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

[Lorenzo Pieralisi]

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