[RFC v4 1/2] file: allow callers to free the old file descriptor after dup2

[aloktiagi <aloktiagi@gmail.com>]

Allow callers of do_dup2 to get a reference to the file pointer being dup'ed
over. The callers can then replace the file with the new file in the eventpoll
interface or the file table before freeing it.

Signed-off-by: aloktiagi <aloktiagi@gmail.com>
---
Changes in v4:
  - Address review comment for a cleaner if else block in do_dup2() to free the
    file pointer.

Changes in v2:
  - Make the commit message more clearer.
  - Address review comment to make the interface cleaner so that the caller cannot
    forget to initialize the fdfile.
---
 fs/file.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 4b2346b8a5ee..cbc258504a64 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1086,7 +1086,7 @@ bool get_close_on_exec(unsigned int fd)
 }
 
 static int do_dup2(struct files_struct *files,
-	struct file *file, unsigned fd, unsigned flags)
+	struct file *file, unsigned fd, struct file **fdfile, unsigned flags)
 __releases(&files->file_lock)
 {
 	struct file *tofree;
@@ -1119,7 +1119,9 @@ __releases(&files->file_lock)
 		__clear_close_on_exec(fd, fdt);
 	spin_unlock(&files->file_lock);
 
-	if (tofree)
+	if (fdfile)
+		*fdfile = tofree;
+	else if (tofree)
 		filp_close(tofree, files);
 
 	return fd;
@@ -1132,6 +1134,7 @@ __releases(&files->file_lock)
 int replace_fd(unsigned fd, struct file *file, unsigned flags)
 {
 	int err;
+	struct file *fdfile = NULL;
 	struct files_struct *files = current->files;
 
 	if (!file)
@@ -1144,7 +1147,10 @@ int replace_fd(unsigned fd, struct file *file, unsigned flags)
 	err = expand_files(files, fd);
 	if (unlikely(err < 0))
 		goto out_unlock;
-	return do_dup2(files, file, fd, flags);
+	err = do_dup2(files, file, fd, &fdfile, flags);
+	if (fdfile)
+		filp_close(fdfile, files);
+	return err;
 
 out_unlock:
 	spin_unlock(&files->file_lock);
@@ -1237,7 +1243,7 @@ static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 			goto Ebadf;
 		goto out_unlock;
 	}
-	return do_dup2(files, file, newfd, flags);
+	return do_dup2(files, file, newfd, NULL, flags);
 
 Ebadf:
 	err = -EBADF;
-- 
2.34.1