Potential config regression after 89cde455 ("kexec: consolidate kexec and crash options into kernel/Kconfig.kexec")

[Ignat Korchagin]

On Tue, Nov 21, 2023 at 7:53 AM Ignat Korchagin <ignat at cloudflare.com> wrote:
>
> On Tue, Nov 21, 2023 at 1:50 AM Baoquan He <bhe at redhat.com> wrote:
> >
> > Eric DeVolder's Oracle mail address is not available anymore, add his
> > current mail address he told me.
>
> Thank you!
>
> > On 11/20/23 at 10:52pm, Ignat Korchagin wrote:
> > > Good day!
> > >
> > > We have recently started to evaluate Linux 6.6 and noticed that we
> > > cannot disable CONFIG_KEXEC anymore, but keep CONFIG_CRASH_DUMP
> > > enabled. It seems to be related to commit 89cde455 ("kexec:
> > > consolidate kexec and crash options into kernel/Kconfig.kexec"), where
> > > a CONFIG_KEXEC dependency was added to CONFIG_CRASH_DUMP.
> > >
> > > In our current kernel (Linux 6.1) we only enable CONFIG_KEXEC_FILE
> > > with enforced signature check to support the kernel crash dumping
> > > functionality and would like to keep CONFIG_KEXEC disabled for
> > > security reasons [1].
> > >
> > > I was reading the long commit message, but the reason for adding
> > > CONFIG_KEXEC as a dependency for CONFIG_CRASH_DUMP evaded me. And I
> > > believe from the implementation perspective CONFIG_KEXEC_FILE should
> > > suffice here (as we successfully used it for crashdumps on Linux 6.1).
> > >
> > > Is there a reason for adding this dependency or is it just an
> > > oversight? Would some solution of requiring either CONFIG_KEXEC or
> > > CONFIG_KEXEC_FILE work here?
> >
> > I searched the patch history, found Eric didn't add the dependency on
> > CONFIG_KEXEC at the beginning. Later a linux-next building failure with
> > randconfig was reported, in there CONFIG_CRASH_DUMP enabled, while
> > CONFIG_KEXEC is disabled. Finally Eric added the KEXEC dependency for
> > CRASH_DUMP. Please see below link for more details:
> >
> > https://lore.kernel.org/all/3e8eecd1-a277-2cfb-690e-5de2eb7b988e@oracle.com/T/#u
>
> Thank you for digging this up. However I'm still confused, because
> this is exactly how we configure Linux 6.1 (although we do have
> CONFIG_KEXEC_FILE enabled) and we don't have any problems. I believe
> we did not investigate this issue properly.

I did some preliminary investigation for this. If I patch out the
dependency on CONFIG_KEXEC the kernel builds just fine for x86
(without CONFIG_CRASH_HOTPLUG - which is probably another issue) - so
this was the previous behaviour. I can see that the reported error is
for arm architecture and was able to reproduce it with a simple cross
compiler in Debian. However, I think it is still somehow related to
this patchset as the previous kernels (up to 6.5) build fine with just
CONFIG_CRASH_DUMP and without CONFIG_KEXEC for arm as well. So even
for arm it was introduced in 6.6.

> > And besides, the newly added CONFIG_CRASH_HOTPLUG also needs
> > CONFIG_KEXEC if the elfcorehdr is allowed to be manipulated when
> > cpu/memory hotplug hapened.
>
> This still feels like a regression to me: any crash dump support
> should be independent of KEXEC syscalls being present. While probably
> the common case (including us) that the crashing kernel and recovery
> kernel are the same, they don't have to be. We need kexec syscall in
> the crashing kernel, but crashdump support in the recovery kernel (but
> the recovery kernel not having the kexec syscalls should be totally
> fine). If we do require some code definitions from kexec - at most we
> should put them under CONFIG_KEXEC_CORE.
>
> > Thanks
> > Baoquan
> >

Ignat