1 files changed, 56 insertions, 12 deletions

diff --git a/fs/file.c b/fs/file.c
index 8627dacfc424..97d212a9b814 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -841,24 +841,68 @@ void do_close_on_exec(struct files_struct *files)
 	spin_unlock(&files->file_lock);
 }
 
+static inline struct file *__fget_files_rcu(struct files_struct *files,
+	unsigned int fd, fmode_t mask, unsigned int refs)
+{
+	for (;;) {
+		struct file *file;
+		struct fdtable *fdt = rcu_dereference_raw(files->fdt);
+		struct file __rcu **fdentry;
+
+		if (unlikely(fd >= fdt->max_fds))
+			return NULL;
+
+		fdentry = fdt->fd + array_index_nospec(fd, fdt->max_fds);
+		file = rcu_dereference_raw(*fdentry);
+		if (unlikely(!file))
+			return NULL;
+
+		if (unlikely(file->f_mode & mask))
+			return NULL;
+
+		/*
+		 * Ok, we have a file pointer. However, because we do
+		 * this all locklessly under RCU, we may be racing with
+		 * that file being closed.
+		 *
+		 * Such a race can take two forms:
+		 *
+		 *  (a) the file ref already went down to zero,
+		 *      and get_file_rcu_many() fails. Just try
+		 *      again:
+		 */
+		if (unlikely(!get_file_rcu_many(file, refs)))
+			continue;
+
+		/*
+		 *  (b) the file table entry has changed under us.
+		 *       Note that we don't need to re-check the 'fdt->fd'
+		 *       pointer having changed, because it always goes
+		 *       hand-in-hand with 'fdt'.
+		 *
+		 * If so, we need to put our refs and try again.
+		 */
+		if (unlikely(rcu_dereference_raw(files->fdt) != fdt) ||
+		    unlikely(rcu_dereference_raw(*fdentry) != file)) {
+			fput_many(file, refs);
+			continue;
+		}
+
+		/*
+		 * Ok, we have a ref to the file, and checked that it
+		 * still exists.
+		 */
+		return file;
+	}
+}
+
 static struct file *__fget_files(struct files_struct *files, unsigned int fd,
 				 fmode_t mask, unsigned int refs)
 {
 	struct file *file;
 
 	rcu_read_lock();
-loop:
-	file = files_lookup_fd_rcu(files, fd);
-	if (file) {
-		/* File object ref couldn't be taken.
-		 * dup2() atomicity guarantee is the reason
-		 * we loop to catch the new file (or NULL pointer)
-		 */
-		if (file->f_mode & mask)
-			file = NULL;
-		else if (!get_file_rcu_many(file, refs))
-			goto loop;
-	}
+	file = __fget_files_rcu(files, fd, mask, refs);
 	rcu_read_unlock();
 
 	return file;