linux-kernel - Re: [RFC][PATCH] printk: increase devkmsg write() ratelimit

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Message-ID: <20181218123748.3aadd16c@gandalf.local.home>
Date:   Tue, 18 Dec 2018 12:37:48 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] printk: increase devkmsg write() ratelimit

On Tue, 18 Dec 2018 18:21:09 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Dec 19, 2018 at 01:52:17AM +0900, Sergey Senozhatsky wrote:
> > On (12/18/18 16:24), Borislav Petkov wrote:  
> > > On Wed, Dec 19, 2018 at 12:14:55AM +0900, Sergey Senozhatsky wrote:  
> > > > Right, but unlike log_buf_len, devkmsg is a bit close to a "binary" knob:
> > > > either all messages or none;  
> > > 
> > > ... which is perfectly fine for a debugging session.  
> > 
> > But devkmsg ratelimits systemd errors, so one does not even know that
> > "some debugging is required". For instance from my x86 box:
> > 
> >  Unmounting /home...
> >  [..]
> >  home.mount: Mount process exited, code=exited status=32
> >  Failed unmounting /home.
> > 
> > I don't want to debug systemd, I want to know that something didn't
> > work out. 10 messages max and 5 seconds interval looks a bit too strict.  
> 
> Again, complain to system-doofus for printing so much crap to somewhere
> it should not print to begin with.

I've been saying that it would be good to make the kmsg be a separate
buffer that just gets interleaved with the kernel buffer. Userspace
processes should never be able to overwrite messages from the kernel.

-- Steve