trace_event names with more than NAME_MAX chars

[Beau Belgrave <beaub@linux.microsoft.com>]

While developing some unrelated features I happened to create a
trace_event that was more than NAME_MAX (255) characters. When this
happened the creation worked, but tracefs would hang any task that tried
to list the directory of the trace_event or remove it.

I followed the code down to the reason being eventfs would call
simple_lookup(), and if it failed, it would still try to create the
dentry. In this case DCACHE_PAR_LOOKUP would get set and never cleared.
This caused d_wait_lookup() to loop forever, since that flag is used in
d_in_lookup().

Both tracefs and eventfs use simple_lookup() and it fails for
dentries that exceed NAME_MAX. Should we even allow trace_events to
be created that exceed this limit? Or should tracefs/eventfs allow
this but somehow represent these differently?

I have a fix that appears to work for myself, but unsure if there are
other locations (attached at the end of this mail).

Thanks,
-Beau

diff --git a/fs/tracefs/event_inode.c b/fs/tracefs/event_inode.c
index f8a594a50ae6..d2c06ba26db4 100644
--- a/fs/tracefs/event_inode.c
+++ b/fs/tracefs/event_inode.c
@@ -561,6 +561,8 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
                if (strcmp(ei_child->name, name) != 0)
                        continue;
                ret = simple_lookup(dir, dentry, flags);
+               if (IS_ERR(ret))
+                       goto out;
                create_dir_dentry(ei, ei_child, ei_dentry, true);
                created = true;
                break;
@@ -583,6 +585,8 @@ static struct dentry *eventfs_root_lookup(struct inode *dir,
                        if (r <= 0)
                                continue;
                        ret = simple_lookup(dir, dentry, flags);
+                       if (IS_ERR(ret))
+                               goto out;
                        create_file_dentry(ei, i, ei_dentry, name, mode, cdata,
                                           fops, true);
                        break;