From: Miklos Szeredi <mszeredi@xxxxxxxxxx>
If mounts are deleted after a read(2) call on /proc/self/mounts (or its
kin), the subsequent read(2) could miss a mount that comes after the
deleted one in the list. This is because the file position is interpreted
as the number mount entries from the start of the list.
E.g. first read gets entries #0 to #9; the seq file index will be 10. Then
entry #5 is deleted, resulting in #10 becoming #9 and #11 becoming #10,
etc... The next read will continue from entry #10, and #9 is missed.
Solve this by adding a cursor entry for each open instance. Taking the
global namespace_sem for write seems excessive, since we are only dealing
with a per-namespace list. Instead add a per-namespace spinlock and use
that together with namespace_sem taken for read to protect against
concurrent modification of the mount list. This may reduce parallelism of
is_local_mountpoint(), but it's hardly a big contention point. We could
also use RCU freeing of cursors to make traversal not need additional
locks, if that turns out to be neceesary.
Reported-by: Karel Zak <kzak@xxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
Differences from v1:
- removed unnecessary code that wanted to handle lseeks
- fixed double entry at the start of a read
fs/mount.h | 12 +++++--
fs/namespace.c | 76 ++++++++++++++++++++++++++++++++++++++------------
fs/proc_namespace.c | 4 +-
include/linux/mount.h | 4 +-
4 files changed, 74 insertions(+), 22 deletions(-)
--- a/fs/mount.h
+++ b/fs/mount.h
@@ -9,7 +9,13 @@ struct mnt_namespace {
atomic_t count;
struct ns_common ns;
struct mount * root;
+ /*
+ * Traversal and modification of .list is protected by either
+ * - taking namespace_sem for write, OR
+ * - taking namespace_sem for read AND taking .ns_lock.
+ */
struct list_head list;
+ spinlock_t ns_lock;
struct user_namespace *user_ns;
struct ucounts *ucounts;
u64 seq; /* Sequence number to prevent loops */
@@ -133,9 +139,7 @@ struct proc_mounts {
struct mnt_namespace *ns;
struct path root;
int (*show)(struct seq_file *, struct vfsmount *);
- void *cached_mount;
- u64 cached_event;
- loff_t cached_index;
+ struct mount cursor;
};
extern const struct seq_operations mounts_op;
@@ -153,3 +157,5 @@ static inline bool is_anon_ns(struct mnt
{
return ns->seq == 0;
}
+
+extern void mnt_cursor_del(struct mnt_namespace *ns, struct mount *cursor);
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -648,6 +648,30 @@ struct vfsmount *lookup_mnt(const struct
return m;
}
+static inline void lock_ns_list(struct mnt_namespace *ns)
+{
+ spin_lock(&ns->ns_lock);
+}
+
+static inline void unlock_ns_list(struct mnt_namespace *ns)
+{
+ spin_unlock(&ns->ns_lock);
+}
+
+static inline bool mnt_is_cursor(struct mount *mnt)
+{
+ return mnt->mnt.mnt_flags & MNT_CURSOR;
+}
+
+static struct mount *mnt_skip_cursors(struct mnt_namespace *ns,
+ struct mount *mnt)
+{
+ list_for_each_entry_from(mnt, &ns->list, mnt_list)
+ if (!mnt_is_cursor(mnt))
+ return mnt;
+ return NULL;
+}
+
/*
* __is_local_mountpoint - Test to see if dentry is a mountpoint in the
* current mount namespace.
