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

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Stephen Hemminger <stephen@networkplumber.org>
Date: Tue, 20 May 2025 12:28:38 -0700
> 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?

The coredump only works with the pathname socket, so ideally
the prefix should be '/', but it's same with the direct-file
coredump.  We can distinguish the socket by S_ISSOCK() though.