Re: arm64/acpi: NULL dereference reports from UBSAN at boot

From: Will Deacon
Date: Tue May 26 2020 - 16:22:05 EST

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:

and its version 4.2.50 (v4.2.0-779-g4354edb6dcc7).

My clang is version 11.0.1.