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

[Ard Biesheuvel]

On 29 July 2016 at 12:49, Will Deacon <will.deacon at 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 at linaro.org> wrote:
>> > On 29 July 2016 at 11:06, Will Deacon <will.deacon at 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 at 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.

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.