linux-kernel - [RFC 2/3] file: allow callers to free the old file descriptor after dup2

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Date:   Thu,  2 Mar 2023 18:22:06 +0000
From:   aloktiagi <aloktiagi@...il.com>
To:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     keescook@...omium.org, hch@...radead.org,
        aloktiagi <aloktiagi@...il.com>,
        Tycho Andersen <tycho@...ho.pizza>
Subject: [RFC 2/3] file: allow callers to free the old file descriptor after dup2

Allow callers of do_dup2 to free the old file descriptor in case they need to
make additional operations on it.

Signed-off-by: aloktiagi <aloktiagi@...il.com>
---
 fs/file.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index 4b2346b8a5ee..04c8775d337a 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;
@@ -1120,7 +1120,7 @@ __releases(&files->file_lock)
 	spin_unlock(&files->file_lock);
 
 	if (tofree)
-		filp_close(tofree, files);
+		*fdfile = tofree;
 
 	return fd;
 
@@ -1132,6 +1132,7 @@ __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 +1145,10 @@ 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);
@@ -1216,6 +1220,7 @@ static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 {
 	int err = -EBADF;
 	struct file *file;
+	struct file *fdfile = NULL;
 	struct files_struct *files = current->files;
 
 	if ((flags & ~O_CLOEXEC) != 0)
@@ -1237,7 +1242,10 @@ static int ksys_dup3(unsigned int oldfd, unsigned int newfd, int flags)
 			goto Ebadf;
 		goto out_unlock;
 	}
-	return do_dup2(files, file, newfd, flags);
+	err = do_dup2(files, file, newfd, &fdfile, flags);
+	if (fdfile)
+		filp_close(fdfile, files);
+	return err;
 
 Ebadf:
 	err = -EBADF;
-- 
2.34.1