LINUX: Avoid symlink-y resolution limits

Implementing the d_automount or follow_link function pointers for our
directories means that we can hit symlink resolution limits during
lookup, since we look like a "symlink". We can hit these limits pretty
easily if there are just too many directories in the lookup path.

Our pseudo-symlink directories cannot contribute to an infinite
resolution loop, since our destination is always an actual directory,
not a symlink that will result in more redirection. So, decrement the
total_link_count counter when our d_automount or follow_link code is
reached, so we do not contribute to hitting the max resolution limit.

Note that this is not related to recursive symlink lookup (link_count)
but only to the iterative symlink limit (total_link_count). Our
lookups are not recursive here, and we are not causing more recursive
lookups like a normal text-based symlink would do.

Reviewed-on: http://gerrit.openafs.org/8009
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 238b88624a8fef39557d397cc336c88bd8efc5b1)

Change-Id: Iec6e3fac25ab86ef5f10f359344908398bef8828
Reviewed-on: http://gerrit.openafs.org/8194
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>

diff --git a/src/afs/LINUX/osi_vnodeops.c b/src/afs/LINUX/osi_vnodeops.c
index b1ce5df49d4927d706db921a96ae37403113b2ed..10289083b65c5616aaaf41724529fc40e5b77568 100644 (file)
--- a/src/afs/LINUX/osi_vnodeops.c
+++ b/src/afs/LINUX/osi_vnodeops.c
@@ -1150,6 +1150,10 @@ afs_dentry_automount(struct path *path)
 {
     struct dentry *target;
 
+    /* avoid symlink resolution limits when resolving; we cannot contribute to
+     * an infinite symlink loop */
+    current->total_link_count--;
+
     target = canonical_dentry(path->dentry->d_inode);
 
     if (target == path->dentry) {
@@ -2614,6 +2618,17 @@ afs_linux_dir_follow_link(struct dentry *dentry, struct nameidata *nd)
     struct dentry **dpp;
     struct dentry *target;
 
+    if (current->total_link_count > 0) {
+       /* avoid symlink resolution limits when resolving; we cannot contribute to
+        * an infinite symlink loop */
+       /* only do this for follow_link when total_link_count is positive to be
+        * on the safe side; there is at least one code path in the Linux
+        * kernel where it seems like it may be possible to get here without
+        * total_link_count getting incremented. it is not clear on how that
+        * path is actually reached, but guard against it just to be safe */
+       current->total_link_count--;
+    }
+
     target = canonical_dentry(dentry->d_inode);
 
 # ifdef STRUCT_NAMEIDATA_HAS_PATH