Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.33 as permitted sender) client-ip=23.128.96.33;
Date:   Wed, 22 Nov 2023 17:34:33 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Ignat Korchagin <ignat@cloudflare.com>
Cc:     eric_devolder@yahoo.com, linux@armlinux.org.uk,
        catalin.marinas@arm.com, will@kernel.org, chenhuacai@kernel.org,
        geert@linux-m68k.org, tsbogend@alpha.franken.de,
        James Bottomley <James.Bottomley@hansenpartnership.com>,
        deller@gmx.de, ysato@users.sourceforge.jp, dalias@libc.org,
        glaubitz@physik.fu-berlin.de, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        dave.hansen@linux.intel.com, x86@kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org,
        loongarch@lists.linux.dev, linux-m68k@lists.linux-m68k.org,
        linux-mips@vger.kernel.org, linux-parisc@vger.kernel.org,
        linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
        linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
        kernel@xen0n.name, mpe@ellerman.id.au, npiggin@gmail.com,
        christophe.leroy@csgroup.eu, paul.walmsley@sifive.com,
        palmer@dabbelt.com, aou@eecs.berkeley.edu, hca@linux.ibm.com,
        gor@linux.ibm.com, agordeev@linux.ibm.com,
        borntraeger@linux.ibm.com, svens@linux.ibm.com, hpa@zytor.com,
        keescook@chromium.org, paulmck@kernel.org,
        Peter Zijlstra <peterz@infradead.org>, frederic@kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Ard Biesheuvel <ardb@kernel.org>, samitolvanen@google.com,
        juerg.haefliger@canonical.com, arnd@arndb.de,
        rmk+kernel@armlinux.org.uk, linus.walleij@linaro.org,
        sebastian.reichel@collabora.com, rppt@kernel.org,
        kirill.shutemov@linux.intel.com, anshuman.khandual@arm.com,
        ziy@nvidia.com, masahiroy@kernel.org, ndesaulniers@google.com,
        mhiramat@kernel.org, ojeda@kernel.org, thunder.leizhen@huawei.com,
        xin3.li@intel.com, tj@kernel.org,
        Greg KH <gregkh@linuxfoundation.org>, tsi@tuyoix.net,
        hbathini@linux.ibm.com, sourabhjain@linux.ibm.com,
        boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
        kernel-team <kernel-team@cloudflare.com>
Subject: Re: Potential config regression after 89cde455 ("kexec: consolidate
 kexec and crash options into kernel/Kconfig.kexec")
Message-ID: <ZV3LKVOokpx2WvKp@MiWiFi-R3L-srv>
References: <CALrw=nHpRQQaQTP_jZfREgrQEMpS8jBF8JQCv4ygqXycE-StaA@mail.gmail.com>
 <ZVwMzXxWkgonIAfc@MiWiFi-R3L-srv>
 <CALrw=nG8xsYw7XKyL_VMHtKiaBcQCKvC8UVp-C9-BdeN4A1Daw@mail.gmail.com>
 <CALrw=nH-vcROja2W23rUKEEZMZhxsQiNB4P_ZZQ-XhPHAJGxrg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <CALrw=nH-vcROja2W23rUKEEZMZhxsQiNB4P_ZZQ-XhPHAJGxrg@mail.gmail.com>
Precedence: bulk

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.

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
>