fork: release the files of a failed fork after sched_cancel_fork()

Reorder copy_process() cleanup so sched_fork() taking scx_fork_rwsem
scx_pre_fork() is safe and isn't held around exiting files. Right now, a
fork that fails while another thread closed the bpf link fd will
deadlock against its own read side. It also blocks every fork on the
system:

    copy_process()                  holds scx_fork_rwsem for read
      exit_files()
        close_files()
          filp_close_sync()
            bpf_scx_unreg()
              kthread_flush_work()  waits for the disable work
                scx_root_disable()
                  percpu_down_write(&scx_fork_rwsem)

Simply release the child's files after sched_cancel_fork() dropped the
lock. The task starts out as a copy of its parent so p->files points to
the parent's table until copy_files() replaces it. exit_files() must not
run for a fork that failed before that. Clear p->files up front so
exit_files() is a no-op for those and make copy_files() set it
explicitly for CLONE_FILES.

Reported-by: Chris Mason <mason@kernel.org>
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-7-8e4adb1619e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

0 files changed, 0 insertions, 0 deletions