On Thu, May 16, 2019 at 04:03:27PM +0200, Jann Horn wrote:

On Thu, May 16, 2019 at 3:08 PM Christian Brauner [off-list ref] wrote:

quoted

On Wed, May 15, 2019 at 10:45:06AM -0700, Daniel Colascione wrote:

quoted

On Wed, May 15, 2019 at 3:04 AM Christian Brauner [off-list ref] wrote:

quoted

This adds the pidfd_open() syscall. It allows a caller to retrieve pollable
pidfds for a process which did not get created via CLONE_PIDFD, i.e. for a
process that is created via traditional fork()/clone() calls that is only
referenced by a PID:

[...]

quoted

quoted

quoted

+/**
+ * pidfd_open() - Open new pid file descriptor.
+ *
+ * @pid:   pid for which to retrieve a pidfd
+ * @flags: flags to pass
+ *
+ * This creates a new pid file descriptor with the O_CLOEXEC flag set for
+ * the process identified by @pid. Currently, the process identified by
+ * @pid must be a thread-group leader. This restriction currently exists
+ * for all aspects of pidfds including pidfd creation (CLONE_PIDFD cannot
+ * be used with CLONE_THREAD) and pidfd polling (only supports thread group
+ * leaders).
+ *
+ * Return: On success, a cloexec pidfd is returned.
+ *         On error, a negative errno number will be returned.
+ */
+SYSCALL_DEFINE2(pidfd_open, pid_t, pid, unsigned int, flags)
+{

[...]

quoted

quoted

quoted

+       if (pid <= 0)
+               return -EINVAL;

WDYT of defining pid == 0 to mean "open myself"?

I'm torn. It be a nice shortcut of course but pid being 0 is usually an
indicator for child processes. So unless the getpid() before
pidfd_open() is an issue I'd say let's leave it as is. If you really
want the shortcut might -1 be better?

Joining the bikeshed painting club: Please don't allow either 0 or -1
as shortcut for "self". James Forshaw found an Android security bug a
while back (https://bugs.chromium.org/p/project-zero/issues/detail?id=727)
that passed a PID to getpidcon(), except that the PID was 0
(placeholder for oneway binder transactions), and then the service
thought it was talking to itself. You could pick some other number and
provide a #define for that, but I think pidfd_open(getpid(), ...)
makes more sense.

Yes, I agree. I left it as is for v1, i.e. no shortcut; getpid() should
do.

Christian