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

Ard Biesheuvel
2016-07-29T11:01:55Z

On 29 July 2016 at 12:57, Will Deacon <will.deacon@arm.com> wrote:
> 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?
>

As I said, the fixes tag is accurate. So it should still go into
4.8-late but with a cc:stable. The [tweaked] "suppress R_AARCH64_ABS64
relocations in vmlinux" could simply go on top, with no backport (it
is just an optimization, not a bugfix)

>> 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.
>

That was my reasoning as well, but I was just highlighting that
nothing is broken currently with released toolchains.