Re: [PATCH] ARC: uaccess: get_user to zero out dest in cause of fault

[Linus Torvalds]

On Sat, Aug 20, 2016 at 5:11 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> Interesting...  BTW, how's this in the "really vile tricks" department?
>
>         if (!uaccess_begin())
>                 goto fail;

So I slightly considered it, because gcc actually has support for that
kind of behavior thanks to setjmp/longjmp (and yes, the compiler
actually needs to know about the magic "this code can be entered a
second time from elsewhere" - it _used_ to be purely a library thing
back in the days of stupid compilers, but no more).

And I'm not saying it's wrong, but I'm not a huge fan of
setjmp/longjmp. Afaik it tends to make gcc generate potentially much
worse code in the function that uses setjmp.

That said, you have a really strong argument that I hadn't even thought about:

> AFAICS, it should avoid the problems with asm goto, right?

Yes. That was something I never even thought about. I just thought
"asm goto has some limitations, but they aren't _fundamental_, so
hopefully they get fixed". But they may not be fundamental, but it
will take a long time. If ever.

And you're right, using setjmp semantics would avoid all that and
"just work". Even for "get_user()" that needs to return a value.

Hmm.

You have to save the stack pointer at the setjmp point too. And there
might be other architecture-specific ABI rules for that. But you're
right, it might be worth it.

I *would* be a bit worried about code generation issues.
setjmp/longjmp is so seldom used that it's one of those things where
it might be best to verify with some gcc person that it doesn't cause
huge code-gen problems.

Adding Jakub just to check: Jakub, would a setjump/longjump kind of
interface for exception handling going to cause us problems
(performance or correctness) with gcc?

            Linus