Re: [PATCH] tcp_lp: use 64-bit arithmetic instead of 32-bit

[Gustavo A. R. Silva]

Hi Andrew,

Quoting Andrew Lunn <andrew@xxxxxxx>:

> On Wed, Jan 31, 2018 at 07:07:49PM -0600, Gustavo A. R. Silva wrote:
> > Hi Alan,
> >
> > Quoting Alan Cox <gnomes@xxxxxxxxxxxxxxxxxxx>:
> >
> > >On Wed, 31 Jan 2018 18:24:07 -0600
> > >"Gustavo A. R. Silva" <gustavo@xxxxxxxxxxxxxx> wrote:
> > >
> > >>Cast to s64 some variables and a macro in order to give the
> > >>compiler complete information about the proper arithmetic to
> > >>use. Notice that these elements are used in contexts that
> > >>expect expressions of type s64 (64 bits, signed).
> > >>
> > >>Currently such expression are being evaluated using 32-bit
> > >>arithmetic.
> > >
> > >The question you need to ask is 'can it overflow 32bit maths', otherwise
> > >you are potentially making the system do extra work for no reason.
> > >
> >
> > Yeah, I get your point and it seems that in this particular case there is no
> > risk of a 32bit overflow, but in general and IMHO as the code evolves, the
> > use of incorrect arithmetic may have security implications in the future, so
> > I advocate for code correctness in this case.
>
> Hi Gustavo
>
> Is this on the hotpath? How much overhead does it add to 32 bit
> architectures which don't have 64 bit arithmetic in hardware? There
> are a lot of embedded systems which are 32 bit.

I'm sorry, I don't have access to 32-bit hardware at the moment.

Thanks
--
Gustavo