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    SubjectRe: arm64/acpi: NULL dereference reports from UBSAN at boot
    On Wed, May 27, 2020 at 02:41:04PM +0100, Lorenzo Pieralisi wrote:
    > 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).

    Cheers, Lorenzo.

    > Now on what to do with it - thoughts welcome.

    Nick -- any idea what to do about the above? The '#define' pasted by
    Lorenzo is causing a couple of spurious UBSAN splats when compiling with
    clang 11.

    Cheers,

    Will

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