[PATCH] arm64: vmlinux.lds: make __rela_offset and __dynsym_offset ABSOLUTE

Will Deacon
2016-07-29T10:57:47Z

On Fri, Jul 29, 2016 at 12:52:12PM +0200, Ard Biesheuvel wrote:
> On 29 July 2016 at 12:49, Will Deacon <will.deacon@arm.com> wrote:
> > On Fri, Jul 29, 2016 at 12:46:49PM +0200, Ard Biesheuvel wrote:
> >> On 29 July 2016 at 11:30, Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:
> >> > On 29 July 2016 at 11:06, Will Deacon <will.deacon@arm.com> wrote:
> >> >> Hi Ard,
> >> >>
> >> >> On Thu, Jul 28, 2016 at 04:15:14PM +0200, Ard Biesheuvel wrote:
> >> >>> Due to the untyped KIMAGE_VADDR constant, the linker may not notice
> >> >>> that the __rela_offset and __dynsym_offset expressions are absolute
> >> >>> values (i.e., are not subject to relocation). This does not matter for
> >> >>> KASLR, but it does confuse kallsyms in relative mode, since it uses
> >> >>> the lowest non-absolute symbol address as the anchor point, and expects
> >> >>> all other symbol addresses to be within 4 GB of it.
> >> >>>
> >> >>> Fix this by qualifying these expressions as ABSOLUTE() explicitly.
> >> >>>
> >> >>> Fixes: 0cd3defe0af4 ("arm64: kernel: perform relocation processing from ID map")
> >> >>> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> >> >>> ---
> >> >>> arch/arm64/kernel/vmlinux.lds.S | 4 ++--
> >> >>> 1 file changed, 2 insertions(+), 2 deletions(-)
> >> >>>
> >> >>> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
> >> >>> index 89d6e177ecbd..d49492179967 100644
> >> >>> --- a/arch/arm64/kernel/vmlinux.lds.S
> >> >>> +++ b/arch/arm64/kernel/vmlinux.lds.S
> >> >>> @@ -184,9 +184,9 @@ SECTIONS
> >> >>> *(.hash)
> >> >>> }
> >> >>>
> >> >>> - __rela_offset = ADDR(.rela) - KIMAGE_VADDR;
> >> >>> + __rela_offset = ABSOLUTE(ADDR(.rela) - KIMAGE_VADDR);
> >> >>> __rela_size = SIZEOF(.rela);
> >> >>> - __dynsym_offset = ADDR(.dynsym) - KIMAGE_VADDR;
> >> >>> + __dynsym_offset = ABSOLUTE(ADDR(.dynsym) - KIMAGE_VADDR);
> >> >>
> >> >> You removed the __dynsym_offset entry entirely in "arm64: relocatable:
> >> >> suppress R_AARCH64_ABS64 relocations in vmlinux", so I'll drop this part
> >> >> of the patch and assume you'll do a separate backport for stable. Ok?
> >> >>
> >> >
> >> > If you are ok with fixing up patches manually while applying, could we
> >> > just reorder them instead?
> >>
> >> Actually, this should go into v4.8-late not -stable
> >
> > Then the fixes tag is pretty confusing, since that's a commit from v4.7.
> >
>
> Sorry, my bad. I managed to confuse myself here.
> The tag is accurate, I was wrong. This should go into v4.7 as well.

Haha, ok! So if this is v4.7 material, what *should* the fixes tag be?

> Note that the breakage I experienced was only with an LD built from
> binutils-gdb HEAD (2.28), so I guess there is no urgency here.

Sure, but people might upgrade their binutils before their kernel Image,
so the backport is worthwhile.

Will