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

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Date:	Mon, 5 Oct 2015 16:33:54 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <bp@...en8.de>,
	open list <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] string: Improve the generic strlcpy() implementation


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

> On Oct 5, 2015 14:15, "Ingo Molnar" <mingo@...nel.org> wrote:
> >
> > Hm, so GCC (v4.9.2) will only warn about this bug if -Wtype-limits is
> enabled
> > explicitly:
> 
> Some of the warnings are really nasty, and cause people to write worse code.
> 
> For example, this is inherently good code:
> 
>     if (x < 0 || x > MAXLEN)
>         return -EINVAL;
> 
> and a compiler that warns about that is pure and utter crap. Obviously.
> Agreed?
> 
> Now, imagine that "x" here is some random type. Maybe it's s "char" and you
> don't even know the sign. Maybe it's "loff_t". Maybe it's "size_t", or
> whatever.
> 
> Note how that test is correct *regardless* of the sign of the type. A
> compiler that warns about the "x < 0" part just because x happens to be
> unsigned is a bad bad compiler, and makes people remove that check, even
> though it's good for readability, and good for robustness wrt changing the
> type.

Hm, so there's a flip side here - if we consider 'example 6)' in my previous mail:

  kernel/auditsc.c:1027:23: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

        size_t len, len_left, to_send;

        ...

        if (WARN_ON_ONCE(len < 0 || len > MAX_ARG_STRLEN - 1)) {

Now if this code was written as:

        if (WARN_ON_ONCE(len < 0)) {

then it would be a clear bug, right?

So we could solve that by adding a generic range check:

 static inline int range_ok(unsigned long low, unsigned long val, unsigned long high)
 {               
        if (val < low)
                return 0;
        if (val >= high)
                return 0;

        return 1;
 }

and we could write this:

        if (len < 0 || len > MAX_ARG_STRLEN - 1) {

as:

        if (!range_ok(0, len, MAX_ARG_STRLEN)) {

?

That kind of construct:

 - is robust against a changed type for 'len'

 - is robust against these common typos for open coded security checks:

        if (len <= 0 || len > MAX_ARG_STRLEN - 1) {

        if (len < 0 || len >= MAX_ARG_STRLEN - 1) {

        if (len < 0 || len > MAX_ARG_STRLEN) {

   the first and second ones over-check and are harmless in this context, the 
   third one is harmful because it does not catch the MAX_ARG_STRLEN case.

 - it would also clearly document range checking performed in a function that gets
   untrusted data.

Hypothetically, if this was acceptable then we could use this in the cases where 
GCC generates a bogus warning.

But ... no strong feelings. Just found it weird that GCC let my bug slip through.

Thanks,

	Ingo
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