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

[Fangrui Song]

On 2020-12-03, Nick Desaulniers wrote:
> On Thu, Dec 3, 2020 at 9:05 AM Fangrui Song <maskray@xxxxxxxxxx> 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@xxxxxxxxx>
> > Signed-off-by: Fangrui Song <maskray@xxxxxxxxxx>
> > ---
> >  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