[PATCH] kbuild: Always link with '-z norelro'

[Nick Desaulniers]

On Wed, Nov 18, 2020 at 3:07 PM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> On Thu, 19 Nov 2020 at 00:05, Nick Desaulniers <ndesaulniers at google.com> wrote:
> >
> > > > > > > To avoid playing whack-a-mole with different architectures over time,
> > > > > > > hoist '-z norelro' into the main Makefile. This does not affect ld.bfd
> > > > > > > because '-z norelro' is the default for it.
> >
> > Fangrui pointed out off list that this might need an ld-option wrapper
> > for older versions of GNU binutils.  Dan was showing me some build
> > logs today, and I thought I spotted such warnings about `-z norelro
> > will be ignored`.
>
> Does ld-option catch options that cause warnings but no errors?

$ ld.bfd -z foo /dev/null
ld.bfd: warning: -z foo ignored
ld.bfd: warning: cannot find entry symbol _start; not setting start address
➜ echo $?
0

Probably not. Can be a version check then (yuck); next is to find when
ld.bfd supported `-z norelro`.

commit 8c37241be3b1 in binutils looks like it.
Date:   Tue May 11 17:08:38 2004 +0000

which looks like either
2004-05-17 19:46:23 +0000  (tag: binutils-2_15)
or
2005-05-02 22:04:18 +0000  (tag: binutils-2_16)

So I think that would be fine then, since the kernel only supports 2.23+.

Though maybe it's
commit 5fd104addfddb68844fb8df67be832ee98ad9888
    Emit a warning when -z relro is unsupported

    ld silently accepts -z relro and -z norelro for targets that lack the
    necessary GNU_RELRO support.  This patch makes those targets emit a
    warning instead, and adds testsuite infrastructure to detect when
    relro is unsupported.

So maybe then alpha and xtensa are getting new warnings (IIUC).  If
that's the case, then we might not be able to set `-z norelro`
globally, and instead have to play whack a mole per architecture.
-- 
Thanks,
~Nick Desaulniers