linux-kernel - Re: [PATCH] scripts: Introduce ARCH_WANTS_RELA_DYN to avoid stripping .rela.dyn section

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Message-ID: <CAMj1kXGaUZZoKUMEJtv4Fvckub_kRcHBZbpMxet-skqOLD7SNw@mail.gmail.com>
Date: Thu, 3 Apr 2025 09:38:39 +0300
From: Ard Biesheuvel <ardb@...nel.org>
To: Alexandre Ghiti <alexghiti@...osinc.com>
Cc: Paul Walmsley <paul.walmsley@...ive.com>, Palmer Dabbelt <palmer@...belt.com>, 
	Alexandre Ghiti <alex@...ti.fr>, Björn Töpel <bjorn@...osinc.com>, 
	Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Charlie Jenkins <charlie@...osinc.com>, linux-kernel@...r.kernel.org, 
	linux-riscv@...ts.infradead.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH] scripts: Introduce ARCH_WANTS_RELA_DYN to avoid stripping
 .rela.dyn section

On Fri, 28 Mar 2025 at 08:55, Alexandre Ghiti <alexghiti@...osinc.com> wrote:
>
> riscv uses the .rela.dyn section to relocate the kernel at runtime but
> that section is stripped from vmlinux. That prevents kexec to
> successfully load vmlinux since it does not contain the relocations info
> needed.
>
> Fix this by introducing a new config ARCH_WANTS_RELA_DYN that prevents
> .rela.dyn to be stripped.
>
> Fixes: 71d815bf5dfd ("kbuild: Strip runtime const RELA sections correctly")
> Signed-off-by: Alexandre Ghiti <alexghiti@...osinc.com>

Apologies for the breakage.



> ---
>  arch/Kconfig         | 7 +++++++
>  arch/riscv/Kconfig   | 1 +
>  scripts/Makefile.lib | 4 +++-
>  3 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/arch/Kconfig b/arch/Kconfig
> index b8a4ff365582..d93f50ecb8f9 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -1544,6 +1544,13 @@ config HAVE_ARCH_PREL32_RELOCATIONS
>           architectures, and don't require runtime relocation on relocatable
>           kernels.
>
> +config ARCH_WANTS_RELA_DYN

I don't think we need this - just preserve .rela.dyn in all cases.

> +       bool
> +       depends on RELOCATABLE
> +       help
> +         This prevents the .rela.dyn section to be stripped from vmlinux since
> +         it is needed to relocate the kernel at runtime.
> +
>  config ARCH_USE_MEMREMAP_PROT
>         bool
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 7612c52e9b1e..d58053efa4e4 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -84,6 +84,7 @@ config RISCV
>         select ARCH_WANT_OPTIMIZE_DAX_VMEMMAP
>         select ARCH_WANT_OPTIMIZE_HUGETLB_VMEMMAP
>         select ARCH_WANTS_NO_INSTR
> +       select ARCH_WANTS_RELA_DYN if RELOCATABLE
>         select ARCH_WANTS_THP_SWAP if HAVE_ARCH_TRANSPARENT_HUGEPAGE
>         select ARCH_WEAK_RELEASE_ACQUIRE if ARCH_USE_QUEUED_SPINLOCKS
>         select BINFMT_FLAT_NO_DATA_START_OFFSET if !MMU
> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> index cad20f0e66ee..e59210a9fde0 100644
> --- a/scripts/Makefile.lib
> +++ b/scripts/Makefile.lib
> @@ -376,8 +376,10 @@ quiet_cmd_ar = AR      $@
>  quiet_cmd_objcopy = OBJCOPY $@
>  cmd_objcopy = $(OBJCOPY) $(OBJCOPYFLAGS) $(OBJCOPYFLAGS_$(@F)) $< $@
>
> +objcopy-args-$(CONFIG_ARCH_WANTS_RELA_DYN) += --remove-section=!.rela.dyn
> +
>  quiet_cmd_strip_relocs = RSTRIP  $@
> -cmd_strip_relocs = $(OBJCOPY) --remove-section='.rel*' $@
> +cmd_strip_relocs = $(OBJCOPY) --remove-section='.rel*' $(objcopy-args-y) $@
>
>  # Gzip
>  # ---------------------------------------------------------------------------
> --
> 2.39.2
>