lkml.org 
[lkml]   [2024]   [May]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [RFC] Mitigating unexpected arithmetic overflow
On Wed, 8 May 2024 at 16:47, Linus Torvalds
<torvalds@linux-foundation.org> 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

\
 
 \ /
  Last update: 2024-05-09 02:23    [W:0.165 / U:0.380 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site