On 11/26/2014 01:23 PM, Linus Torvalds wrote:
> On Wed, Nov 26, 2014 at 9:55 AM, Sasha Levin <[email protected]> wrote:
>> >
>> > Good point. Do you think there's an advantage in using GCC's overflow
>> > checker in this case?
> No. However, if your coccinelle script can be changed to verify that
> the type of the expression is unsigned, _that_ would be useful.
I'm pretty sure that this is something GCC will warn you about in the
compilation stage.
> And the "multiplication overflow" may actually be a way to generate
> better code. Possibly. I'm not entirely sure exactly what gcc actually
> does. How many multiplication overflow tests do we actually have,
> though?
Well, there are two straightforward checks in the kcalloc() family. They're
not the issue though. The problem is the *unchecked* *signed* integer overflows
lurking around.
kernel/time/ntp.c:process_adjtimex_modes():
if (txc->modes & ADJ_FREQUENCY) {
time_freq = txc->freq * PPM_SCALE; <=== Undefined overflow
time_freq = min(time_freq, MAXFREQ_SCALED);
time_freq = max(time_freq, -MAXFREQ_SCALED);
/* update pps_freq */
pps_set_freq(time_freq);
}
The multiplication is between signed integers, and it overflows (user
triggerable).
Thanks,
Sasha
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