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

From Ingo Molnar <mingo@kernel.org>
Newsgroups linux.kernel
Subject Re: [PATCH] string: Improve the generic strlcpy() implementation
Date 2015-10-05 16:40 +0200
Message-ID <qgf5M-1ED-9@gated-at.bofh.it> (permalink)
References (1 earlier) <qfTRD-4Np-7@gated-at.bofh.it> <qgc7U-5TG-9@gated-at.bofh.it> <qgcAW-6rH-11@gated-at.bofh.it> <qgdQm-8oB-23@gated-at.bofh.it> <qgeCJ-16H-9@gated-at.bofh.it>
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:

> On Oct 5, 2015 14:15, "Ingo Molnar" <mingo@kernel.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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Re: [GIT PULL] strscpy string copy function Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-04 18:00 +0200
  [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 13:30 +0200
    Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 14:00 +0200
      Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 15:20 +0200
        Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 16:10 +0200
        Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 16:10 +0200
        Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 16:40 +0200
          Re: [PATCH] string: Improve the generic strlcpy() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-05 17:40 +0200
            Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 18:10 +0200
    Re: [PATCH] string: Improve the generic strlcpy() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-05 14:30 +0200
      Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-05 15:20 +0200
    Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-06 00:30 +0200
      Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-06 10:00 +0200
      Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-06 10:10 +0200
        Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-07 00:10 +0200
          Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-07 09:20 +0200
            Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-07 11:10 +0200
              Re: [PATCH] string: Improve the generic strlcpy() implementation Linus Torvalds <torvalds@linux-foundation.org> - 2015-10-07 11:30 +0200
                Re: [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar <mingo@kernel.org> - 2015-10-08 10:50 +0200
                Re: [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 10:20 +0200
                [RFC 0/3] eliminate potential race in string() (was: [PATCH] string: Improve the generic strlcpy() implementation) Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                [RFC 1/3] lib/vsprintf.c: pull out padding code from dentry_name() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                [RFC 3/3] lib/vsprintf.c: eliminate potential race in string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                [RFC 2/3] lib/vsprintf.c: move string() below widen_string() Rasmus Villemoes <linux@rasmusvillemoes.dk> - 2015-10-09 11:20 +0200
                Re: [RFC 0/3] eliminate potential race in string() (was: [PATCH]  string: Improve the generic strlcpy() implementation) Ingo Molnar <mingo@kernel.org> - 2015-10-10 09:50 +0200

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