linux-kernel - Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime relocations

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Date:   Tue, 30 Jun 2020 19:27:59 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Arvind Sankar <nivedita@...m.mit.edu>,
        Ard Biesheuvel <ardb@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Dmitry Golovin <dima@...ovin.in>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Daniel Kiper <daniel.kiper@...cle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "H . J . Lu" <hjl@...rceware.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 7/7] x86/boot: Check that there are no runtime
 relocations

On Tue, Jun 30, 2020 at 03:00:43PM -0700, Fangrui Song wrote:
> * Ard Biesheuvel
> > On Tue, 30 Jun 2020 at 01:34, Fangrui Song <maskray@...gle.com> wrote:
> 
> If the executable is purely static, it does not need to have PLT. All
> calls to a PLT can be redirected to the function itself.  Some range
> extension thunks (other terms: stub groups, veneers, etc) may still be
> needed if the distance is too large.
> 
> There are cases where a GOT cannot be avoided, e.g.
> 
> extern char foo[] __attribute__((weak, visibility("hidden")));
> char *fun() { return foo; }
> 
> If foo is a SHN_ABS, `movq foo@...PCREL(%rip), %rax` can't be optimized
> by GOTPCRELX (https://sourceware.org/bugzilla/show_bug.cgi?id=25749 binutils>=2.35 will be good)
> Many other architectures don't even have a GOT optimization.

Urk -- the example given in that bug report isn't even weak. Are you
guys proposing to pessimize every access to a global symbol, regardless
of visibility, by going through the GOT on the off chance that somebody
might define one of them as SHN_ABS? Can we at least gate it behind
something like __attribute__((might_be_shn_abs))?

> 
> >I don't think it's really relevant for the kernel build -- all we get is
> >ld -static --no-dynamic-linker, all -static does is prevent searching
> >shared libraries, and we already pass --no-dynamic-linker if it's
> >supported.
> >
> >[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81498