lkml.org 
[lkml]   [2015]   [May]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH v3 105/110] namei: make unlazy_walk and terminate_walk handle nd->stack, add unlazy_link
On Tue, May 12, 2015 at 12:43:33AM +0100, Al Viro wrote:
> On Mon, May 11, 2015 at 07:08:05PM +0100, Al Viro wrote:
> > +static bool legitimize_links(struct nameidata *nd)
> > +{
> > + int i;
> > + for (i = 0; i < nd->depth; i++) {
> > + struct saved *last = nd->stack + i;
> > + if (unlikely(!legitimize_path(nd, &last->link, last->seq))) {
> > + drop_links(nd);
> > + nd->depth = i;
>
> Broken, actually - it should be i + 1. What happens is that we attempt to
> grab references on nd->stack[...].link; if everything succeeds, we'd won.
> If legitimizing nd->stack[i].link fails (e.g. ->d_seq has changed on us),
> we
> * put_link everything in stack and clear nd->stack[...].cookie, making
> sure that nobody will call ->put_link() on it later.
> * leave the things for terminate_walk() so that it would do
> path_put() on everything we have grabbed and ignored everything we hadn't
> even got around to.
>
> But this failed legitimize_path() requires path_put() - we *can't* block
> there (we wouldn't be able to do ->put_link() afterwards if we did), so
> we just zero what we didn't grab and leave what we had for subsequent
> path_put(). Which may be anything from "nothing" (mount_lock has been
> touched) to "both vfsmount and dentry" (->d_seq mismatch).
>
> So we need to set nd->depth to i + 1 here, not i. As it is, we are risking
> a vfsmount (and possibly dentry) leak. Fixed and force-pushed...

FWIW, below is a better replacement; tested and force-pushed. And seeing that
we just got nd->root_seq, I wonder if we really need messing with
current->fs there - something like
if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
if (unlikely(!legitimize_path(&nd->root, nd->root_seq))) {
rcu_read_unlock();
dput(dentry);
return -ECHILD;
}
}
should do better than playing with fs->lock, etc. we do right now...

diff --git a/fs/namei.c b/fs/namei.c
index 92bf031..6db14f2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -554,6 +554,68 @@ static inline int nd_alloc_stack(struct nameidata *nd)
return __nd_alloc_stack(nd);
}

+static void drop_links(struct nameidata *nd)
+{
+ int i = nd->depth;
+ while (i--) {
+ struct saved *last = nd->stack + i;
+ struct inode *inode = last->inode;
+ if (last->cookie && inode->i_op->put_link) {
+ inode->i_op->put_link(inode, last->cookie);
+ last->cookie = NULL;
+ }
+ }
+}
+
+static void terminate_walk(struct nameidata *nd)
+{
+ drop_links(nd);
+ if (!(nd->flags & LOOKUP_RCU)) {
+ int i;
+ path_put(&nd->path);
+ for (i = 0; i < nd->depth; i++)
+ path_put(&nd->stack[i].link);
+ } else {
+ nd->flags &= ~LOOKUP_RCU;
+ if (!(nd->flags & LOOKUP_ROOT))
+ nd->root.mnt = NULL;
+ rcu_read_unlock();
+ }
+ nd->depth = 0;
+}
+
+/* path_put is needed afterwards regardless of success or failure */
+static bool legitimize_path(struct nameidata *nd,
+ struct path *path, unsigned seq)
+{
+ int res = __legitimize_mnt(path->mnt, nd->m_seq);
+ if (unlikely(res)) {
+ if (res > 0)
+ path->mnt = NULL;
+ path->dentry = NULL;
+ return false;
+ }
+ if (unlikely(!lockref_get_not_dead(&path->dentry->d_lockref))) {
+ path->dentry = NULL;
+ return false;
+ }
+ return !read_seqcount_retry(&path->dentry->d_seq, seq);
+}
+
+static bool legitimize_links(struct nameidata *nd)
+{
+ int i;
+ for (i = 0; i < nd->depth; i++) {
+ struct saved *last = nd->stack + i;
+ if (unlikely(!legitimize_path(nd, &last->link, last->seq))) {
+ drop_links(nd);
+ nd->depth = i + 1;
+ return false;
+ }
+ }
+ return true;
+}
+
/*
* Path walking has 2 modes, rcu-walk and ref-walk (see
* Documentation/filesystems/path-lookup.txt). In situations when we can't
@@ -575,6 +637,8 @@ static inline int nd_alloc_stack(struct nameidata *nd)
* unlazy_walk attempts to legitimize the current nd->path, nd->root and dentry
* for ref-walk mode. @dentry must be a path found by a do_lookup call on
* @nd or NULL. Must be called from rcu-walk context.
+ * Nothing should touch nameidata between unlazy_walk() failure and
+ * terminate_walk().
*/
static int unlazy_walk(struct nameidata *nd, struct dentry *dentry, unsigned seq)
{
@@ -583,22 +647,13 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry, unsigned seq