@@ -673,11 +697,15 @@ bool __is_local_mountpoint(struct dentry
goto out;
down_read(&namespace_sem);
+ lock_ns_list(ns);
list_for_each_entry(mnt, &ns->list, mnt_list) {
+ if (mnt_is_cursor(mnt))
+ continue;
is_covered = (mnt->mnt_mountpoint == dentry);
if (is_covered)
break;
}
+ unlock_ns_list(ns);
up_read(&namespace_sem);
out:
return is_covered;
@@ -1249,31 +1277,30 @@ struct vfsmount *mnt_clone_internal(cons
static void *m_start(struct seq_file *m, loff_t *pos)
{
struct proc_mounts *p = m->private;
+ struct mount *mnt;
down_read(&namespace_sem);
- if (p->cached_event == p->ns->event) {
- void *v = p->cached_mount;
- if (*pos == p->cached_index)
- return v;
- if (*pos == p->cached_index + 1) {
- v = seq_list_next(v, &p->ns->list, &p->cached_index);
- return p->cached_mount = v;
- }
- }
+ lock_ns_list(p->ns);
+ if (!*pos)
+ list_move(&p->cursor.mnt_list, &p->ns->list);
+ mnt = mnt_skip_cursors(p->ns, &p->cursor);
+ unlock_ns_list(p->ns);
- p->cached_event = p->ns->event;
- p->cached_mount = seq_list_start(&p->ns->list, *pos);
- p->cached_index = *pos;
- return p->cached_mount;
+ return mnt;
}
static void *m_next(struct seq_file *m, void *v, loff_t *pos)
{
struct proc_mounts *p = m->private;
+ struct mount *mnt = v;
- p->cached_mount = seq_list_next(v, &p->ns->list, pos);
- p->cached_index = *pos;
- return p->cached_mount;
+ lock_ns_list(p->ns);
+ list_move(&p->cursor.mnt_list, &mnt->mnt_list);
+ mnt = mnt_skip_cursors(p->ns, &p->cursor);
+ unlock_ns_list(p->ns);
+ ++*pos;
+
+ return mnt;
}
static void m_stop(struct seq_file *m, void *v)
@@ -1284,7 +1311,7 @@ static void m_stop(struct seq_file *m, v
static int m_show(struct seq_file *m, void *v)
{
struct proc_mounts *p = m->private;
- struct mount *r = list_entry(v, struct mount, mnt_list);
+ struct mount *r = v;
return p->show(m, &r->mnt);
}
@@ -1294,6 +1321,15 @@ const struct seq_operations mounts_op =
.stop = m_stop,
.show = m_show,
};
+
+void mnt_cursor_del(struct mnt_namespace *ns, struct mount *cursor)
+{
+ down_read(&namespace_sem);
+ lock_ns_list(ns);
+ list_del(&cursor->mnt_list);
+ unlock_ns_list(ns);
+ up_read(&namespace_sem);
+}
#endif /* CONFIG_PROC_FS */
/**
@@ -3202,6 +3238,7 @@ static struct mnt_namespace *alloc_mnt_n
atomic_set(&new_ns->count, 1);
INIT_LIST_HEAD(&new_ns->list);
init_waitqueue_head(&new_ns->poll);
+ spin_lock_init(&new_ns->ns_lock);
new_ns->user_ns = get_user_ns(user_ns);
new_ns->ucounts = ucounts;
return new_ns;
@@ -3842,10 +3879,14 @@ static bool mnt_already_visible(struct m
bool visible = false;
down_read(&namespace_sem);
+ lock_ns_list(ns);
list_for_each_entry(mnt, &ns->list, mnt_list) {
struct mount *child;
int mnt_flags;
+ if (mnt_is_cursor(mnt))
+ continue;
+
if (mnt->mnt.mnt_sb->s_type != sb->s_type)
continue;
@@ -3893,6 +3934,7 @@ static bool mnt_already_visible(struct m
next: ;
}
found:
+ unlock_ns_list(ns);
up_read(&namespace_sem);
return visible;
}
--- a/fs/proc_namespace.c
+++ b/fs/proc_namespace.c
@@ -279,7 +279,8 @@ static int mounts_open_common(struct ino
p->ns = ns;
p->root = root;
p->show = show;
- p->cached_event = ~0ULL;
+ INIT_LIST_HEAD(&p->cursor.mnt_list);
+ p->cursor.mnt.mnt_flags = MNT_CURSOR;
return 0;
@@ -296,6 +297,7 @@ static int mounts_release(struct inode *
struct seq_file *m = file->private_data;
struct proc_mounts *p = m->private;
path_put(&p->root);
+ mnt_cursor_del(p->ns, &p->cursor);
put_mnt_ns(p->ns);
return seq_release_private(inode, file);
}
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -50,7 +50,8 @@ struct fs_context;
#define MNT_ATIME_MASK (MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME )
#define MNT_INTERNAL_FLAGS (MNT_SHARED | MNT_WRITE_HOLD | MNT_INTERNAL | \
- MNT_DOOMED | MNT_SYNC_UMOUNT | MNT_MARKED)
+ MNT_DOOMED | MNT_SYNC_UMOUNT | MNT_MARKED | \
+ MNT_CURSOR)
#define MNT_INTERNAL 0x4000
@@ -64,6 +65,7 @@ struct fs_context;
#define MNT_SYNC_UMOUNT 0x2000000
#define MNT_MARKED 0x4000000
#define MNT_UMOUNT 0x8000000
+#define MNT_CURSOR 0x10000000
struct vfsmount {
struct dentry *mnt_root; /* root of the mounted tree */
