linux-kernel - Re: [PATCH printk 2/3] printk: move dictionary keys to dev_printk

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Date:   Fri, 18 Sep 2020 16:24:50 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
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>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 2/3] printk: move dictionary keys to
 dev_printk_info

On Fri 2020-09-18 14:32:41, Rasmus Villemoes wrote:
> On 18/09/2020 14.13, Petr Mladek wrote:
> > On Fri 2020-09-18 08:16:37, Rasmus Villemoes wrote:
> >> On 17/09/2020 15.16, John Ogness wrote:
> >>
> >>>  	if (dev->class)
> >>>  		subsys = dev->class->name;
> >>>  	else if (dev->bus)
> >>>  		subsys = dev->bus->name;
> >>>  	else
> >>> -		return 0;
> >>> +		return;
> >>>  
> >>> -	pos += snprintf(hdr + pos, hdrlen - pos, "SUBSYSTEM=%s", subsys);
> >>> -	if (pos >= hdrlen)
> >>> -		goto overflow;
> >>> +	snprintf(dev_info->subsystem, sizeof(dev_info->subsystem), subsys);
> >>
> >> It's unlikely that subsys would contain a %, but this will be yet
> >> another place to spend brain cycles ignoring if doing static analysis.
> >> So can we not do this. Either of strXcpy() for X=s,l will do the same
> >> thing, and likely faster.
> > 
> > Good point! Better be on the safe size in a generic printk() API.
> > 
> > Well, I am afraid that this would be only small drop in a huge lake.
> > class->name and bus->name seems to be passed to %s in so many
> > *print*() calls all over the kernel code.
> 
> So what? printf("%s", some_string_that_might_contain_percent_chars) is
> not a problem.

Grr, shame on me. I have completely messed this. The combination of
Friday afternoon and noisy kids did not help me much to get it right.

> printf(some_string_that_might_contain_percent_chars) is.

I fully agree that passing unknown string as "fmt" is dangerous and
must be used carefully. It is not needed here.

> And yes, one could do
> 
>   snprintf(dev_info->subsystem, sizeof(dev_info->subsystem), "%s", subsys);
>
> but one might as well avoid the snprintf overhead and use one of the
> strX functions that have the exact same semantics (copy as much as
> there's room for, ensure nul-termination).

Yes, we should use either snprinf() with %s or strXcpy().

Best Regards,
Petr