* Linus Torvalds <torva...@linux-foundation.org> wrote:
> On Oct 5, 2015 14:15, "Ingo Molnar" <mi...@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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