linux-kernel - Re: [PATCH next v2 0/2] printk: fix reading beyond buffer

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Date:   Wed, 30 Sep 2020 15:30:55 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v2 0/2] printk: fix reading beyond buffer

On Wed 2020-09-30 11:07:32, John Ogness wrote:
> Hello,
> 
> Marek Szyprowski reported [0] a problem with a particular printk
> usage. This particular usage performs thousands of LOG_CONT calls.
> The printk.c implementation was only limiting the growing record by
> the maximum size available in the ringbuffer, thus creating a record
> that was several kilobytes in size. This in and of itself is not
> a problem.
> 
> However, the various readers used buffers that were about 1KB in
> size. The ringbuffer would only fill the reader's 1KB buffer, but the
> meta data stated that the message was actually much larger. The
> reader code was not checking this and assumed its buffer contained
> the full message.
> 
> I have solved this problem by adding the necessary check to the
> functions where the situation can occur and also adding an argument
> when extending records so that a maximum size is specified. This
> will prevent the records from growing beyond the size that we know
> our readers are using.
> 
> I did not add the check where it is certain that the reader's
> buffer is large enough to contain the largest possible message.
> 
> The 2nd patch in this series reduces the size of the initial setup
> buffer. I noticed it was too big while verifying all the sizes for
> this series.
> 
> John Ogness
> 
> [0] https://lkml.kernel.org/r/f1651593-3579-5820-6863-5f4973d2bfdc@samsung.com
> 
> John Ogness (2):
>   printk: avoid and/or handle record truncation
>   printk: reduce setup_text_buf size to LOG_LINE_MAX

The patchset is committed in printk/linux.git, branch printk-rework.

Best Regards,
Petr