[RFC][PATCH v2 26/34] new helper: traverse_mounts()

Al Viro
2020-02-23T01:24:03Z

common guts of follow_down() and follow_managed() taken to a new
helper - traverse_mounts(). The remnants of follow_managed()
are folded into its sole remaining caller (handle_mounts()).
Calling conventions of handle_mounts() slightly sanitized -
instead of the weird "1 for success, -E... for failure" that used
to be imposed by the calling conventions of walk_component() et.al.
we can use the normal "0 for success, -E... for failure".

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
---
fs/namei.c | 177 +++++++++++++++++++++++++------------------------------------
1 file changed, 72 insertions(+), 105 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index e83071d25fae..a7730bbee162 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1168,91 +1168,79 @@ static int follow_automount(struct path *path, int *count, unsigned lookup_flags
}

/*
- * Handle a dentry that is managed in some way.
- * - Flagged for transit management (autofs)
- * - Flagged as mountpoint
- * - Flagged as automount point
- *
- * This may only be called in refwalk mode.
- * On success path->dentry is known positive.
- *
- * Serialization is taken care of in namespace.c
+ * mount traversal - out-of-line part. One note on ->d_flags accesses -
+ * dentries are pinned but not locked here, so negative dentry can go
+ * positive right under us. Use of smp_load_acquire() provides a barrier
+ * sufficient for ->d_inode and ->d_flags consistency.
*/
-static int follow_managed(struct path *path, struct nameidata *nd)
+static int __traverse_mounts(struct path *path, unsigned flags, bool *jumped,
+ int *count, unsigned lookup_flags)
{
- struct vfsmount *mnt = path->mnt; /* held by caller, must be left alone */
- unsigned flags;
+ struct vfsmount *mnt = path->mnt;
bool need_mntput = false;
int ret = 0;

- /* Given that we're not holding a lock here, we retain the value in a
- * local variable for each dentry as we look at it so that we don't see
- * the components of that value change under us */
- while (flags = smp_load_acquire(&path->dentry->d_flags),
- unlikely(flags & DCACHE_MANAGED_DENTRY)) {
+ while (flags & DCACHE_MANAGED_DENTRY) {
/* Allow the filesystem to manage the transit without i_mutex
* being held. */
if (flags & DCACHE_MANAGE_TRANSIT) {
- BUG_ON(!path->dentry->d_op);
- BUG_ON(!path->dentry->d_op->d_manage);
ret = path->dentry->d_op->d_manage(path, false);
flags = smp_load_acquire(&path->dentry->d_flags);
if (ret < 0)
break;
}

- /* Transit to a mounted filesystem. */
- if (flags & DCACHE_MOUNTED) {
+ if (flags & DCACHE_MOUNTED) { // something's mounted on it..
struct vfsmount *mounted = lookup_mnt(path);
- if (mounted) {
+ if (mounted) { // ... in our namespace
dput(path->dentry);
if (need_mntput)
mntput(path->mnt);
path->mnt = mounted;
path->dentry = dget(mounted->mnt_root);
+ // here we know it's positive
+ flags = path->dentry->d_flags;
need_mntput = true;
continue;
}
-
- /* Something is mounted on this dentry in another
- * namespace and/or whatever was mounted there in this
- * namespace got unmounted before lookup_mnt() could
- * get it */
}

- /* Handle an automount point */
- if (flags & DCACHE_NEED_AUTOMOUNT) {
- ret = follow_automount(path, &nd->total_link_count,
- nd->flags);
- if (ret < 0)
- break;
- continue;
- }
+ if (!(flags & DCACHE_NEED_AUTOMOUNT))
+ break;

- /* We didn't change the current path point */
- break;
+ // uncovered automount point
+ ret = follow_automount(path, count, lookup_flags);
+ flags = smp_load_acquire(&path->dentry->d_flags);
+ if (ret < 0)
+ break;
}

- if (need_mntput) {
- if (path->mnt == mnt)
- mntput(path->mnt);
- if (unlikely(nd->flags & LOOKUP_NO_XDEV))
- ret = -EXDEV;
- else
- nd->flags |= LOOKUP_JUMPED;
- }
- if (ret == -EISDIR || !ret)
- ret = 1;
- if (ret > 0 && unlikely(d_flags_negative(flags)))
+ if (ret == -EISDIR)
+ ret = 0;
+ // possible if you race with several mount --move
+ if (need_mntput && path->mnt == mnt)
+ mntput(path->mnt);
+ if (!ret && unlikely(d_flags_negative(flags)))
ret = -ENOENT;
- if (unlikely(ret < 0)) {
- dput(path->dentry);
- if (path->mnt != nd->path.mnt)
- mntput(path->mnt);
- }
+ *jumped = need_mntput;
return ret;
}

