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SubjectRe: arm64/acpi: NULL dereference reports from UBSAN at boot
On Tue, May 26, 2020 at 09:21:57PM +0100, Will Deacon wrote:
> Hi Lorenzo, Hanjun, [+Nick]
>
> On Thu, May 21, 2020 at 06:37:38PM +0100, Lorenzo Pieralisi wrote:
> > On Thu, May 21, 2020 at 11:09:53AM +0100, Will Deacon wrote:
> > > Hi folks,
> > >
> > > I just tried booting the arm64 for-kernelci branch under QEMU (version
> > > 4.2.50 (v4.2.0-779-g4354edb6dcc7)) with UBSAN enabled, and I see a
> > > couple of NULL pointer dereferences reported at boot. I think they're
> > > both GIC related (log below). I don't see a panic with UBSAN disabled,
> > > so something's fishy here.
> >
> > May I ask you the QEMU command line please - just to make sure I can
> > replicate it.
>
> As it turns out, I'm only able to reproduce this when building with Clang,
> but I don't know whether that's because GCC is missing something of Clang
> is signalling a false positive. You also don't need all of those whacky
> fuzzing options enabled.
>
> Anyway, to reproduce:
>
> $ git checkout for-next/kernelci
> $ make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu- defconfig
> <then do a menuconfig and enable UBSAN>
> $ make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu- Image
>
> I throw that at QEMU using:
>
> qemu-system-aarch64 -M virt -machine virtualization=true \
> -machine virt,gic-version=3 \
> -cpu max,sve=off -smp 2 -m 4096 \
> -drive if=pflash,format=raw,file=efi.img,readonly \
> -drive if=pflash,format=raw,file=varstore.img \
> -drive if=virtio,format=raw,file=disk.img \
> -device virtio-scsi-pci,id=scsi0 \
> -device virtio-rng-pci \
> -device virtio-net-pci,netdev=net0 \
> -netdev user,id=net0,hostfwd=tcp::8222-:22 \
> -nographic \
> -kernel ~/work/linux/arch/arm64/boot/Image \
> -append "earlycon root=/dev/vda2"
>
> I built QEMU a while ago according to:
>
> https://mirrors.edge.kernel.org/pub/linux/kernel/people/will/docs/qemu/qemu-arm64-howto.html
>
> and its version 4.2.50 (v4.2.0-779-g4354edb6dcc7).
>
> My clang is version 11.0.1.

Thanks a lot Will.

I *think* I was right - it is the ACPI_OFFSET() macro:

#define ACPI_OFFSET(d, f) ACPI_PTR_DIFF (&(((d *) 0)->f), (void *) 0)

that triggers the warnings (I suspected it because at least in one of
the warnings I could not see any dereference of any dynamically
allocated data).

Now on what to do with it - thoughts welcome.

Lorenzo

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