next/master bisection: baseline.login on rk3288-rock2-square

[Ard Biesheuvel]

On Thu, 4 Feb 2021 at 22:31, Guillaume Tucker
<guillaume.tucker at collabora.com> wrote:
>
> On 04/02/2021 18:23, Nick Desaulniers wrote:
> > On Thu, Feb 4, 2021 at 10:12 AM Nathan Chancellor <nathan at kernel.org> wrote:
> >>
> >> On Thu, Feb 04, 2021 at 10:06:08AM -0800, 'Nick Desaulniers' via Clang Built Linux wrote:
> >>> On Thu, Feb 4, 2021 at 8:02 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> >>>>
> >>>> On Thu, 4 Feb 2021 at 16:53, Guillaume Tucker
> >>>> <guillaume.tucker at collabora.com> wrote:
> >>>>>
> >>>>> On 04/02/2021 15:42, Ard Biesheuvel wrote:
> >>>>>> On Thu, 4 Feb 2021 at 12:32, Guillaume Tucker
> >>>>>> <guillaume.tucker at collabora.com> wrote:
> >>>>>>>
> >>>>>>> Essentially:
> >>>>>>>
> >>>>>>>   make -j18 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- LLVM=1 CC="ccache clang" zImage
> >>>
> >>> This command should link with BFD (and assemble with GAS; it's only
> >>> using clang as the compiler.
> >>
> >> I think you missed the 'LLVM=1' before CC="ccache clang". That should
> >> use all of the LLVM utilities minus the integrated assembler while
> >> wrapping clang with ccache.
> >
> > You're right, I missed `LLVM=1`. Adding `LD=ld.bfd` I think should
> > permit fallback to BFD.
>
> That was close, except we're cross-compiling with GCC for arm.
> So I've now built a plain next-20210203 (without Ard's fix) using
> this command line:
>
>     make LD=arm-linux-gnueabihf-ld.bfd -j18 ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- LLVM=1 CC="ccache clang" zImage
>
> I'm using a modified Docker image gtucker/kernelci-build-clang-11
> with the very latest LLVM 11 and gcc-8-arm-linux-gnueabihf
> packages added to be able to use the GNU linker.  BTW I guess we
> should enable this kind of hybrid build setup on kernelci.org as
> well.
>
> Full build log + kernel binaries can be found here:
>
>     https://storage.staging.kernelci.org/gtucker/next-20210203-ard-fix/v5.10-rc4-24722-g58b6c0e507b7-gtucker_single-staging-41/arm/multi_v7_defconfig/clang-11/
>
> And this booted fine, which confirms it's really down to how
> ld.lld puts together the kernel image.  Does it actually solve
> the debate whether this is an issue to fix in the assembly code
> or at link time?
>
> Full test job details for the record:
>
>     https://lava.collabora.co.uk/scheduler/job/3176004
>


So the issue appears to be in the way the linker generates the
_kernel_bss_size symbol, which obviously has an impact, given that the
queued fix takes it into account in the cache_clean operation.

On GNU ld, I see

   479: 00065e14     0 NOTYPE  GLOBAL DEFAULT  ABS _kernel_bss_size

whereas n LLVM ld.lld, I see

   433: c1c86e98     0 NOTYPE  GLOBAL DEFAULT  ABS _kernel_bss_size

and adding this value may cause the cache clean to operate on unmapped
addresses, or cause the addition to wrap and not perform a cache clean
at all.

AFAICT, this also breaks the appended DTB case in LLVM, so this needs
a separate fix in any case.