+static inline int traverse_mounts(struct path *path, bool *jumped,
+ int *count, unsigned lookup_flags)
+{
+ unsigned flags = smp_load_acquire(&path->dentry->d_flags);
+
+ /* fastpath */
+ if (likely(!(flags & DCACHE_MANAGED_DENTRY))) {
+ *jumped = false;
+ if (unlikely(d_flags_negative(flags)))
+ return -ENOENT;
+ return 0;
+ }
+ return __traverse_mounts(path, flags, jumped, count, lookup_flags);
+}
+
int follow_down_one(struct path *path)
{
struct vfsmount *mounted;
@@ -1270,6 +1258,23 @@ int follow_down_one(struct path *path)
EXPORT_SYMBOL(follow_down_one);

/*
+ * Follow down to the covering mount currently visible to userspace. At each
+ * point, the filesystem owning that dentry may be queried as to whether the
+ * caller is permitted to proceed or not.
+ */
+int follow_down(struct path *path)
+{
+ struct vfsmount *mnt = path->mnt;
+ bool jumped;
+ int ret = traverse_mounts(path, &jumped, NULL, 0);
+
+ if (path->mnt != mnt)
+ mntput(mnt);
+ return ret;
+}
+EXPORT_SYMBOL(follow_down);
+
+/*
* Try to skip to top of mountpoint pile in rcuwalk mode. Fail if
* we meet a managed dentry that would need blocking.
*/
@@ -1325,6 +1330,7 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
struct path *path, struct inode **inode,
unsigned int *seqp)
{
+ bool jumped;
int ret;

path->mnt = nd->path.mnt;
@@ -1334,15 +1340,25 @@ static inline int handle_mounts(struct nameidata *nd, struct dentry *dentry,
if (unlikely(!*inode))
return -ENOENT;
if (likely(__follow_mount_rcu(nd, path, inode, seqp)))
- return 1;
+ return 0;
if (unlazy_child(nd, dentry, seq))
return -ECHILD;
// *path might've been clobbered by __follow_mount_rcu()
path->mnt = nd->path.mnt;
path->dentry = dentry;
}
- ret = follow_managed(path, nd);
- if (likely(ret >= 0)) {
+ ret = traverse_mounts(path, &jumped, &nd->total_link_count, nd->flags);
+ if (jumped) {
+ if (unlikely(nd->flags & LOOKUP_NO_XDEV))
+ ret = -EXDEV;
+ else
+ nd->flags |= LOOKUP_JUMPED;
+ }
+ if (unlikely(ret)) {
+ dput(path->dentry);
+ if (path->mnt != nd->path.mnt)
+ mntput(path->mnt);
+ } else {
*inode = d_backing_inode(path->dentry);
*seqp = 0; /* out of RCU mode, so the value doesn't matter */
}
@@ -1411,55 +1427,6 @@ static int follow_dotdot_rcu(struct nameidata *nd)
}

/*
- * Follow down to the covering mount currently visible to userspace. At each
- * point, the filesystem owning that dentry may be queried as to whether the
- * caller is permitted to proceed or not.
- */
-int follow_down(struct path *path)
-{
- unsigned managed;
- int ret;
-
- while (managed = READ_ONCE(path->dentry->d_flags),
- unlikely(managed & DCACHE_MANAGED_DENTRY)) {
- /* Allow the filesystem to manage the transit without i_mutex
- * being held.
- *
- * We indicate to the filesystem if someone is trying to mount
- * something here. This gives autofs the chance to deny anyone
- * other than its daemon the right to mount on its
- * superstructure.
- *
- * The filesystem may sleep at this point.
- */
- if (managed & DCACHE_MANAGE_TRANSIT) {
- BUG_ON(!path->dentry->d_op);
- BUG_ON(!path->dentry->d_op->d_manage);
- ret = path->dentry->d_op->d_manage(path, false);
- if (ret < 0)
- return ret == -EISDIR ? 0 : ret;
- }
-
- /* Transit to a mounted filesystem. */
- if (managed & DCACHE_MOUNTED) {
- struct vfsmount *mounted = lookup_mnt(path);
- if (!mounted)
- break;
- dput(path->dentry);
- mntput(path->mnt);
- path->mnt = mounted;
- path->dentry = dget(mounted->mnt_root);
- continue;
- }
-
- /* Don't handle automount points here */
- break;
- }
- return 0;
-}
-EXPORT_SYMBOL(follow_down);
-
-/*
* Skip to top of mountpoint pile in refwalk mode for follow_dotdot()
*/
static void follow_mount(struct path *path)
--
2.11.0