Commit 84f0cd9e authored Mar 03, 2020 by Al Viro

pick_link(): pass it struct path already with normal refcounting rules



step_into() tries to avoid grabbing and dropping mount references
on the steps that do not involve crossing mountpoints (which is
obviously the majority of cases).  So it uses a local struct path
with unusual refcounting rules - path.mnt is pinned if and only if
it's not equal to nd->path.mnt.

We used to have similar beasts all over the place and we had quite
a few bugs crop up in their handling - it's easy to get confused
when changing e.g. cleanup on failure exits (or adding a new check,
etc.)

Now that's mostly gone - the step_into() instance (which is what
we need them for) is the only one left.  It is exposed to mount
traversal and it's (shortly) seen by pick_link().  Since pick_link()
needs to store it in link stack, where the normal rules apply,
it has to make sure that mount is pinned regardless of nd->path.mnt
value.  That's done on all calls of pick_link() and very early
in those.  Let's do that in the caller (step_into()) instead -
that way the fewer places need to be aware of such struct path
instances.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

parent 19f6028a

fs/namei.c

+6 −6

Original line number	Diff line number	Diff line
		@@ -1602,13 +1602,10 @@ static const char pick_link(struct nameidata nd, struct path *link,
		int error;

		if (unlikely(nd->total_link_count++ >= MAXSYMLINKS)) {
		path_to_nameidata(link, nd);
		if (!(nd->flags & LOOKUP_RCU))
		path_put(link);
		return ERR_PTR(-ELOOP);
		}
		if (!(nd->flags & LOOKUP_RCU)) {
		if (link->mnt == nd->path.mnt)
		mntget(link->mnt);
		}
		error = nd_alloc_stack(nd);
		if (unlikely(error)) {
		if (error == -ECHILD) {
		@@ -1713,10 +1710,13 @@ static const char step_into(struct nameidata nd, int flags,
		nd->seq = seq;
		return NULL;
		}
		/* make sure that d_is_symlink above matches inode */
		if (nd->flags & LOOKUP_RCU) {
		/* make sure that d_is_symlink above matches inode */
		if (read_seqcount_retry(&path.dentry->d_seq, seq))
		return ERR_PTR(-ECHILD);
		} else {
		if (path.mnt == nd->path.mnt)
		mntget(path.mnt);
		}
		return pick_link(nd, &path, inode, seq, flags);
		}

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