RE: [PATCH] Fix chance of sign extension to nsec after its msb is set during calculation.

[Thomas Gleixner <tglx@linutronix.de>]

On Sun, 4 Sep 2016, Liav Rehana wrote:

Please do not top post and trim your reply.

> The root of the problem is that in case the multiplication of delta and
> tkr->mult in the line that I've changed is too big that the MSB of the
> result is set, then the shift will cause an unwanted sign extension.

I completely understand that, but as I said before:

> > This typecast is just a baindaid. What happens if you double the
> > suspend time?  The multiplication will simply overflow. So the proper
> > fix is to sanity check delta and do multiple conversions if delta is
> > big enough.  Preferrably this happens somewhere at the call site and
> > not in this hotpath function.

> That sign extension will be avoided completely if the variable nsec was
> unsigned (u64 instead of s64), so I think the correct solution for this
> is to change the type of nsec to u64.

That's a different story and its not a solution for the general problem of

       delta * mult >= (1 << 31) or delta * mult >= (1 << 32)

Thanks,

	tglx