Messages in this thread | | | Date | Thu, 3 Dec 2020 12:27:37 -0800 | From | Fangrui Song <> | Subject | Re: [PATCH] firmware_loader: Align .builtin_fw to 8 |
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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
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