Re: Direct dumped kernel cores

On Thu, Oct 31, 2024 at 4:24 PM Justin Hibbits <jhibbits@freebsd.org> wrote:

> Hi everyone,
>
> At Juniper we've been using a so-called 'rescue' kernel for dumping
> vmcores directly to the filesystem after a panic.  We're now
> contributing this feature, implemented by Klara Systems, to FreeBSD, and
> looking for feedback. I posted a review
> at https://reviews.freebsd.org/D47358 for anyone interested.
>
> Interesting bits to keep in mind:
> * It requires a 2-stage build process, one to build the rescue kernel,
>   the other to build the main kernel, which embeds the rescue kernel
>   inside its image.  This might need some further work.
> * Thus far it's been implemented for amd64 and arm64, once proven out,
>   other architectures (powerpc64/le, riscv64) can follow suit.
> * Kernel environment bits to pass down to the rescue kernel are
>   prefixed `debug.rescue.`, for instance
>   `debug.rescue.vfs.root.mountfrom`.
>

First off, this is kinda cool. I've wanted this occasionally when my swap
partition is too small (though in my case, it was easy enough to add another
drive to the system that was panicking and dump to that).

I do have a question: I'm curious why you didn't follow the Linux lead of
having
a kexec_load(2) system call to load the 'rescue kernel' to make this more
generic.
That would make the leap to having full kexec support (eg reboot(CMD_KEXEC)
a lot easier to implement.

Warner


> There are many more details in the review summary.
>
> We'd love to get feedback from anyone interested.
>
> Thanks,
> Justin Hibbits
>
>