock. deactivate_locked_super()
called by foofs_get_sb() will just unlock the sucker since
we are holding another active reference to it.

What we need is a way to tell if superblock has been successfully
set up. Unfortunately, neither ->s_root nor the check for
MS_ACTIVE quite fit. Cheap and easy way, suitable for backport:
new flag set by the (only) caller of ->get_sb(). If that flag
isn't present by the time sget() grabbed s_umount on preexisting
superblock it has found, it's seeing a stillborn and should
just bury it with deactivate_locked_super() (and repeat the search).

Longer term we want to set that flag in ->get_sb() instances (and
check for it to distinguish between "sget() found us a live sb"
and "sget() has allocated an sb, we need to set it up" in there,
instead of checking ->s_root as we do now).

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
fs/namespace.c | 2 +-
fs/super.c | 6 ++++++
include/linux/fs.h | 1 +
3 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index f20cb57..bfe3f3e 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1996,7 +1996,7 @@ long do_mount(char *dev_name, char *dir_name, char *type_page,
if (flags & MS_RDONLY)
mnt_flags |= MNT_READONLY;

- flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
+ flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE | MS_BORN |
MS_NOATIME | MS_NODIRATIME | MS_RELATIME| MS_KERNMOUNT |
MS_STRICTATIME);

diff --git a/fs/super.c b/fs/super.c
index 1527e6a..53040a6 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -356,6 +356,11 @@ retry:
if (s) {
up_write(&s->s_umount);
destroy_super(s);
+ s = NULL;
+ }
+ if (unlikely(!(old->s_flags & MS_BORN))) {
+ deactivate_locked_super(old);
+ goto retry;
}
return old;
}
@@ -957,6 +962,7 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
goto out_free_secdata;
BUG_ON(!mnt->mnt_sb);
WARN_ON(!mnt->mnt_sb->s_bdi);
+ mnt->mnt_sb->s_flags |= MS_BORN;

error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
if (error)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e93529d..8aa6bd9 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -209,6 +209,7 @@ struct inodes_stat_t {
#define MS_KERNMOUNT (1<<22) /* this is a kern_mount call */
#define MS_I_VERSION (1<<23) /* Update inode I_version field */
#define MS_STRICTATIME (1<<24) /* Always perform atime updates */
+#define MS_BORN (1<<29)
#define MS_ACTIVE (1<<30)
#define MS_NOUSER (1<<31)

--
1.7.4.4


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