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

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Message-ID: <20200624161643.73x6navnwryckuit@google.com>
Date:   Wed, 24 Jun 2020 09:16:43 -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 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.

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.)

>> order[1].)
>>
>> ld.lld: warning: <internal>:(.symtab) is being placed in '.symtab'
>> ld.lld: warning: <internal>:(.shstrtab) is being placed in '.shstrtab'
>> ld.lld: warning: <internal>:(.strtab) is being placed in '.strtab'
>>
>> [1] https://lore.kernel.org/lkml/20200622224928.o2a7jkq33guxfci4@google.com/
>>
>> Reported-by: Fangrui Song <maskray@...gle.com>
>> Reviewed-by: Fangrui Song <maskray@...gle.com>
>> Signed-off-by: Kees Cook <keescook@...omium.org>
>> ---
>>  include/asm-generic/vmlinux.lds.h | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
>> index 1248a206be8d..8e71757f485b 100644
>> --- a/include/asm-generic/vmlinux.lds.h
>> +++ b/include/asm-generic/vmlinux.lds.h
>> @@ -792,7 +792,10 @@
>>  		.stab.exclstr 0 : { *(.stab.exclstr) }			\
>>  		.stab.index 0 : { *(.stab.index) }			\
>>  		.stab.indexstr 0 : { *(.stab.indexstr) }		\
>> -		.comment 0 : { *(.comment) }
>> +		.comment 0 : { *(.comment) }				\
>> +		.symtab 0 : { *(.symtab) }				\
>> +		.strtab 0 : { *(.strtab) }				\
>> +		.shstrtab 0 : { *(.shstrtab) }
>>
>>  #ifdef CONFIG_GENERIC_BUG
>>  #define BUG_TABLE							\
>> --
>> 2.25.1
>>