sys: Add sys/ckdint.h

[markj]

Summary

We have a C23 stdckdint.h header for userspace, which provides checked
addition, subtraction and multiplication. We lack similar helpers in
the kernel, where they are regularly needed.

Let's just adopt the C23 macros. For bonus points, I added a wrapper to
ensure that ignored an return value is raised as an error by the
compiler. I also converted a couple of places in the kernel to use the
new macros. The netmap change in particular fixes a bug in my commit
319414a926af1515e2572f89f0636e5505e762d5.

[emaste]

sys/dev/netmap/netmap_mem2.c

2017–2024 ↗

(On Diff #183839)

Or even

			if (ckd_mul(&len, ndesc, sizeof(struct netmap_slot)) ||
			    ckd_add(&len, len, sizeof(struct netmap_ring))) {
				error = EINVAL;
				goto cleanup;
			}

To me at least this reads close to ndesc * sizeof(struct netmap_slot) + sizeof(struct netmap_ring) checked for overflow.

[markj]

Review feedback

[fuz]

I like it. Please land the new header and kernel fixes separately in case reversal is needed.

sys/kern/kern_malloc.c

827 ↗

(On Diff #183910)

Should we get rid of the WOULD_OVERFLOW macro entirely?

[markj]

In D58773#1351426, @fuz wrote:

I like it. Please land the new header and kernel fixes separately in case reversal is needed.

Yep, I will. I put everything together just for demonstration.

sys/kern/kern_malloc.c

827 ↗

(On Diff #183910)

Once other consumers are converted, yes.