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

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Message-ID: <87y1er6hl4.fsf@mail.lhotse>
Date:   Tue, 21 Nov 2023 12:05:59 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Ignat Korchagin <ignat@...udflare.com>, eric.devolder@...cle.com
Cc:     linux@...linux.org.uk, catalin.marinas@....com, will@...nel.org,
        chenhuacai@...nel.org, geert@...ux-m68k.org,
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Subject: Re: Potential config regression after 89cde455 ("kexec: consolidate
 kexec and crash options into kernel/Kconfig.kexec")

Ignat Korchagin <ignat@...udflare.com> writes:
> 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 don't actually see any reason for CRASH_DUMP to depend on KEXEC or
KEXEC_FILE.

None of the old CRASH_DUMP symbols depended on KEXEC AFAICS. Using
something like:

 $ git diff 89cde455..95d1fef5 | grep -A 3 "^-.*config CRASH_DUMP"

It's reasonable to want to build a kernel that supports CRASH_DUMP (ie.
can be a dump kernel), but doesn't support kexec and requires a regular
reboot. Though I doubt anyone does that in practice?

cheers