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Subject: Re: [PATCH] printk: Add caller information to printk() output.
To:     Petr Mladek <pmladek@suse.com>
Cc:     Dmitry Vyukov <dvyukov@google.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>
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 <20181210130930.bas4fozjhe3dc5nu@pathway.suse.cz>
From:   Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Message-ID: <c5b5d660-d217-35f0-c4f1-7252367934d1@i-love.sakura.ne.jp>
Date:   Mon, 10 Dec 2018 23:01:39 +0900
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On 2018/12/10 22:09, Petr Mladek wrote:
>> +#ifdef CONFIG_PRINTK_FROM
>> +#define PREFIX_FROM_MAX		16
>> +#define PREFIX_MAX		(32 + PREFIX_FROM_MAX)
>> +#define LOG_LINE_MAX		(1024 - 32)
> 
> This looks suspicious. We either need to limit LOG_LINE_MAX
> by the real PREFIX_MAX (48). Or we must make sure that
> the code is able to handle an eventual overflow.

Reducing LOG_LINE_MAX makes devkmsg_write() to return -EINVAL.
Is such user visible change acceptable?

> BTW: The limit 48 looks right. If I count correctly, the longest
> prefix might be:
> 
>     <2048>[4294967296,xxxxxx][T4294967296]
> 
> 38 = 6+19+13

Theoretical max (based on variable's bit width) is
6 + 20 + 13 + 1 (for space) + 1 (for '\0') = 41.

We allocate 32 for "<2047>[18446744073.709551] ". Thus,
I chose 48 for "<2047>[18446744073.709551][T4294967295] ".