Re: [PATCH v8 0/9] coredump: add coredump socket

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 16 May 2025 13:25:27 +0200
Christian Brauner <brauner@kernel.org> 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 AF_UNIX socket.
> 
> Userspace can set /proc/sys/kernel/core_pattern to:
> 
>         @/path/to/coredump.socket
> 
> The "@" at the beginning indicates to the kernel that an AF_UNIX
> coredump socket will be used to process coredumps.
> 
> The coredump socket must be located in the initial mount namespace.
> When a task coredumps it opens a client socket in the initial network
> namespace and connects to the coredump socket.


There is a problem with using @ as naming convention.
The starting character of @ is already used to indicate abstract
unix domain sockets in some programs like ss.
And will the new coredump socekt allow use of abstrace unix
domain sockets?