linux-kernel - Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections from bzImage

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Date:   Mon, 24 Feb 2020 12:51:14 -0800
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Michael Matz <matz@...e.de>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Fangrui Song <maskray@...gle.com>,
        Borislav Petkov <bp@...en8.de>,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 2/2] x86/boot/compressed: Remove unnecessary sections from bzImage

On Mon, Feb 24, 2020 at 5:28 AM Michael Matz <matz@...e.de> wrote:
>
> Hello,
>
> On Sat, 22 Feb 2020, Nick Desaulniers wrote:
>
> > > > > In GNU ld, it seems that .shstrtab .symtab and .strtab are special
> > > > > cased. Neither the input section description *(.shstrtab) nor *(*)
> > > > > discards .shstrtab . I feel that this is a weird case (probably even a bug)
> > > > > that lld should not implement.
> > > >
> > > > Ok, forget what the tools do for a second: why is .shstrtab special and
> > > > why would one want to keep it?
> > > >
> > > > Because one still wants to know what the section names of an object are
> > > > or other tools need it or why?
> > > >
> > > > Thx.
> > > >
> > > > --
> > > > Regards/Gruss,
> > > >     Boris.
> > > >
> > > > https://people.kernel.org/tglx/notes-about-netiquette
> > >
> > > .shstrtab is required by the ELF specification. The e_shstrndx field in
> > > the ELF header is the index of .shstrtab, and each section in the
> > > section table is required to have an sh_name that points into the
> > > .shstrtab.
> >
> > Yeah, I can see it both ways.  That `*` doesn't glob all remaining
> > sections is surprising to me, but bfd seems to be "extra helpful" in
> > not discarding sections that are required via ELF spec.
>
> In a way the /DISCARD/ assignment should be thought of as applying to
> _input_ sections (as all such section references on the RHS), not
> necessarily to output sections.  What this then means for sections that
> are synthesized by the link editor is less clear.  Some of them are
> generated regardless (as you noted, e.g. the symbol table and associated
> string sections, including section name string table), some of them are
> suppressed, and either lead to an followup error (e.g. with .gnu.hash), or
> to invalid output (e.g. missing .dynsym for executables simply lead to
> segfaults when running them).
>
> That's the reason for the perceived inconsistency with behaviour on '*':
> it's application to synthesized sections.  Arguably bfd should be fixed to
> also not discard the other essential sections (or alternatively to give an
> error when an essential section is discarded).  The lld behaviour of e.g.
> discarding .shstrtab (or other synthesized sections necessary for valid
> ELF output) doesn't make much sense either, though.

Hi Michael, thank you for the precise feedback.  Do you have a list of
"synthesized sections necessary for valid ELF output?" Also, could you
point me to the documentation about `*` and its relation to
"synthesized sections necessary for valid ELF output?"  This will help
me file a precise bug against LLD.
-- 
Thanks,
~Nick Desaulniers