Messages in this thread | | | From | Linus Torvalds <> | Date | Sat, 20 Aug 2016 17:45:00 -0700 | Subject | Re: [PATCH] ARC: uaccess: get_user to zero out dest in cause of fault |
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On Sat, Aug 20, 2016 at 5:11 PM, Al Viro <viro@zeniv.linux.org.uk> 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
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