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

[Nick Desaulniers]

On Thu, Nov 12, 2020 at 10:06 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Fri, 13 Nov 2020 at 01:53, Nathan Chancellor
> <natechancellor@xxxxxxxxx> wrote:
> >
> > On Thu, Nov 12, 2020 at 04:44:46PM -0800, Nick Desaulniers wrote:
> > > On Thu, Nov 12, 2020 at 10:41 AM Nathan Chancellor
> > > <natechancellor@xxxxxxxxx> wrote:
> > > >
> > > > Commit 3bbd3db86470 ("arm64: relocatable: fix inconsistencies in linker
> > > > script and options") added '-z norelro' to the arm64 Makefile when
> > > > CONFIG_RELOCATABLE was set to help support ld.lld because ld.lld
> > > > defaults to '-z relro' but the kernel does not use program headers or
> > > > adhere to the section layout that is required for RELRO to work.
> > > >
> > > > Commit 3b92fa7485eb ("arm64: link with -z norelro regardless of
> > > > CONFIG_RELOCATABLE") unconditionally added it to LDFLAGS_vmlinux because
> > > > an error occurs with CONFIG_KASAN set even when CONFIG_RELOCATABLE is
> > > > unset.
> > > >
> > > > As it turns out, ARM experiences the same error after CONFIG_KASAN was
> > > > implemented, meaning that '-z norelro' needs to be added to that
> > > > Makefile as well (multi_v7_defconfig + CONFIG_KASAN=y + LD=ld.lld):
> > > >
> > > > $ make ARCH=arm CROSS_COMPILE=arm-linux-gnueabi- LLVM=1 zImage
> > > > ld.lld: error: section: .exit.data is not contiguous with other relro sections
> > > >
> > > > 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.
> > > >
> > > > Link: https://github.com/ClangBuiltLinux/linux/issues/1189
> > > > Suggested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > > > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > >
> > > Why not add it additionally to KBUILD_LDFLAGS_MODULE a la
> > > `--build-id=sha1` a few lines above? (or `LDFLAGS_MODULE`, but that
> > > looks unused?)  We probably don't want this for modules either.  In
> > > that case, you could add -z norelo to the two existing lines with
> > > `--build-id=sha1` above?
> >
> > Yes, I can do that. I will send a v2 along tomorrow morning to let
> > others comment.
> >
>
> Modules are partially linked objects, so there is no point in passing
> -z options for them.

Ok then.

Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Tested-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

More work to be done to boot this config, but at least we can link
without error.
-- 
Thanks,
~Nick Desaulniers