linux-kernel - Re: [PATCH] string: Improve the generic strlcpy() implementation

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Date:	Thu, 8 Oct 2015 10:48:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Chris Metcalf <cmetcalf@...hip.com>,
	open list <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH] string: Improve the generic strlcpy() implementation


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> So I really refuse to worry about the snprintf() family of functions wrt this 
> race. I don't think it was hugely important for strlcpy() either - more of a 
> "quality of implementation" issue rather than anything fundamental - but for 
> snprintf and friends it's an almost unavoidable issue because of how snprintf 
> works.
> 
> Saying that 'strlcpy()' and 'snprintf("%s")' are equivalent is true only in the 
> loosest sense. Yes, they return the same return value. Yes, the result string 
> should be the same. But the two are completely different despite that.
> 
> snprintf() has to handle all the *other* cases than just "%s", including 
> right-justification, string precision handling, etc etc. It is effectively 
> impossible to do without doing "strlen()" on the source of the string 
> beforehand. As a result, snprintf() is fundamentally always going to be racy wrt 
> the string changing during the call.
> 
> So the simple end result is that we shouldn't worry about it, and if you are 
> doing snprintf() on a changing string, you should just be aware of it. We *do* 
> actually do that, for things like "current->comm" that really can change while 
> being printed out. We just don't care deeply, and have in fact been removing 
> locks in this area, because the end result is still guaranteed to be 
> NUL-terminated etc.
> 
> Can we get odd truncated printouts in the (very very very unlikely) case that 
> the string is being changed? Yes. We just don't care.

I do agree mostly, but I think we should still try to achieve the following two 
properties, if possible sanely+cheaply+cleanly:

 - the printed string should not contain spurious \0 bytes even if the %s source
   'races'. [I think this is true currently.]

 - the return code should correctly represent what snprintf did to the target
   string. [This might not be the case currently. But I'm not sure!]

Because that's a real concern I think: snprintf() return is used frequently to 
iterate over buffers, and it should correctly and reliably represent what it did, 
regardless of what the source buffer does - because snprintf obviously knows what 
it did to the output buffer, it has full, race-free control over it.

Whether left-alignment and other formatting details were calculated correctly, 
etc. is a secondary concern and cannot be guaranteed, but we should at least 
guarantee that we generated a single string, that we did nothing else, and that we 
correctly returned its length.

Agreed?

Thanks,

	Ingo
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