Re: [PATCH v2 0/5] coredump: allow for flexible coredump handling

[Jeff Layton <jlayton@kernel.org>]

On Tue, 2025-06-03 at 15:31 +0200, Christian Brauner wrote:
> In addition to the extensive selftests I've already written a
> (non-production ready) simple Rust coredump server for this in
> userspace:
> 
> https://github.com/brauner/dumdum.git
>
> Extend the coredump socket to allow the coredump server to tell the
> kernel how to process individual coredumps. This allows for fine-grained
> coredump management. Userspace can decide to just let the kernel write
> out the coredump, or generate the coredump itself, or just reject it.
>
> When the crashing task connects to the coredump socket the kernel will
> send a struct coredump_req to the coredump server. The kernel will set
> the size member of struct coredump_req allowing the coredump server how
> much data can be read.
> 
> The coredump server uses MSG_PEEK to peek the size of struct
> coredump_req. If the kernel uses a newer struct coredump_req the
> coredump server just reads the size it knows and discard any remaining
> bytes in the buffer. If the kernel uses an older struct coredump_req
> the coredump server just reads the size the kernel knows.
> 
> The returned struct coredump_req will inform the coredump server what
> features the kernel supports. The coredump_req->mask member is set to
> the currently know features.
> 
> The coredump server may only use features whose bits were raised by the
> kernel in coredump_req->mask.
> 
> In response to a coredump_req from the kernel the coredump server sends
> a struct coredump_ack to the kernel. The kernel informs the coredump
> server what version of struct coredump_ack it supports by setting struct
> coredump_req->size_ack to the size it knows about. The coredump server
> may only send as many bytes as coredump_req->size_ack indicates (a
> smaller size is fine of course). The coredump server must set
> coredump_ack->size accordingly.
> 
> The coredump server sets the features it wants to use in struct
> coredump_ack->mask. Only bits returned in struct coredump_req->mask may
> be used.
> 
> In case an invalid struct coredump_ack is sent to the kernel an
> out-of-band byte will be sent by the kernel indicating the reason why
> the coredump_ack was rejected.
> 
> The out-of-band markers allow advanced userspace to infer failure. They
> are optional and can be ignored by not listening for POLLPRI events and
> aren't necessary for the coredump server to function correctly.
> 
> In the initial version the following features are supported in
> coredump_{req,ack}->mask:
> 
> * COREDUMP_KERNEL
>   The kernel will write the coredump data to the socket.
> 
> * COREDUMP_USERSPACE
>   The kernel will not write coredump data but will indicate to the
>   parent that a coredump has been generated. This is used when userspace
>   generates its own coredumps.
> 
> * COREDUMP_REJECT
>   The kernel will skip generating a coredump for this task.
> 
> * COREDUMP_WAIT
>   The kernel will prevent the task from exiting until the coredump
>   server has shutdown the socket connection.
> 

How do you envision COREDUMP_WAIT being used? I took a look at the
trivial server, but it wasn't clear to me why you'd want to block the
task from exiting.

> The flexible coredump socket can be enabled by using the "@@" prefix
> instead of the single "@" prefix for the regular coredump socket:
> 
>   @@/run/systemd/coredump.socket
> 
> will enable flexible coredump handling. Current kernels already enforce
> that "@" must be followed by "/" and will reject anything else. So
> extending this is backward and forward compatible.
> 
> Signed-off-by: Christian Brauner <brauner@kernel.org>
> ---
> Changes in v2:
> - Add epoll-based concurrent coredump handling selftests.
> - Improve cover letter.
> - Ensure that enum coredump_oob is packed aka a single byte and add a
>   static_assert() verifying that.
> - Simplify helper functions making the patch even smaller.
> - Link to v1: https://lore.kernel.org/20250530-work-coredump-socket-protocol-v1-0-20bde1cd4faa@kernel.org
> 
> ---
> Christian Brauner (5):
>       coredump: allow for flexible coredump handling
>       selftests/coredump: fix build
>       selftests/coredump: cleanup coredump tests
>       tools: add coredump.h header
>       selftests/coredump: add coredump server selftests
> 
>  fs/coredump.c                                     |  130 +-
>  include/uapi/linux/coredump.h                     |  104 ++
>  tools/include/uapi/linux/coredump.h               |  104 ++
>  tools/testing/selftests/coredump/Makefile         |    2 +-
>  tools/testing/selftests/coredump/config           |    4 +
>  tools/testing/selftests/coredump/stackdump_test.c | 1705 ++++++++++++++++++---
>  6 files changed, 1799 insertions(+), 250 deletions(-)
> ---
> base-commit: 3e406741b19890c3d8a2ed126aa7c23b106ca9e1
> change-id: 20250520-work-coredump-socket-protocol-6980d1f54c2f

-- 
Jeff Layton <jlayton@kernel.org>