Re: riscv+KASAN does not boot

Dmitry Vyukov
2021-02-16T11:26:47Z

On Tue, Feb 16, 2021 at 12:17 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Fri, Jan 29, 2021 at 9:11 AM Dmitry Vyukov <dvyukov@google.com> wrote:
> > > I was fixing KASAN support for my sv48 patchset so I took a look at your
> > > issue: I built a kernel on top of the branch riscv/fixes using
> > > https://github.com/google/syzkaller/blob/269d24e857a757d09a898086a2fa6fa5d827c3e1/dashboard/config/linux/upstream-riscv64-kasan.config
> > > and Buildroot 2020.11. I have the warnings regarding the use of
> > > __virt_to_phys on wrong addresses (but that's normal since this function
> > > is used in virt_addr_valid) but not the segfaults you describe.
> >
> > Hi Alex,
> >
> > Let me try to rebuild buildroot image. Maybe there was something wrong
> > with my build, though, I did 'make clean' before doing. But at the
> > same time it worked back in June...
> >
> > Re WARNINGs, they indicate kernel bugs. I am working on setting up a
> > syzbot instance on riscv. If there a WARNING during boot then the
> > kernel will be marked as broken. No further testing will happen.
> > Is it a mis-use of WARN_ON? If so, could anybody please remove it or
> > replace it with pr_err.
>
>
> Hi,
>
> I've localized one issue with riscv/KASAN:
> KASAN breaks VDSO and that's I think the root cause of weird faults I
> saw earlier. The following patch fixes it.
> Could somebody please upstream this fix? I don't know how to add/run
> tests for this.
> Thanks
>
> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> index 0cfd6da784f84..cf3a383c1799d 100644
> --- a/arch/riscv/kernel/vdso/Makefile
> +++ b/arch/riscv/kernel/vdso/Makefile
> @@ -35,6 +35,7 @@ CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_FTRACE) -Os
> # Disable gcov profiling for VDSO code
> GCOV_PROFILE := n
> KCOV_INSTRUMENT := n
> +KASAN_SANITIZE := n
>
> # Force dependency
> $(obj)/vdso.o: $(obj)/vdso.so

Second issue I am seeing seems to be related to text segment size.
I check out v5.11 and use this config:
https://gist.github.com/dvyukov/6af25474d455437577a84213b0cc9178

Then trying to boot it using:
QEMU emulator version 5.2.0 (Debian 1:5.2+dfsg-3)
$ qemu-system-riscv64 -machine virt -smp 2 -m 4G ...

It shows no output from the kernel whatsoever, even though I have
earlycon and output shows very early with other configs.
Kernel boots fine with defconfig and other smaller configs.

If I enable KASAN_OUTLINE and CC_OPTIMIZE_FOR_SIZE, then this config
also boots fine. Both of these options significantly reduce kernel
size. However, I can also boot the kernel without these 2 configs, if
I disable a whole lot of subsystem configs. This makes me think that
there is an issue related to kernel size somewhere in
qemu/bootloader/kernel bootstrap code.
Does it make sense to you? Can somebody reproduce what I am seeing?

Thanks