[RFC][PATCHSET] VM_FAULT_RETRY fixes

[Linus Torvalds]

On Tue, Jan 31, 2023 at 1:10 PM Al Viro <viro at zeniv.linux.org.uk> wrote:
>
> Umm...  What about the semantics of get_user() of unmapped address?
> Some architectures do quiet EFAULT; some (including alpha) hit
> the sucker with SIGBUS, no matter what.

I think we should strive to just make this all common.

The reason alpha is different is almost certainly not intentional, but
a combination of "pure accident" and "nobody actually cares".

> Are we free to modify that behaviour, or is that part of arch-specific
> ABI?

I'd just unify this all, probably with a preference for existing
semantics on x86 (because of "biggest and most varied user base").

That whole "send SIGBUS even for kernel faults" is certainly bogus and
against the usual rules. And I may well be to blame for it (I have
this memory of disliking how EFAULT as a return code didn't actually
return the faulting address). And realistically, it's also just not
something that any normal application will ever hit.  Giving invalid
addresses to system calls is basically always a bug, although there
are always special software that do all the crazy corner cases (ie
things like emulators tend to do odd things).

I doubt such special software exists on Linux/alpha, though.

So I wouldn't worry about those kinds of oddities overmuch.

*If* somebody then finds a load that cares, we can always fix it
later, and I'll go "mea culpa, I didn't think it would matter, and I
was wrong".

         Linus