linux-kernel - Re: [PATCH] Kbuild: implement support for DWARF5

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Date:   Tue, 3 Nov 2020 14:27:05 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-toolchains@...r.kernel.org
Subject: Re: [PATCH] Kbuild: implement support for DWARF5

On Wed, Oct 21, 2020 at 6:44 PM Fangrui Song <maskray@...gle.com> wrote:
>
> On 2020-10-21, 'Nick Desaulniers' via Clang Built Linux wrote:
> >DWARF5 is the latest standard of the DWARF debug info format.
> >
> >Feature detection of DWARF5 is onerous, especially given that we've
> >removed $(AS), so we must query $(CC) for DWARF5 assembler directive
> >support. Further -gdwarf-X where X is an unsupported value doesn't
> >produce an error in $(CC). GNU `as` only recently gained support for
> >specifying -gdwarf-5.
> >
> >The DWARF version of a binary can be validated with:
>
> To be more correct: this is just the version number of the .debug_info section.
> Other sections can use different version numbers.
> (For example, GNU as still does not support version 5 .debug_line)

How do you recommend we validate that then?

>
> >$ llvm-dwarfdump vmlinux | head -n 5 | grep version
> >or
> >$ readelf --debug-dump=info vmlinux 2>/dev/null | grep Version
> >diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> >index cd14444bf600..0382808ef9fe 100644
> >--- a/include/asm-generic/vmlinux.lds.h
> >+++ b/include/asm-generic/vmlinux.lds.h
> >@@ -828,7 +828,11 @@
> >               .debug_types    0 : { *(.debug_types) }                 \
> >               /* DWARF 5 */                                           \
> >               .debug_macro    0 : { *(.debug_macro) }                 \
> >-              .debug_addr     0 : { *(.debug_addr) }
> >+              .debug_addr     0 : { *(.debug_addr) }                  \
> >+              .debug_line_str 0 : { *(.debug_line_str) }              \
> >+              .debug_loclists 0 : { *(.debug_loclists) }              \
> >+              .debug_rnglists 0 : { *(.debug_rnglists) }              \
> >+              .debug_str_offsets 0 : { *(.debug_str_offsets) }
>
> Consider adding .debug_names for the accelerator table.
> It is the DWARF v5 version of .debug_pub{names,types} (which are mentioned
> a few lines above).

I hadn't seen that section produced in my limited testing.  Being a
fan of TDD, I kind of would like to see the linker warn on orphan
section placement, then add it to the list, as I did with the above.
Do you have more info on when or how .debug_pub* can be produced?

Thanks for the rest of the feedback, I'll incorporate it into v2.

-- 
Thanks,
~Nick Desaulniers