Re: [PATCH v7 0/9] coredump: add coredump socket

Christian Brauner
2025-05-15T09:17:10Z

On Thu, May 15, 2025 at 12:03:33AM +0200, Christian Brauner wrote:
> Coredumping currently supports two modes:
>
> (1) Dumping directly into a file somewhere on the filesystem.
> (2) Dumping into a pipe connected to a usermode helper process
> spawned as a child of the system_unbound_wq or kthreadd.
>
> For simplicity I'm mostly ignoring (1). There's probably still some
> users of (1) out there but processing coredumps in this way can be
> considered adventurous especially in the face of set*id binaries.
>
> The most common option should be (2) by now. It works by allowing
> userspace to put a string into /proc/sys/kernel/core_pattern like:
>
> |/usr/lib/systemd/systemd-coredump %P %u %g %s %t %c %h
>
> The "|" at the beginning indicates to the kernel that a pipe must be
> used. The path following the pipe indicator is a path to a binary that
> will be spawned as a usermode helper process. Any additional parameters
> pass information about the task that is generating the coredump to the
> binary that processes the coredump.
>
> In the example core_pattern shown above systemd-coredump is spawned as a
> usermode helper. There's various conceptual consequences of this
> (non-exhaustive list):
>
> - systemd-coredump is spawned with file descriptor number 0 (stdin)
> connected to the read-end of the pipe. All other file descriptors are
> closed. That specifically includes 1 (stdout) and 2 (stderr). This has
> already caused bugs because userspace assumed that this cannot happen
> (Whether or not this is a sane assumption is irrelevant.).
>
> - systemd-coredump will be spawned as a child of system_unbound_wq. So
> it is not a child of any userspace process and specifically not a
> child of PID 1. It cannot be waited upon and is in a weird hybrid
> upcall which are difficult for userspace to control correctly.
>
> - systemd-coredump is spawned with full kernel privileges. This
> necessitates all kinds of weird privilege dropping excercises in
> userspace to make this safe.
>
> - A new usermode helper has to be spawned for each crashing process.
>
> This series adds a new mode:
>
> (3) Dumping into an abstract AF_UNIX socket.

s/abstract//
Forgot to remove that. Fixed in-tree.