Reserve Arm MTE SIGSEGV codes

[emaste]

Summary

LLDB is adding support for sync MTE exception and async MTE reporting. Claim the same values as used on Linux for future compatibility.

[markj]

sys/sys/signal.h

323–325

This line is part of the SEGV_ACCERR description.

[kib]

I am curious what is the value of having FreeBSD si_code numerically equal to the Linux' one ?

When I added PKU si_code, I effectively started MD values at 100.

[emaste]

fix comment, whitespace

[emaste]

I am curious what is the value of having FreeBSD si_code numerically equal to the Linux' one ?

Application developers might have a tendency to do things like

#ifndef SEGV_MTEAERR
#define SEGV_MTEAERR 8
#endif

(from https://reviews.llvm.org/D93495)

Even though this is not really valid it seems sensible to me to just use the Linux values.

[kib]

I do not think this is a valid practice, and we should not encourage it.

IMO it buys us nothing, while making some havoc in the SEGV_ value space. I would prefer to keep new values around SEGV_PKUERR, as any other new MD si_codes.

[emaste]

OK.