[PATCH 3/9] VFS/namei: new flag to support RCU symlinks: LOOKUP_LINK_RCU.

[NeilBrown <neilb@suse.de>]

When we support ->follow_link in RCU-walk we will not want to
take a reference to the 'struct path *link' passed to follow_link,
and correspondingly will not want to drop that reference.

As link_path_walk will complete_walk() in the case of an error,
and as complete_walk() will clear LOOKUP_RCU, we cannot test
LOOKUP_RCU to determine if the path should be 'put'.

So introduce a new flag: LOOKUP_LINK_RCU.  This is set on
entry to follow_link() if appropriate and put_link() will
only call path_put() if it is clear.

Also, unlazy_walk() will fail if LOOKUP_LINK_RCU is set.
This is because there is no way for unlazy_walk to get references
on all the "struct path *link"s that are protected by that flag.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/namei.c            |   18 +++++++++++++-----
 include/linux/namei.h |    1 +
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 515adc4594be..a4879bb3b4ee 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -533,6 +533,9 @@ static int unlazy_walk(struct nameidata *nd, struct dentry *dentry)
 	struct dentry *parent = nd->path.dentry;
 
 	BUG_ON(!(nd->flags & LOOKUP_RCU));
+	if (nd->flags & LOOKUP_LINK_RCU)
+		/* Cannot unlazy in the middle of following a symlink */
+		return -ECHILD;
 
 	/*
 	 * After legitimizing the bastards, terminate_walk()
@@ -743,7 +746,8 @@ static inline void put_link(struct nameidata *nd, struct path *link, void *cooki
 	struct inode *inode = link->dentry->d_inode;
 	if (inode->i_op->put_link)
 		inode->i_op->put_link(link->dentry, nd, cookie);
-	path_put(link);
+	if (!(nd->flags & LOOKUP_LINK_RCU))
+		path_put(link);
 }
 
 int sysctl_protected_symlinks __read_mostly = 0;
@@ -869,9 +873,10 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 	int error;
 	char *s;
 
-	BUG_ON(nd->flags & LOOKUP_RCU);
-
-	if (link->mnt == nd->path.mnt)
+	nd->flags &= ~LOOKUP_LINK_RCU;
+	if (nd->flags & LOOKUP_RCU)
+		nd->flags |= LOOKUP_LINK_RCU;
+	else if (link->mnt == nd->path.mnt)
 		mntget(link->mnt);
 
 	error = -ELOOP;
@@ -921,7 +926,8 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 out_put_nd_path:
 	*p = NULL;
 	terminate_walk(nd);
-	path_put(link);
+	if (!(nd->flags & LOOKUP_LINK_RCU))
+		path_put(link);
 	return error;
 }
 
@@ -1644,6 +1650,8 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)
 
 	current->link_count--;
 	nd->depth--;
+	if (!nd->depth)
+		nd->flags &= ~LOOKUP_LINK_RCU;
 	return res;
 }
 
diff --git a/include/linux/namei.h b/include/linux/namei.h
index c8990779f0c3..34bc87845b0e 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -32,6 +32,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_PARENT		0x0010
 #define LOOKUP_REVAL		0x0020
 #define LOOKUP_RCU		0x0040
+#define LOOKUP_LINK_RCU		0x0080
 
 /*
  * Intent data