pidfs: ensure that PIDFS_INFO_EXIT is available

When we currently create a pidfd we check that the task hasn't been reaped right before we create the pidfd. But it is of course possible that by the time we return the pidfd to userspace the task has already been reaped since we don't check again after having created a dentry for it. This was fine until now because that race was meaningless. But now that we provide PIDFD_INFO_EXIT it is a problem because it is possible that the kernel returns a reaped pidfd and it depends on the race whether PIDFD_INFO_EXIT information is available. This depends on if the task gets reaped before or after a dentry has been attached to struct pid. Make this consistent and only returned pidfds for reaped tasks if PIDFD_INFO_EXIT information is available. This is done by performing another check whether the task has been reaped right after we attached a dentry to struct pid. Since pidfs_exit() is called before struct pid's task linkage is removed the case where the task got reaped but a dentry was already attached to struct pid and exit information was recorded and published can be handled correctly. In that case we do return a pidfd for a reaped task like we would've before.

CBChristian Brauner committed 1 year ago