vfs: make d_path() get the root path under RCU

This avoids the spinlocks and refcounts in the d_path() sequence too
(used by /proc and various other entities).  See commit 8b19e34188a3 for
the equivalent getcwd() system call path.

And unlike getcwd(), d_path() doesn't copy the result to user space, so
I don't need to fear _that_ particular bug happening again.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 2 deletions

diff --git a/fs/dcache.c b/fs/dcache.c
index 91e551b5af59..dddc67fed732 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -2869,6 +2869,16 @@ static int prepend_unreachable(char **buffer, int *buflen)
         return prepend(buffer, buflen, "(unreachable)", 13);
 }
 
+static void get_fs_root_rcu(struct fs_struct *fs, struct path *root)
+{
+        unsigned seq;
+
+        do {
+                seq = read_seqcount_begin(&fs->seq);
+                *root = fs->root;
+        } while (read_seqcount_retry(&fs->seq, seq));
+}
+
 /**
  * d_path - return the path of a dentry
  * @path: path to report
@@ -2901,13 +2911,15 @@ char *d_path(const struct path *path, char *buf, int buflen)
         if (path->dentry->d_op && path->dentry->d_op->d_dname)
                 return path->dentry->d_op->d_dname(path->dentry, buf, buflen);
 
-        get_fs_root(current->fs, &root);
+        rcu_read_lock();
+        get_fs_root_rcu(current->fs, &root);
         br_read_lock(&vfsmount_lock);
         error = path_with_deleted(path, &root, &res, &buflen);
         br_read_unlock(&vfsmount_lock);
+        rcu_read_unlock();
+
         if (error < 0)
                 res = ERR_PTR(error);
-        path_put(&root);
         return res;
 }
 EXPORT_SYMBOL(d_path);