[RFC][PATCH v4 57/69] pick_link(): pass it struct path already with normal refcounting rules

Al Viro
2020-03-13T23:55:27Z

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>
---
fs/namei.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index a2b5dbe432d6..a94ff3d58b51 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -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) {
@@ -1712,10 +1709,13 @@ 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);
}
--
2.11.0