Re: riscv+KASAN does not boot

Alex Ghiti
2021-02-18T09:20:16Z

Hi Dmitry,

> On Wed, Feb 17, 2021 at 5:36 PM Alex Ghiti <alex@ghiti.fr> wrote:
>>
>> Le 2/16/21 à 11:42 PM, Dmitry Vyukov a écrit :
>>> On Tue, Feb 16, 2021 at 9:42 PM Alex Ghiti <alex@ghiti.fr> wrote:
>>>>
>>>> Hi Dmitry,
>>>>
>>>> Le 2/16/21 à 6:25 AM, Dmitry Vyukov a écrit :
>>>>> 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
>>>>
>>>> What's weird is that I don't have any issue without this patch with the
>>>> following config whereas it indeed seems required for KASAN. But when
>>>> looking at the segfaults you got earlier, the segfault address is 0xbb0
>>>> and the cause is an instruction page fault: this address is the PLT base
>>>> address in vdso.so and an instruction page fault would mean that someone
>>>> tried to jump at this address, which is weird. At first sight, that does
>>>> not seem related to your patch above, but clearly I may be wrong.
>>>>
>>>> Tobias, did you observe the same segfaults as Dmitry ?
>>>
>>>
>>> I noticed that not all buildroot images use VDSO, it seems to be
>>> dependent on libc settings (at least I think I changed it in the
>>> past).
>>
>> Ok, I used uClibc but then when using glibc, I have the same segfaults,
>> only when KASAN is enabled. And your patch fixes the problem. I will try
>> to take a look later to better understand the problem.
>>
>>> I also booted an image completely successfully including dhcpd/sshd
>>> start, but then my executable crashed in clock_gettime. The executable
>>> was build on linux/amd64 host with "riscv64-linux-gnu-gcc -static"
>>> (10.2.1).
>>>
>>>
>>>>> 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
>>>>
>>>> This config gave my laptop a hard time ! Finally I was able to boot
>>>> correctly to userspace, but I realized I used my sv48 branch...Either I
>>>> fixed your issue along the way or I can't reproduce it, I'll give it a
>>>> try tomorrow.
>>>
>>> Where is your branch? I could also test in my setup on your branch.
>>>
>>
>> You can find my branch int/alex/riscv_kernel_end_of_address_space_v2
>> here: https://github.com/AlexGhiti/riscv-linux.git
>
> No, it does not work for me.
>
> Source is on b61ab6c98de021398cd7734ea5fc3655e51e70f2 (HEAD,
> int/alex/riscv_kernel_end_of_address_space_v2)
> Config is https://gist.githubusercontent.com/dvyukov/6af25474d455437577a84213b0cc9178/raw/55b116522c14a8a98a7626d76df740d54f648ce5/gistfile1.txt
>
> riscv64-linux-gnu-gcc -v
> gcc version 10.2.1 20210110 (Debian 10.2.1-6+build1)
>
> qemu-system-riscv64 --version
> QEMU emulator version 5.2.0 (Debian 1:5.2+dfsg-3)
>
> qemu-system-riscv64 \
> -machine virt -smp 2 -m 2G \
> -device virtio-blk-device,drive=hd0 \
> -drive file=image-riscv64,if=none,format=raw,id=hd0 \
> -kernel arch/riscv/boot/Image \
> -nographic \
> -device virtio-rng-device,rng=rng0 -object
> rng-random,filename=/dev/urandom,id=rng0 \
> -netdev user,id=net0,host=10.0.2.10,hostfwd=tcp::10022-:22 -device
> virtio-net-device,netdev=net0 \
> -append "root=/dev/vda earlyprintk=serial console=ttyS0 oops=panic
> panic_on_warn=1 panic=86400 earlycon"

It still works for me but I had to disable CONFIG_DEBUG_INFO_BTF (I
don't think that changes anything at runtime). But your above command
line does not work for me as it appears you do not load any firmware, if
I add -bios images/fw_jump.elf, it works. But then I don't know where
your opensbi output below comes from...

And regarding your issue with calling clock_gettime 'directly' compared
to using the syscall, I have the same consistent output from both calls.

I have an older gcc (9.3.0) and the same qemu. I think what is missing
here is your buildroot config, so that we have the exact same
environment: could you post your buildroot config as well ?

Thanks,

>
> OpenSBI v0.8
> ____ _____ ____ _____
> / __ \ / ____| _ \_ _|
> | | | |_ __ ___ _ __ | (___ | |_) || |
> | | | | '_ \ / _ \ '_ \ \___ \| _ < | |
> | |__| | |_) | __/ | | |____) | |_) || |_
> \____/| .__/ \___|_| |_|_____/|____/_____|
> | |
> |_|
>
> Platform Name : riscv-virtio,qemu
> Platform Features : timer,mfdeleg
> Platform HART Count : 2
> Boot HART ID : 1
> Boot HART ISA : rv64imafdcsu
> BOOT HART Features : pmp,scounteren,mcounteren,time
> BOOT HART PMP Count : 16
> Firmware Base : 0x80000000
> Firmware Size : 104 KB
> Runtime SBI Version : 0.2
>
> MIDELEG : 0x0000000000000222
> MEDELEG : 0x000000000000b109
> PMP0 : 0x0000000080000000-0x000000008001ffff (A)OpenSBI v0.6

>
>
> no output after this
> PMP1 : 0x0000000000000000-0xffffffffffffffff (A,R,W,X)
>
>
>
>> Thanks,
>>
>>>
>>>>> 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? >
>>>>
>>>> I did not bring any answer to your question, but at least you know I'm
>>>> working on it, I'll keep you posted.
>>>>
>>>> Thanks for taking the time to setup syzkaller.
>>>>
>>>> Alex
>>>>
>>>>> Thanks
>>>>>
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>>>>>
>>>
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