linux-ext4 - Re: [syzbot] [ext4?] kernel panic: EXT4-fs (device loop0): panic forced after error (3)

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Date:   Thu, 17 Aug 2023 16:28:33 +0200
From:   Aleksandr Nogikh <nogikh@...gle.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     syzbot <syzbot+27eece6916b914a49ce7@...kaller.appspotmail.com>,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, nathan@...nel.org, ndesaulniers@...gle.com,
        syzkaller-bugs@...glegroups.com, trix@...hat.com
Subject: Re: [syzbot] [ext4?] kernel panic: EXT4-fs (device loop0): panic
 forced after error (3)

On Thu, Aug 17, 2023 at 4:21 PM Theodore Ts'o <tytso@....edu> wrote:
>
> On Wed, Aug 16, 2023 at 03:48:49PM -0700, syzbot wrote:
> > Hello,
> >
> > syzbot found the following issue on:
> >
> > HEAD commit:    ae545c3283dc Merge tag 'gpio-fixes-for-v6.5-rc6' of git://..
> > git tree:       upstream
> > console+strace: https://syzkaller.appspot.com/x/log.txt?x=13e5d553a80000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=171b698bc2e613cf
> > dashboard link: https://syzkaller.appspot.com/bug?extid=27eece6916b914a49ce7
> > compiler:       Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13433207a80000
> > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=109cd837a80000
> >
> > EXT4-fs error (device loop0): ext4_validate_block_bitmap:430: comm syz-executor211: bg 0: block 46: invalid block bitmap
> > Kernel panic - not syncing: EXT4-fs (device loop0): panic forced after error
>
> #syz invalid
>
> This is fundamentally a syzbot bug.  The file system is horrifically
> corrupted, *and* the superblock has the "panic on error" (aka "panic
> onfile system corruption") bit set.

The console log has the following line:

[   60.708717][ T5061] Kernel panic - not syncing: EXT4-fs (device
loop0): panic forced after error

Can we consider a "panic forced after error" line to be a reliable
indicator that syzbot must ignore the report?


>
> This can be desireable because in a failover situation, if the file
> system is found to be corrupted, you *want* the primary server to
> fail, and let the secondary server to take over.  This is a technique
> which is decades old.
>
> So this is Working As Intended, and is a classic example of (a) if you
> are root, you can force the file system to crash, and (b) a classic
> example of syzbot noise.  (Five minutes of my life that I'm never
> getting back.  :-)
>
>                                                 - Ted
>
> --
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