Subject: Re: WARNING in bpf_jit_free

From: Daniel Vetter <[email-masked]>
Date: Tue, 11 Jun 2019 11:01:10 +0200

On Tue, Jun 11, 2019 at 10:51:23AM +0200, Daniel Vetter wrote:
> On Tue, Jun 11, 2019 at 10:33:21AM +0200, Dmitry Vyukov wrote:
> > On Tue, Jun 11, 2019 at 10:04 AM Daniel Vetter <[email-masked]> wrote:
> > >
> > > On Sat, Jun 08, 2019 at 04:22:06AM -0700, syzbot wrote:
> > > > syzbot has found a reproducer for the following crash on:
> > > >
> > > > HEAD commit:    79c3ba32 Merge tag 'drm-fixes-2019-06-07-1' of git://anong..
> > > > git tree:       upstream
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1201b971a00000
> > > > kernel config:  https://syzkaller.appspot.com/x/.config?x=60564cb52ab29d5b
> > > > dashboard link: https://syzkaller.appspot.com/bug?extid=2ff1e7cb738fd3c41113
> > > > compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
> > > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=14a3bf51a00000
> > > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=120d19f2a00000
> > >
> > > Looking at the reproducer I don't see any calls to ioctl which could end
> > > up anywhere in drm.
> > > >
> > > > The bug was bisected to:
> > > >
> > > > commit 0fff724a33917ac581b5825375d0b57affedee76
> > > > Author: Paul Kocialkowski <[email-masked]>
> > > > Date:   Fri Jan 18 14:51:13 2019 +0000
> > > >
> > > >     drm/sun4i: backend: Use explicit fourcc helpers for packed YUV422 check
> > >
> > > And most definitely not in drm/sun4i. You can only hit this if you have
> > > sun4i and run on arm, which per your config isn't the case.
> > >
> > > tldr; smells like bisect gone wrong.
> > > -Daniel
> > 
> > From the bisection log it looks like the bug is too hard to trigger
> > for reliable bisection. So it probably classified one bad commit as
> > good. But it should got quite close to the right one.
> 
> Well statistically it'll get close, since there's a fair chance that it's
> one of the later bisect results that got mischaracterized.
> 
> But you can be equally unlucky, and if it's one of the earliers, then it
> can easily be a few thousand commits of. Looking at the log it's mostly
> bad, with a few good sprinkled in, which could just be reproduction
> failures. So might very well be that the very first "good" result is
> wrong. And that very first "good" decision cuts away a big pile of bpf
> related commits. The next "good" decision then only cuts away a pile of
> drm commits, but at that point you're already off the rails most likely.
> 
> I'd say re-test on f90d64483ebd394958841f67f8794ab203b319a7 a few times,
> I'm willing to bet that one is actually bad.

btw if this theory is right, we have a 1-in-4 chance of a spurious "good"
with your test. If you get 10 repeated "good" then that should be good
enough to make playing the lottery a better endeavor :-)
-Daniel

> 
> Cheers, Daniel
> 
> > 
> > > > bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=1467550f200000
> > > > final crash:    https://syzkaller.appspot.com/x/report.txt?x=1667550f200000
> > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1267550f200000
> > > >
> > > > IMPORTANT: if you fix the bug, please add the following tag to the commit:
> > > > Reported-by: [email-masked]
> > > > Fixes: 0fff724a3391 ("drm/sun4i: backend: Use explicit fourcc helpers for
> > > > packed YUV422 check")
> > > >
> > > > WARNING: CPU: 0 PID: 8951 at kernel/bpf/core.c:851 bpf_jit_free+0x157/0x1b0
> > > > Kernel panic - not syncing: panic_on_warn set ...
> > > > CPU: 0 PID: 8951 Comm: kworker/0:0 Not tainted 5.2.0-rc3+ #23
> > > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> > > > Google 01/01/2011
> > > > Workqueue: events bpf_prog_free_deferred
> > > > Call Trace:
> > > >  __dump_stack lib/dump_stack.c:77 [inline]
> > > >  dump_stack+0x172/0x1f0 lib/dump_stack.c:113
> > > >  panic+0x2cb/0x744 kernel/panic.c:219
> > > >  __warn.cold+0x20/0x4d kernel/panic.c:576
> > > >  report_bug+0x263/0x2b0 lib/bug.c:186
> > > >  fixup_bug arch/x86/kernel/traps.c:179 [inline]
> > > >  fixup_bug arch/x86/kernel/traps.c:174 [inline]
> > > >  do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
> > > >  do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
> > > >  invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:986
> > > > RIP: 0010:bpf_jit_free+0x157/0x1b0
> > > > Code: 00 fc ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 75 5d 48 b8 00 02 00 00
> > > > 00 00 ad de 48 39 43 70 0f 84 05 ff ff ff e8 f9 b5 f4 ff <0f> 0b e9 f9 fe ff
> > > > ff e8 bd 53 2d 00 e9 d9 fe ff ff 48 89 7d e0 e8
> > > > RSP: 0018:ffff88808886fcb0 EFLAGS: 00010293
> > > > RAX: ffff88808cb6c480 RBX: ffff88809051d280 RCX: ffffffff817ae68d
> > > > RDX: 00000000> >
> > >
> > > --
> > > Daniel Vetter
> > > Software Engineer, Intel Corporation
> > > http://blog.ffwll.ch
> > >
> > > --
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> > > To view this discussion on the web visit https://groups.google.com/d/msgid/syzkaller-bugs/20190611080431.GP21222%40phenom.ffwll.local.
> > > For more options, visit https://groups.google.com/d/optout.00000000 RSI: ffffffff817bf0f7 RDI: ffff88809051d2f0
> > > > RBP: ffff88808886fcd0 R08: 1ffffffff14ccaa8 R09: fffffbfff14ccaa9
> > > > R10: fffffbfff14ccaa8 R11: ffffffff8a665547 R12: ffffc90001925000
> > > > R13: ffff88809051d2e8 R14: ffff8880a0e43900 R15: ffff8880ae834840
> > > >  bpf_prog_free_deferred+0x27a/0x350 kernel/bpf/core.c:1984
> > > >  process_one_work+0x989/0x1790 kernel/workqueue.c:2269
> > > >  worker_thread+0x98/0xe40 kernel/workqueue.c:2415
> > > >  kthread+0x354/0x420 kernel/kthread.c:255
> > > >  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
> > > > Kernel Offset: disabled
> > > > Rebooting in 86400 seconds..
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch