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    SubjectRe: Potential config regression after 89cde455 ("kexec: consolidate kexec and crash options into kernel/Kconfig.kexec")
    On Tue, Nov 21, 2023 at 7:53 AM Ignat Korchagin <ignat@cloudflare.com> wrote:
    >
    > On Tue, Nov 21, 2023 at 1:50 AM Baoquan He <bhe@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

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