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From:   Ignat Korchagin <ignat@cloudflare.com>
Date:   Thu, 23 Nov 2023 08:23:34 +0000
Message-ID: <CALrw=nEO3A2MPeSXhO3=cPdrAFB07sQcnjNN5V5jj2YcqAZ+bQ@mail.gmail.com>
Subject: Re: [PATCH 1/3] kernel/Kconfig.kexec: drop select of KEXEC for CRASH_DUMP
To:     Baoquan He <bhe@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kexec@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org,
        akpm@linux-foundation.org, eric_devolder@yahoo.com,
        kernel-team <kernel-team@cloudflare.com>
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On Thu, Nov 23, 2023 at 7:37=E2=80=AFAM Baoquan He <bhe@redhat.com> wrote:
>
> Ignat Korchagin complained that a potential config regression was
> introduced by commit 89cde455915f ("kexec: consolidate kexec and
> crash options into kernel/Kconfig.kexec"). Before the commit,
> CONFIG_CRASH_DUMP has no dependency on CONFIG_KEXEC. After the commit,
> CRASH_DUMP selects KEXEC. That enforces system to have CONFIG_KEXEC=3Dy
> as long as CONFIG_CRASH_DUMP=3DY which people may not want.
>
> In Ignat's case, he sets CONFIG_CRASH_DUMP=3Dy, CONFIG_KEXEC_FILE=3Dy and
> CONFIG_KEXEC=3Dn because kexec_load interface could have security issue i=
f
> kernel/initrd has no chance to be signed and verified.
>
> CRASH_DUMP has select of KEXEC because Eric, author of above commit,
> met a LKP report of build failure when posting patch of earlier version.
> Please see below link to get detail of the LKP report:
>
>     https://lore.kernel.org/all/3e8eecd1-a277-2cfb-690e-5de2eb7b988e@orac=
le.com/T/#u
>
> In fact, that LKP report is triggered because arm's <asm/kexec.h> is
> wrapped in CONFIG_KEXEC ifdeffery scope. That is wrong. CONFIG_KEXEC
> controls the enabling/disabling of kexec_load interface, but not kexec
> feature. Removing the wrongly added CONFIG_KEXEC ifdeffery scope in
> <asm/kexec.h> of arm allows us to drop the select KEXEC for CRASH_DUMP.

Hm... With the patch, when cross compiling for arm and
CONFIG_KEXEC_CORE=3Dy
# CONFIG_KEXEC is not set
CONFIG_CRASH_DUMP=3Dy

I get the following linker error at the end:

  CALL    scripts/checksyscalls.sh
  UPD     include/generated/utsversion.h
  CC      init/version-timestamp.o
  LD      .tmp_vmlinux.kallsyms1
arm-linux-gnueabi-ld: kernel/kexec_core.o: in function `kimage_free':
kexec_core.c:(.text+0xf5c): undefined reference to `machine_kexec_cleanup'
arm-linux-gnueabi-ld: kernel/kexec_core.o: in function `__crash_kexec':
kexec_core.c:(.text+0x15bc): undefined reference to `machine_crash_shutdown=
'
arm-linux-gnueabi-ld: kexec_core.c:(.text+0x15c4): undefined reference
to `machine_kexec'
arm-linux-gnueabi-ld: kernel/kexec_core.o: in function `kernel_kexec':
kexec_core.c:(.text+0x1a04): undefined reference to `machine_kexec'
make[2]: *** [scripts/Makefile.vmlinux:37: vmlinux] Error 1
make[1]: *** [/home/ignat/git/linux-upstream/Makefile:1154: vmlinux] Error =
2
make: *** [Makefile:234: __sub-make] Error 2

> Signed-off-by: Baoquan He <bhe@redhat.com>
> ---
>  arch/arm/include/asm/kexec.h | 4 ----
>  kernel/Kconfig.kexec         | 1 -
>  2 files changed, 5 deletions(-)
>
> diff --git a/arch/arm/include/asm/kexec.h b/arch/arm/include/asm/kexec.h
> index e62832dcba76..a8287e7ab9d4 100644
> --- a/arch/arm/include/asm/kexec.h
> +++ b/arch/arm/include/asm/kexec.h
> @@ -2,8 +2,6 @@
>  #ifndef _ARM_KEXEC_H
>  #define _ARM_KEXEC_H
>
> -#ifdef CONFIG_KEXEC
> -
>  /* Maximum physical address we can use pages from */
>  #define KEXEC_SOURCE_MEMORY_LIMIT (-1UL)
>  /* Maximum address we can reach in physical address mode */
> @@ -82,6 +80,4 @@ static inline struct page *boot_pfn_to_page(unsigned lo=
ng boot_pfn)
>
>  #endif /* __ASSEMBLY__ */
>
> -#endif /* CONFIG_KEXEC */
> -
>  #endif /* _ARM_KEXEC_H */
> 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
> --
> 2.41.0
>