linux-kernel - Re: [RFC] Recursive printk

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Date:	Sat, 6 Dec 2008 09:16:19 +0100
From:	Willy Tarreau <w@....eu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nick Andrew <nick@...k-andrew.net>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Takashi Iwai <tiwai@...e.de>
Subject: Re: [RFC] Recursive printk

On Fri, Dec 05, 2008 at 11:41:05PM -0800, Andrew Morton wrote:
> On Sat, 6 Dec 2008 08:33:05 +0100 Willy Tarreau <w@....eu> wrote:
> 
> > On Fri, Dec 05, 2008 at 11:20:16PM -0800, Andrew Morton wrote:
> > > > void snd_verbose_printk(const char *file, int line, const char *format, ...)
> > > > {
> > > >         va_list args;
> > > > 
> > > >         if (format[0] == '<' && format[1] >= '0' && format[1] <= '7' && format[2] == '>') {
> > > >                 char tmp[] = "<0>";
> > > >                 tmp[1] = format[1];
> > > >                 printk("%sALSA %s:%d: ", tmp, file, line);
> > > >                 format += 3;
> > > 
> > > That's racy.  Two threads can fight over tmp[1].  It should do:
> > > 
> > > 		printk("<%c>ALSA %s:%d: ", format[1], tmp, file, line);
> > > 
> > > (I didn't know that you can even modify literal strings - shouldn't
> > > they be in read-only storage?)
> > 
> > no Andrew, this tmp[] is declared on the stack, and gcc emits code to
> > copy the constant "<0>" onto the stack every time this code is called
> 
> Good heavens, so it does.  It is unusual for C to cater to such stupid
> code by generating such inefficient code.  A bit of a trap, really.

especially since teachers tend to explain students that char foo[] is the
same thing as char *foo, which obviously it is not.

Willy

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