linux-kernel - Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel panic

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Message-ID: <CACT4Y+ZhHvsVZh1pKzK1tn-P78rOssOz=7eWkXz7z2Sh1JscdA@mail.gmail.com>
Date:   Tue, 19 Mar 2019 14:35:07 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: [syzbot? printk?] no WARN_ON() messages printed before "Kernel
 panic - not syncing: panic_on_warn set ..."

On Tue, Mar 19, 2019 at 1:35 PM Sergey Senozhatsky
<sergey.senozhatsky@...il.com> wrote:
>
> On (03/19/19 09:10), Dmitry Vyukov wrote:
>
> [..]
>
> > In some setups we read out output with "dmesg -w" (should stream
> > output as well) in absence of a better solution. Will this be affected
> > by SYSLOG_ACTION_CLEAR?
>
> There is a small chance that syslog/devkmsg can be affected
>
>         syslog_clear()
>         {
>                 clear_seq = log_next_seq;
>                 clear_idx = log_next_idx;
>         }
>
>         syslog_print_all()
>         {
>                 ...
>                 seq = clear_seq;
>                 idx = clear_idx;
>                 while (seq < log_next_seq) {
>                 ...
>                         len += msg_print_text(msg, true, time, NULL, 0);
>                 }
>                 ...
>         }
>
> devkmsg also may be affected
>
>         syslog_clear()
>         {
>                 clear_seq = log_next_seq;
>                 clear_idx = log_next_idx;
>         }
>
>         devkmsg_llseek()
>         {
>         ...
>                 case SEEK_DATA:
>                         user->idx = clear_idx;
>                         user->seq = clear_seq;
>                         break;
>         ...
>         }
>
> user->idx/user->seq then are used in devkmsg poll()/read(). syslog is
> more likely to be affected, I think.

SYSLOG_ACTION_CLEAR is disabled:
https://github.com/google/syzkaller/commit/28c04d39e052ee2960eebc16a3f7fba88920cd52