Re: [RFC] Mitigating unexpected arithmetic overflow

[Linus Torvalds]

On Wed, 8 May 2024 at 16:47, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> So *that* I feel could be something where you can warn without a ton
> of compiler smarts at all. If you see an *implicit* cast to unsigned
> and then the subsequent operations wraps around, it's probably worth
> being a lot more worried about.

Side note on this part: quite often, because of C promotion rules, you
have "int" as an "intermediate" type.

IOW, while I had that example of

        int a;
        ...
        a * sizeof(xyz);

being questionably not-UB (because "int a" gets promoted to unsigned
as part of C integer promotion, and thus you really had a signed value
that was involved in unsigned wrap-around), if you have

        unsigned short a;
        ...
        a * sizeof(xyz);

then technically that 'a' is first promoted to 'int' (because all
arithmetic on types smaller than int get promoted to int), and then it
gets promoted to size_t because the multiply gets done in the bigger
type.

So in one sense that unsigned multiply may actually have involved a
cast from a signed type, but at the same time it's not at all in that
kind of "accidentally not UB" class.

I suspect most compilers would have combined the two levels of
implicit casts into just one, so at no point outside of perhaps some
very intermediate stage will it show as a signed int cast to unsigned,
but I thought I'd mention it anyway. Implicit casts get nasty not just
in assignments, but also in these kinds of situations.

I still suspect the "implicit truncating cast at assignment" is likely
a much more common case of loss of information than actual arithmetic
wrap-around, but clearly the two have commonalities.

              Linus