Re: rcutorture’s init segfaults in ppc64le VM

[Zhouyi Zhou <zhouzhouyi@gmail.com>]

Dear Paul

On Thu, Mar 10, 2022 at 4:10 PM Paul Menzel <pmenzel@molgen.mpg.de> wrote:
>
> Dear Zhouyi,
>
>
> Thank you for still looking into this.
You are very welcome ;-)
>
>
> Am 10.03.22 um 03:37 schrieb Zhouyi Zhou:
>
> > I try to reproduce the bug in ppc64 VM in Oregon State University
> > using the vmlinux extracted from
> > https://owww.molgen.mpg.de/~pmenzel/rcutorture-2022.02.01-21.52.37-torture-locktorture-kasan-lock01.tar.xz
> >
> > the ppc64 VM in which I run the qemu without hardware acceleration is:
> > Linux version 5.4.0-100-generic (buildd@bos02-ppc64el-021) (gcc version 9.3.0 (Ubuntu 9.3.0-17ubuntu1~20.04)) #113-Ubuntu SMP Thu Feb 3 18:43:11 UTC 2022 (Ubuntu 5.4.0-100.113-generic 5.4.166)
> >
> >
> > The qemu command I use to test:
> > cd /tmp/dev/shm/linux/tools/testing/selftests/rcutorture/res/2022.02.01-21.52.37-torture/results-locktorture-kasan/LOCK01$
> > $qemu-system-ppc64   -nographic -smp cores=2,threads=1 -net none -M
> > pseries -nodefaults -device spapr-vscsi -serial file:/tmp/console.log
> > -m 512 -kernel ./vmlinux -append "debug_boot_weak_hash panic=-1
> > console=ttyS0 rcutorture.onoff_interval=200
> > rcutorture.onoff_holdoff=30 rcutree.gp_preinit_delay=12
> > rcutree.gp_init_delay=3 rcutree.gp_cleanup_delay=3
> > rcutree.kthread_prio=2 threadirqs tree.use_softirq=0
> > rcutorture.n_barrier_cbs=4 rcutorture.stat_interval=15
> > rcutorture.shutdown_secs=1800 rcutorture.test_no_idle_hz=1
> > rcutorture.verbose=1"
> >
> > The console.log is uploaded to:
> > http://154.223.142.244/logs/20220310/console.paul.log
> > The log tells us it is illegal instruction that causes the trouble:
> > [    4.246387][    T1] init[1]: illegal instruction (4) at 1002c308 nip 1002c308 lr 10001684 code 1 in init[10000000+d0000]
> > [    4.251400][    T1] init[1]: code: f90d88c0 f92a0008 f9480008 7c2004ac 2c2d0000 f9490000 386d88d0 380000e8
> > [    4.253416][    T1] init[1]: code: 41820098 e92d8f98 75290010 4182008c <44000001> 2c2d0000 60000000 8902f438
> >
> >
> > Meanwhile, the vmlinux compiled by myself runs smoothly.
>
> How did you build it? Using GCC or clang? I forgot, if the problem was
I built vmlinux(es) using GCC and clang both. The compiled vmlinux(es)
runs smoothly.
> only reproducible if the host Linux kernel was built with clang or the
> VM kernel.
Yes, I also remember this, the dependence of how the host Linux kernel
is built makes things more complex.
>
> > Then I modify mkinitrd.sh to let it panic manually:
> > http://154.223.142.244/logs/20220310/mkinitrd.sh
>
> I only see the change:
>
>      -
>      +  int *ptr = 0;
>      +  *ptr =  0;
>
Yes, I make the segfault happen manually.
> > The log tells us it is a segfault (instead of a illegal instruction):
> > http://154.223.142.244/logs/20220310/console.zhouyi.log
> >
> > Then I use gdb to debug the init in host:
> > ubuntu@zhouzhouyi-1:~/newkernel/linux-next$ gdb
> > tools/testing/selftests/rcutorture/initrd/init
> > (gdb) run
> > Starting program:
> > /home/ubuntu/newkernel/linux-next/tools/testing/selftests/rcutorture/initrd/init
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > 0x0000000010000b2c in ?? ()
> > (gdb) x/10i $pc
> > => 0x10000b2c:    stw     r9,0(r9)
> >     0x10000b30:    trap
> >     0x10000b34:    .long 0x0
> >     0x10000b38:    .long 0x0
> >     0x10000b3c:    .long 0x0
> >     0x10000b40:    lis     r2,4110
> >     0x10000b44:    addi    r2,r2,31488
> >     0x10000b48:    mr      r9,r1
> >     0x10000b4c:    rldicr  r1,r1,0,59
> >     0x10000b50:    li      r0,0
> > (gdb) p $r9
> > $1 = 0
> > (gdb) x/30x $pc - 0x30
> > 0x10000afc:    0x38840040    0x387f0040    0xf8010040    0x48026919
> > 0x10000b0c:    0x60000000    0xe8010040    0x7c0803a6    0x4bffff24
> > 0x10000b1c:    0x00000000    0x01000000    0x00000180    0x39200000
> > 0x10000b2c:    0x91290000    0x7fe00008    0x00000000    0x00000000
> > which matches the hex content of
> > http://154.223.142.244/logs/20220310/console.zhouyi.log:
> > [    5.077431][    T1] init[1]: segfault (11) at 0 nip 10000b2c lr 10001024 code 1 in init[10000000+d0000]
> > [    5.087167][    T1] init[1]: code: 38840040 387f0040 f8010040 48026919 60000000 e8010040 7c0803a6 4bffff24
> > [    5.093987][    T1] init[1]: code: 00000000 01000000 00000180 39200000 <91290000> 7fe00008 00000000 00000000
> >
> >
> > Conclusions: there might be something wrong when packing the init into
> > vmlinux in your environment.
> >
> > I will continue to do research on this interesting problem with you.
>
> As written I think it’s a problem with LLVM/clang. Unfortunately, I
> won’t be able to retest before next week.
Roger that, no need to hurry ;-)

Kind regards
Zhouyi
> Kind regards,
>
> Paul