linux-kernel - Re: [PATCH next v2 1/2] printk: avoid and/or handle record truncation

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Date:   Thu, 01 Oct 2020 00:39:31 -0700
From:   Joe Perches <joe@...ches.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        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 1/2] printk: avoid and/or handle record
 truncation

On Thu, 2020-10-01 at 09:26 +0200, Petr Mladek wrote:
> On Wed 2020-09-30 08:25:24, Joe Perches wrote:
> > On Wed, 2020-09-30 at 11:07 +0206, John Ogness wrote:
> > > If a reader provides a buffer that is smaller than the message text,
> > > the @text_len field of @info will have a value larger than the buffer
> > > size. If readers blindly read @text_len bytes of data without
> > > checking the size, they will read beyond their buffer.
> > > 
> > > Add this check to record_print_text() to properly recognize when such
> > > truncation has occurred.
> > > 
> > > Add a maximum size argument to the ringbuffer function to extend
> > > records so that records can not be created that are larger than the
> > > buffer size of readers.
> > > 
> > > When extending records (LOG_CONT), do not extend records beyond
> > > LOG_LINE_MAX since that is the maximum size available in the buffers
> > > used by consoles and syslog.
> > 
> > I still think it better to support backspace by rewinding
> > the buffer rather than truncation of the output.
> 
> IMHO, backspace support is not worth the complexity. It might do
> some fancy animation on console but it does not bring any advantage
> in static logs (dmesg, journalctl).
> 
> It is possible that it worked in the past when the log buffer was
> just an array of characters that were pushed to the console when
> they appeared.
> 
> But I am pretty sure that it has stopped working many years agl
> variable-length record buffer").

It's more that spinner or timer dots could fill the
buffer and any message after the spinner/dots like
success or failure is lost via truncation.

There aren't many spinners/dots, perhaps it's better
to find and delete them.