[PATCH v3 2/4] pid: Use file_receive helper to copy FDs

[Sargun Dhillon <sargun@sargun.me>]

The code to copy file descriptors was duplicated in pidfd_getfd.
Rather than continue to duplicate it, this hoists the code out of
kernel/pid.c and uses the newly added file_receive helper.

Earlier, when this was implemented there was some back-and-forth
about how the semantics should work around copying around file
descriptors [1], and it was decided that the default behaviour
should be to not modify cgroup data. As a matter of least surprise,
this approach follows the default semantics as presented by SCM_RIGHTS.

In the future, a flag can be added to avoid manipulating the cgroup
data on copy.

[1]: https://lore.kernel.org/lkml/20200107175927.4558-1-sargun@sargun.me/

Signed-off-by: Sargun Dhillon <sargun@sargun.me>
Suggested-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Daniel Wagner <daniel.wagner@bmw-carit.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jann Horn <jannh@google.com>
Cc: John Fastabend <john.r.fastabend@intel.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Tycho Andersen <tycho@tycho.ws>
Cc: stable@vger.kernel.org
Cc: cgroups@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
---
 kernel/pid.c | 20 +++++++++-----------
 1 file changed, 9 insertions(+), 11 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index c835b844aca7..1642cf940aa1 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -606,7 +606,7 @@ static int pidfd_getfd(struct pid *pid, int fd)
 {
 	struct task_struct *task;
 	struct file *file;
-	int ret;
+	int ret, err;
 
 	task = get_pid_task(pid, PIDTYPE_PID);
 	if (!task)
@@ -617,18 +617,16 @@ static int pidfd_getfd(struct pid *pid, int fd)
 	if (IS_ERR(file))
 		return PTR_ERR(file);
 
-	ret = security_file_receive(file);
-	if (ret) {
-		fput(file);
-		return ret;
-	}
-
 	ret = get_unused_fd_flags(O_CLOEXEC);
-	if (ret < 0)
-		fput(file);
-	else
-		fd_install(ret, file);
+	if (ret >= 0) {
+		err = file_receive(ret, file);
+		if (err) {
+			put_unused_fd(ret);
+			ret = err;
+		}
+	}
 
+	fput(file);
 	return ret;
 }
 
-- 
2.25.1