BUG_ON(!(nd->flags & LOOKUP_RCU));

- /*
- * After legitimizing the bastards, terminate_walk()
- * will do the right thing for non-RCU mode, and all our
- * subsequent exit cases should rcu_read_unlock()
- * before returning. Do vfsmount first; if dentry
- * can't be legitimized, just set nd->path.dentry to NULL
- * and rely on dput(NULL) being a no-op.
- */
- if (!legitimize_mnt(nd->path.mnt, nd->m_seq))
- return -ECHILD;
nd->flags &= ~LOOKUP_RCU;
-
- if (!lockref_get_not_dead(&parent->d_lockref)) {
- nd->path.dentry = NULL;
- goto out;
- }
+ if (unlikely(!legitimize_links(nd)))
+ goto out2;
+ if (unlikely(!legitimize_mnt(nd->path.mnt, nd->m_seq)))
+ goto out2;
+ if (unlikely(!lockref_get_not_dead(&parent->d_lockref)))
+ goto out1;

/*
* For a negative lookup, the lookup sequence point is the parents
@@ -628,8 +683,10 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry, unsigned seq
*/
if (nd->root.mnt && !(nd->flags & LOOKUP_ROOT)) {
spin_lock(&fs->lock);
- if (nd->root.mnt != fs->root.mnt || nd->root.dentry != fs->root.dentry)
- goto unlock_and_drop_dentry;
+ if (unlikely(!path_equal(&nd->root, &fs->root))) {
+ spin_unlock(&fs->lock);
+ goto drop_dentry;
+ }
path_get(&nd->root);
spin_unlock(&fs->lock);
}
@@ -637,12 +694,14 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry, unsigned seq
rcu_read_unlock();
return 0;

-unlock_and_drop_dentry:
- spin_unlock(&fs->lock);
drop_dentry:
rcu_read_unlock();
dput(dentry);
goto drop_root_mnt;
+out2:
+ nd->path.mnt = NULL;
+out1:
+ nd->path.dentry = NULL;
out:
rcu_read_unlock();
drop_root_mnt:
@@ -651,6 +710,23 @@ drop_root_mnt:
return -ECHILD;
}

+static int unlazy_link(struct nameidata *nd, struct path *link, unsigned seq)
+{
+ if (unlikely(!legitimize_path(nd, link, seq))) {
+ drop_links(nd);
+ rcu_read_unlock();
+ nd->flags &= ~LOOKUP_RCU;
+ nd->path.mnt = NULL;
+ nd->path.dentry = NULL;
+ if (!(nd->flags & LOOKUP_ROOT))
+ nd->root.mnt = NULL;
+ } else if (likely(unlazy_walk(nd, NULL, 0)) == 0) {
+ return 0;
+ }
+ path_put(link);
+ return -ECHILD;
+}
+
static inline int d_revalidate(struct dentry *dentry, unsigned int flags)
{
return dentry->d_op->d_revalidate(dentry, flags);
@@ -1537,20 +1613,6 @@ static inline int handle_dots(struct nameidata *nd, int type)
return 0;
}

-static void terminate_walk(struct nameidata *nd)
-{
- if (!(nd->flags & LOOKUP_RCU)) {
- path_put(&nd->path);
- } else {
- nd->flags &= ~LOOKUP_RCU;
- if (!(nd->flags & LOOKUP_ROOT))
- nd->root.mnt = NULL;
- rcu_read_unlock();
- }
- while (unlikely(nd->depth))
- put_link(nd);
-}
-
static int pick_link(struct nameidata *nd, struct path *link,
struct inode *inode, unsigned seq)
{
@@ -1561,13 +1623,12 @@ static int pick_link(struct nameidata *nd, struct path *link,
return -ELOOP;
}
if (nd->flags & LOOKUP_RCU) {
- if (unlikely(nd->path.mnt != link->mnt ||
- unlazy_walk(nd, link->dentry, seq))) {
+ if (unlikely(unlazy_link(nd, link, seq)))
return -ECHILD;
- }
+ } else {
+ if (link->mnt == nd->path.mnt)
+ mntget(link->mnt);
}
- if (link->mnt == nd->path.mnt)
- mntget(link->mnt);
error = nd_alloc_stack(nd);
if (unlikely(error)) {
path_put(link);

\
 
 \ /
  Last update: 2015-05-12 06:41    [W:0.132 / U:3.540 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site