[PATCH 12/20] VFS/namei: new flag to support RCU symlinks: LOOKUP_LINK_RCU.

NeilBrown
2015-03-23T02:39:15Z

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 1a8cc0e47df6..3262c8c2e73d 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -552,6 +552,9 @@ 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()
@@ -765,7 +768,8 @@ 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_get_link(nd), cookie);
- path_put(link);
+ if (!(nd->flags & LOOKUP_LINK_RCU))
+ path_put(link);
}

int sysctl_protected_symlinks __read_mostly = 0;
@@ -892,9 +896,10 @@ 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;
@@ -946,7 +951,8 @@ 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;
}

@@ -1669,6 +1675,8 @@ static inline int nested_symlink(struct path *path, struct nameidata *nd)

nd->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 cc8b51a47160..633101964520 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -31,6 +31,7 @@ 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