coredump: hand a pidfd to the usermode coredump helper

Give userspace a way to instruct the kernel to install a pidfd into the
usermode helper process. This makes coredump handling a lot more
reliable for userspace. In parallel with this commit we already have
systemd adding support for this in [1].

We create a pidfs file for the coredumping process when we process the
corename pattern. When the usermode helper process is forked we then
install the pidfs file as file descriptor three into the usermode
helpers file descriptor table so it's available to the exec'd program.

Since usermode helpers are either children of the system_unbound_wq
workqueue or kthreadd we know that the file descriptor table is empty
and can thus always use three as the file descriptor number.

Note, that we'll install a pidfd for the thread-group leader even if a
subthread is calling do_coredump(). We know that task linkage hasn't
been removed due to delay_group_leader() and even if this @current isn't
the actual thread-group leader we know that the thread-group leader
cannot be reaped until @current has exited.

Link: https://github.com/systemd/systemd/pull/37125 [1]
Link: https://lore.kernel.org/20250414-work-coredump-v2-3-685bf231f828@kernel.org
Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/coredump.h b/include/linux/coredump.h
index 77e6e195d1d6..76e41805b92d 100644
--- a/include/linux/coredump.h
+++ b/include/linux/coredump.h
@@ -28,6 +28,7 @@ struct coredump_params {
 	int vma_count;
 	size_t vma_data_size;
 	struct core_vma_metadata *vma_meta;
+	struct pid *pid;
 };
 
 extern unsigned int core_file_note_size_limit;