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

Sargun Dhillon
2020-06-03T01:13:59Z

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