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Date:   Thu, 23 Nov 2023 16:27:09 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Ignat Korchagin <ignat@cloudflare.com>
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Subject: Re: Potential config regression after 89cde455 ("kexec: consolidate
 kexec and crash options into kernel/Kconfig.kexec")
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On 11/22/23 at 09:47am, Ignat Korchagin wrote:
> On Wed, Nov 22, 2023 at 9:34 AM Baoquan He <bhe@redhat.com> wrote:
> >
> > On 11/21/23 at 09:43am, Ignat Korchagin wrote:
> > > 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.
> >
> > Thanks for the information.
> >
> > I haven't run the reproducer of issue reported on Eric's old patchset,
> > while checkout to kernel 6.1, only s390 selected KEXEC for CRASH_DUMP
> > already. And with the ARM building breakage, the simplest idea is
> > to select KEXEC only for ARM or S390 CRASH_DUMP. I plan to try the
> > reproducer later. If you have any idea or draft patch, please feel free
> > to post.
> 
> The thing is - before 6.6 even ARM did not require KEXEC for
> CRASH_DUMP (at least to successfully compile), so I think we should
> understand what changed first before adding a dependency for ARM. I'll
> try to investigate more, if I have time.

I did a cross compiling of arm on x86_64, it clearly requires KEXEC for
CRASH_DUMP if the select of KEXEC for CRASH_DUMP is removed and
CONFIG_KEXEC=n is set.

1) building error, only copy the first one:
==========================================
In file included from ../include/linux/ima.h:13,
                 from ../security/keys/key.c:16:
../include/linux/kexec.h:38:2: error: #error KEXEC_SOURCE_MEMORY_LIMIT not defined
   38 | #error KEXEC_SOURCE_MEMORY_LIMIT not defined
      |  ^~~~~
../include/linux/kexec.h:42:2: error: #error KEXEC_DESTINATION_MEMORY_LIMIT not defined
   42 | #error KEXEC_DESTINATION_MEMORY_LIMIT not defined
      |  ^~~~~
../include/linux/kexec.h:46:2: error: #error KEXEC_CONTROL_MEMORY_LIMIT not defined
   46 | #error KEXEC_CONTROL_MEMORY_LIMIT not defined
      |  ^~~~~
../include/linux/kexec.h:54:2: error: #error KEXEC_CONTROL_PAGE_SIZE not defined
   54 | #error KEXEC_CONTROL_PAGE_SIZE not defined
      |  ^~~~~
../include/linux/kexec.h:58:2: error: #error KEXEC_ARCH not defined
   58 | #error KEXEC_ARCH not defined
      |  ^~~~~
In file included from ../drivers/misc/pvpanic/pvpanic.c:13:

2) Kconfig items I enabled:
====
CONFIG_CRASH_CORE=y
CONFIG_KEXEC_CORE=y
# CONFIG_KEXEC is not set
CONFIG_CRASH_DUMP=y


3) KEXEC select is dropped so as not to enable KEXEC automatically:
=====
diff --git a/kernel/Kconfig.kexec b/kernel/Kconfig.kexec
index 7aff28ded2f4..1cc3b1c595d7 100644
--- a/kernel/Kconfig.kexec
+++ b/kernel/Kconfig.kexec
@@ -97,7 +97,6 @@ config CRASH_DUMP
        depends on ARCH_SUPPORTS_KEXEC
        select CRASH_CORE
        select KEXEC_CORE
-       select KEXEC
        help
          Generate crash dump after being started by kexec.
          This should be normally only set in special crash dump kernels
> 
> > diff --git a/kernel/Kconfig.kexec b/kernel/Kconfig.kexec
> > index 7aff28ded2f4..382dcd8d7a9d 100644
> > --- a/kernel/Kconfig.kexec
> > +++ b/kernel/Kconfig.kexec
> > @@ -97,7 +97,7 @@ config CRASH_DUMP
> >         depends on ARCH_SUPPORTS_KEXEC
> >         select CRASH_CORE
> >         select KEXEC_CORE
> > -       select KEXEC
> > +       select KEXEC if (ARM || S390)
> >
> >
> > arch/s390/Kconfig in kernel 6.1:
> > config CRASH_DUMP
> >         bool "kernel crash dumps"
> >         select KEXEC
> >         help
> >           Generate crash dump after being started by kexec.
> >           Crash dump kernels are loaded in the main kernel with kexec-tools
> >           into a specially reserved region and then later executed after
> >           a crash by kdump/kexec.
> >           Refer to <file:Documentation/s390/zfcpdump.rst> for more details on this.
> >           This option also enables s390 zfcpdump.
> >           See also <file:Documentation/s390/zfcpdump.rst>
> >
> > >
> > > > > 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
> > >
> >
>