From: Ingo Molnar <mingo@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Borislav Petkov <bp@alien8.de>,
open list <linux-kernel@vger.kernel.org>,
"H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] string: Improve the generic strlcpy() implementation
Date: Mon, 5 Oct 2015 16:33:54 +0200 [thread overview]
Message-ID: <20151005143354.GB7478@gmail.com> (raw)
In-Reply-To: <CA+55aFx2McOeEiB7fJ-BV=vBsH=i2cC-qW8_EBEnScfQhugD_w@mail.gmail.com>
* 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
next prev parent reply other threads:[~2015-10-05 14:34 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-10 19:43 [GIT PULL] strscpy string copy function Chris Metcalf
2015-10-04 15:55 ` Linus Torvalds
2015-10-05 11:27 ` [PATCH] string: Improve the generic strlcpy() implementation Ingo Molnar
2015-10-05 11:53 ` Ingo Molnar
2015-10-05 13:15 ` Ingo Molnar
2015-10-05 14:04 ` Ingo Molnar
[not found] ` <CA+55aFx2McOeEiB7fJ-BV=vBsH=i2cC-qW8_EBEnScfQhugD_w@mail.gmail.com>
2015-10-05 14:07 ` Ingo Molnar
2015-10-05 14:33 ` Ingo Molnar [this message]
2015-10-05 15:32 ` Linus Torvalds
2015-10-05 16:03 ` Ingo Molnar
2015-10-05 12:28 ` Linus Torvalds
2015-10-05 13:10 ` Ingo Molnar
2015-10-05 22:28 ` Rasmus Villemoes
2015-10-06 7:54 ` Ingo Molnar
2015-10-06 8:03 ` Ingo Molnar
2015-10-06 22:00 ` Rasmus Villemoes
2015-10-07 7:18 ` Ingo Molnar
2015-10-07 9:04 ` Rasmus Villemoes
2015-10-07 9:22 ` Linus Torvalds
2015-10-08 8:48 ` Ingo Molnar
2015-10-09 8:10 ` Rasmus Villemoes
2015-10-09 9:10 ` [RFC 0/3] eliminate potential race in string() (was: [PATCH] string: Improve the generic strlcpy() implementation) Rasmus Villemoes
2015-10-09 9:14 ` [RFC 1/3] lib/vsprintf.c: pull out padding code from dentry_name() Rasmus Villemoes
2015-10-09 9:14 ` [RFC 2/3] lib/vsprintf.c: move string() below widen_string() Rasmus Villemoes
2015-10-09 9:14 ` [RFC 3/3] lib/vsprintf.c: eliminate potential race in string() Rasmus Villemoes
2015-10-10 7:47 ` [RFC 0/3] eliminate potential race in string() (was: [PATCH] string: Improve the generic strlcpy() implementation) Ingo Molnar
2015-10-19 12:42 ` [PATCH] string: Improve the generic strlcpy() implementation Rasmus Villemoes
2015-10-19 16:24 ` Chris Metcalf
2015-10-05 15:38 Alexey Dobriyan
2015-10-05 16:11 ` Ingo Molnar
2015-10-05 16:13 ` Ingo Molnar
[not found] ` <CA+55aFyTVJfCt00gYJpiQW5kqPaRGJ93JmfRRni-73zCf5ivqg@mail.gmail.com>
2015-10-05 16:22 ` Ingo Molnar
2015-10-05 16:28 ` Ingo Molnar
2015-10-05 20:40 ` Linus Torvalds
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