Re: INFO: task hung in filemap_fault

[Dmitry Vyukov]

On Mon, Jan 15, 2018 at 11:44 AM, Tetsuo Handa
<penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:
>> >>        We can't simply drop info from there. Do you propose to add
>> >> another attached file that contains the same info in a different
>> >> format? What is the exact format you are proposing?
>> >
>> > Plain C program which can be compiled without installing additional
>> > program/library packages (except those needed for building kernels).
>> >
>> >>                                                     Is it just
>> >> dropping the syscall name part after $ sign? Note that it's still not
>> >> C, more complex syscall generally look as follows:
>> >>
>> >> perf_event_open(&(0x7f0000b5a000)={0x4000000002, 0x78, 0x1e2, 0x0,
>> >> 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xffff, 0x0,
>> >> 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
>> >> 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
>> >> @perf_bp={&(0x7f0000000000)=0x0, 0x0}, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
>> >> 0x0, 0x0}, 0x0, 0x0, 0xffffffffffffffff, 0x0)
>> >> recvmmsg(0xffffffffffffffff, &(0x7f0000003000)=[{{0x0, 0x0,
>> >> &(0x7f0000002000)=[{&(0x7f000000a000)=""/193, 0xc1},
>> >> {&(0x7f0000007000-0x3d)=""/61, 0x3d}], 0x2,
>> >> &(0x7f0000005000-0x67)=""/103, 0x67, 0x0}, 0x0}], 0x1, 0x0,
>> >> &(0x7f0000003000-0x10)={0x77359400, 0x0})
>> >> bpf$PROG_LOAD(0x5, &(0x7f0000000000)={0x1, 0x5,
>> >> &(0x7f0000002000)=@framed={{0x18, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
>> >> 0x0}, [@jmp={0x5, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}], {0x95, 0x0, 0x0,
>> >> 0x0}}, &(0x7f0000004000-0xa)='syzkaller\x00', 0x3, 0xc3,
>> >> &(0x7f0000386000)=""/195, 0x0, 0x0, [0x0, 0x0, 0x0, 0x0, 0x0, 0x0,
>> >> 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0], 0x0}, 0x48)
>> >>
>> >> Note: you can convert any syzkaller program to equivalent C code using
>> >> syz-prog2c utility that comes with syzkaller.
>> >
>> > I won't install go language into my environment for analyzing/reproducing your
>> > reports. If syz-prog2c is provided as a CGI service (e.g. receive URL containing
>> > raw.log and print the converted C program), I might try it.
>>
>>
>> raw.log is not a _program_, it's hundreds of separate programs that
>> were executed before the crash. It's also very compressed
>> representation as compared to equivalent C programs. For example for
>> this program:
>>
>> mmap(&(0x7f0000000000/0xfff000)=nil, 0xfff000, 0x3, 0x32,
>> 0xffffffffffffffff, 0x0)
>> r0 = socket$nl_generic(0x10, 0x3, 0x10)
>> sendmsg$nl_generic(r0,
>> &(0x7f0000b3e000-0x38)={&(0x7f0000d4a000-0xc)={0x10, 0x0, 0x0, 0x0},
>> 0xc, &(0x7f0000007000)={&(0x7f0000f7c000-0x15c)={0x24, 0x1c, 0x109,
>> 0xffffffffffffffff, 0xffffffffffffffff, {0x4, 0x0, 0x0},
>> [@nested={0x10, 0x9, [@typed={0xc, 0x0, @u32=0x0}]}]}, 0x24}, 0x1,
>> 0x0, 0x0, 0x0}, 0x0)
>>
>> you can get up to this amount of C code:
>> https://gist.githubusercontent.com/dvyukov/eeaeb4e4ac45c3a251f72098c9295bf9/raw/700cd583507eca90711ba11b42e406f317553371/gistfile1.txt
>>
>> that is, 700 lines of C source for 3 line program. So instead of a 1MB
>> file that will be 100MB, and then it probably should be a gzip archive
>> with hundreds of separate C files. There are people on this list
>> complaining even about 200K of attachments. I don't see that this will
>> be better and well accepted.
>>
>
> No problem. In the "tty: User triggerable soft lockup." case, I manually
> trimmed the reproducer at https://marc.info/?l=linux-mm&m=151368630414963 .
> That is,
>
>  (1) Can the problem be reproduced even if setup_tun(0, true); is commented out?
>
>  (2) Can the problem be reproduced even if NONFAILING(A = B); is replaced with
>      plain A = B; assignment?
>
>  (3) Can the problem be reproduced even if install_segv_handler(); is commented
>      out?
>
>  (4) Can the problem be reproduced even if some syscalls (e.g. __NR_memfd_create,
>      __NR_getsockopt, __NR_perf_event_open) are replaced with no-op?
>
> and so on. Then, I finally reached a reproducer which I sent, and the bug was fixed.
>
> What is important is that everyone can try simplifying the reproducer written
> in plain C in order to narrow down the culprit. Providing a (e.g.) CGI service
> which generates plain C reproducer like gistfile1.txt will be helpful to me.

I am not completely following. You previously mentioned raw.log, which
is a collection of multiple programs, but now you seem to be talking
about a single reproducer. When syzbot manages to reproduce the bug
only with syzkaller program but not with a corresponding C program, it
provides only syzkaller program. It such case it can sense to convert.
But the case you pointed to actually contains C program. So there is
no need to do the conversion at all... What am I missing?