Add timingsafe_bcmp and timingsafe_memcmp.

[delphij]

Summary

Add timingsafe_bcmp and timingsafe_memcmp.

Obtained from: OpenBSD
MFC after: 1 month

[delphij]

Fix typo.

[delphij]

Style.

[delphij]

Style in manpage.

[bjk]

I hate to bikeshed, but did we do any research on what names are used for constant-time functions elsewhere than OpenBSD?
Somehow I had the impression that a constant_time_ prefix was in use several places, though maybe that's just OpenSSL..

lib/libc/string/timingsafe_bcmp.3

18 ↗

(On Diff #18656)

Is this a target merge date? ;)

66 ↗

(On Diff #18656)

I slightly worry that future advances in compiler optimizers will make these semantics hard to support, but have no real objection.

[eadler]

I wonder if it makes more sense to write this in asm than in C? Can we add
explicit "volatile" mentions here to ensure that optimizations don't occur?
If we can't write in asm, can we write a unit test to ensure the asm does
not get optimized out?

[delphij]

In D7280#151588, @bjk wrote:

I hate to bikeshed, but did we do any research on what names are used for constant-time functions elsewhere than OpenBSD?
Somehow I had the impression that a constant_time_ prefix was in use several places, though maybe that's just OpenSSL..

Yes, but we already adopted timingsafe_bcmp in kernel...

[delphij]

lib/libc/string/timingsafe_bcmp.3

18 ↗

(On Diff #18656)

Yes.

66 ↗

(On Diff #18656)

That's possible, but I think we can always hint compiler not to optimize a select subset of library functions when this become a problem?