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

[Linus Torvalds]

On Sat, Aug 20, 2016 at 5:45 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> 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).

Hmm. I may just be full of sh*t.

I was pretty sure that there used to be a "setjmp" attribute that gcc
used to make sure that "setjmp()" really could return twice, without
bad things happening on the stack.

But looking at the normal user space headers, I see nothing like that. It's just

    extern int setjmp (jmp_buf __env) __THROWNL;

where __THROWNL just sets the __nothrow__ attribute, which shouldn't
even matter in the kernel since we use -fno-exceptions.

So my "setjmp does potentially bad things to the optimization of the
function calling it" seems to have been just some drug-induced fever
dream of mine.

Sorry for the bogus noise. I don't know why I was so convinced setjmp
needed special gcc semantics.

             Linus