linux-kernel - Re: [PATCH v3 2/9] vmlinux.lds.h: Add .symtab, .strtab, and .shstrtab to STABS

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Message-ID: <20200624172620.654hhjetiyzpgoxw@google.com>
Date:   Wed, 24 Jun 2020 10:26:20 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>, Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>, x86@...nel.org,
        clang-built-linux@...glegroups.com, linux-arch@...r.kernel.org,
        linux-efi@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/9] vmlinux.lds.h: Add .symtab, .strtab, and
 .shstrtab to STABS_DEBUG


On 2020-06-24, Arvind Sankar wrote:
>On Wed, Jun 24, 2020 at 09:16:43AM -0700, Fangrui Song wrote:
>>
>> On 2020-06-24, Arvind Sankar wrote:
>> >On Tue, Jun 23, 2020 at 06:49:33PM -0700, Kees Cook wrote:
>> >> When linking vmlinux with LLD, the synthetic sections .symtab, .strtab,
>> >> and .shstrtab are listed as orphaned. Add them to the STABS_DEBUG section
>> >> so there will be no warnings when --orphan-handling=warn is used more
>> >> widely. (They are added above comment as it is the more common
>> >
>> >Nit 1: is "after .comment" better than "above comment"? It's above in the
>> >sense of higher file offset, but it's below in readelf output.
>>
>> I mean this order:)
>>
>>    .comment
>>    .symtab
>>    .shstrtab
>>    .strtab
>>
>> This is the case in the absence of a linker script if at least one object file has .comment (mostly for GCC/clang version information) or the linker is LLD which adds a .comment
>>
>> >Nit 2: These aren't actually debugging sections, no? Is it better to add
>> >a new macro for it, and is there any plan to stop LLD from warning about
>> >them?
>>
>> https://reviews.llvm.org/D75149 "[ELF] --orphan-handling=: don't warn/error for unused synthesized sections"
>> described that .symtab .shstrtab .strtab are different in GNU ld.
>> Since many other GNU ld synthesized sections (.rela.dyn .plt ...) can be renamed or dropped
>> via output section descriptions, I don't understand why the 3 sections
>> can't be customized.
>
>So IIUC, lld will now warn about .rela.dyn etc only if they're non-empty?

HEAD and future 11.0.0 will not warn about unused synthesized sections
like .rela.dyn

For most synthesized sections, empty = unused.

>>
>> I created a feature request: https://sourceware.org/bugzilla/show_bug.cgi?id=26168
>> (If this is supported, it is a consistent behavior to warn for orphan
>> .symtab/.strtab/.shstrtab
>>
>> There may be 50% chance that the maintainer decides that "LLD diverges"
>> I would disagree: there is no fundamental problems with .symtab/.strtab/.shstrtab which make them special in output section descriptions or orphan handling.)
>>
>
>.shstrtab is a little special in that it can't be discarded if the ELF
>file contains any sections at all. But yeah, there's no reason they
>can't be renamed or placed in a custom location in the file.

https://sourceware.org/pipermail/binutils/2020-March/000179.html
proposes -z nosectionheader. With this option, I believe .shstrtab is
not needed. /DISCARD/ : { *(.shstrtab) }  should achieve a similar effect.