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Date:   Thu, 3 Dec 2020 12:27:37 -0800
From:   Fangrui Song <maskray@google.com>
To:     Nick Desaulniers <ndesaulniers@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>, dwmw@amazon.co.uk,
        Peter Zijlstra <peterz@infradead.org>,
        Kees Cook <keescook@chromium.org>,
        Sam Ravnborg <sam@ravnborg.org>,
        Jakub Jelinek <jakub@redhat.com>
Subject: Re: [PATCH] firmware_loader: Align .builtin_fw to 8
Message-ID: <20201203202737.7c4wrifqafszyd5y@google.com>
References: <20201203170529.1029105-1-maskray@google.com>
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On 2020-12-03, Nick Desaulniers wrote:
>On Thu, Dec 3, 2020 at 9:05 AM 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.
>>
>> 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>
>> ---
>>  include/asm-generic/vmlinux.lds.h | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>> index b2b3d81b1535..3cd4bd1193ab 100644
>> --- a/include/asm-generic/vmlinux.lds.h
>> +++ b/include/asm-generic/vmlinux.lds.h
>> @@ -459,6 +459,7 @@
>>         }                                                               \
>>                                                                         \
>>         /* Built-in firmware blobs */                                   \
>> +       ALIGN_FUNCTION();                                               \
>
>Thanks for the patch!
>
>I'm going to repeat my question from the above link
>(https://github.com/ClangBuiltLinux/linux/issues/1204#issuecomment-737610582)
>just in case it's not naive:
>
>ALIGN_FUNCTION() C preprocessor macro seems to be used to realign
>code, while STRUCT_ALIGN() seems to be used to realign data.  It looks
>to me like only data is put into .builtin_fw.  If these relocations
>require an alignment of 8, than multiples of 8 should also be fine
>(STRUCT_ALIGN in 32 for all toolchain version, except gcc 4.9 which is
>64; both are multiples of 8 though).  It looks like only structs are
>placed in .builtin_fw; ie. data.  In that case, I worry that using
>ALIGN_FUNCTION/8 might actually be under-aligning data in this
>section.

Regarding STRUCT_ALIGN (32 for GCC>4.9) in
include/asm-generic/vmlinux.lds.h, it is probably not suitable for
.builtin_fw

* Its comment is a bit unclear. It probably should mention that the
   32-byte overalignment is only for global structure variables which are
   at least 32 byte large. But this is just my observation. Adding a GCC
   maintainer to comment on this.
* Even if GCC does overalign defined global struct variables, it is unlikely
   that GCC will leverage this property for undefined `extern struct
   builtin_fw __start_builtin_fw[]` (drivers/base/firmware_loader/main.c)

To make .builtin_fw aligned, I agree that ALIGN_FUNCTION() is probably a
misuse. Maybe I should just use `. = ALIGN(8)` if the kernel linker
script prefers `. = ALIGN(8)` to an output section alignment
(https://sourceware.org/binutils/docs/ld/Output-Section-Description.html#Output-Section-Description
https://lld.llvm.org/ELF/linker_script.html#output-section-alignment)

>Though, in https://github.com/ClangBuiltLinux/linux/issues/1204#issuecomment-737625134
>you're comment:
>
>>> In GNU ld, the empty .builtin_fw is removed
>
>So that's a difference in behavior between ld.bfd and ld.lld, which is
>fine, but it makes me wonder whether we should instead or additionally
>be discarding this section explicitly via linker script when
>CONFIG_FW_LOADER is not set?

Short answer: No, we should not discard .builtin_fw

   .builtin_fw        : AT(ADDR(.builtin_fw) - LOAD_OFFSET) {
   __start_builtin_fw = .; ... }

In LLD, either a section reference (`ADDR(.builtin_fw)`) or a
non-PROVIDE symbol assignment __start_builtin_fw makes the section non-discardable.

It can be argued that discarding an output section with a symbol
assignment (GNU ld) is strange because the symbol (st_shndx) will be
defined relative to an arbitrary unrelated section. Retaining the
section can avoid some other issues.

>>         .builtin_fw        : AT(ADDR(.builtin_fw) - LOAD_OFFSET) {      \
>>                 __start_builtin_fw = .;                                 \
>>                 KEEP(*(.builtin_fw))                                    \
>> --
>> 2.29.2.576.ga3fc446d84-goog
>>
>
>
>-- 
>Thanks,
>~Nick Desaulniers