Re: [PATCH v2] firmware_loader: Align .builtin_fw to 8

From: Nick Desaulniers <ndesaulniers@google.com>
To: Fangrui Song <maskray@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	linux-arch <linux-arch@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	kernel test robot <lkp@intel.com>
Subject: Re: [PATCH v2] firmware_loader: Align .builtin_fw to 8
Date: Tue, 8 Dec 2020 09:59:46 -0800	[thread overview]
Message-ID: <CAKwvOdmuwOPzZZHMh58syoorc4ED5-6_tbxCqhL0Gi65mq58-A@mail.gmail.com> (raw)
In-Reply-To: <20201208054646.2913063-1-maskray@google.com>

On Mon, Dec 7, 2020 at 9:46 PM Fangrui Song <maskray@google.com> wrote:
>
> arm64 references the start address of .builtin_fw (__start_builtin_fw)
> with a pair of R_AARCH64_ADR_PREL_PG_HI21/R_AARCH64_LDST64_ABS_LO12_NC
> relocations. The compiler is allowed to emit the
> R_AARCH64_LDST64_ABS_LO12_NC relocation because struct builtin_fw in
> include/linux/firmware.h is 8-byte aligned.
>
> The R_AARCH64_LDST64_ABS_LO12_NC relocation requires the address to be a
> multiple of 8, which may not be the case if .builtin_fw is empty.
> Unconditionally align .builtin_fw to fix the linker error. 32-bit
> architectures could use ALIGN(4) but that would add unnecessary
> complexity, so just use ALIGN(8).
>
> Fixes: 5658c76 ("firmware: allow firmware files to be built into kernel image")
> Link: https://github.com/ClangBuiltLinux/linux/issues/1204
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Fangrui Song <maskray@google.com>
> Acked-by: Arnd Bergmann <arnd@arndb.de>
>
> ---
> Change in v2:
> * Use output section alignment instead of inappropriate ALIGN_FUNCTION()

Cool, this approach is what we already use for other global arrays;
such as __tracepoints_ptrs.  (I wonder why we don't use 4B alignment
for 32b...but 8 is a multiple of 4, so should be fine for 32b
targets).

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>

Build+Boot tested aarch64 and x86_64 with ld.lld and ld.bfd. Did not
test loading builtin firmware.

> ---
>  include/asm-generic/vmlinux.lds.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index b2b3d81b1535..b97c628ad91f 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -459,7 +459,7 @@
>         }                                                               \
>                                                                         \
>         /* Built-in firmware blobs */                                   \
> -       .builtin_fw        : AT(ADDR(.builtin_fw) - LOAD_OFFSET) {      \
> +       .builtin_fw : AT(ADDR(.builtin_fw) - LOAD_OFFSET) ALIGN(8) {    \
>                 __start_builtin_fw = .;                                 \
>                 KEEP(*(.builtin_fw))                                    \
>                 __end_builtin_fw = .;                                   \
> --
> 2.29.2.576.ga3fc446d84-goog
>

-- 
Thanks,
~Nick Desaulniers