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

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Date:   Sat, 20 Aug 2016 18:00:17 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>, Jakub Jelinek <jakub@...hat.com>
Cc:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] ARC: uaccess: get_user to zero out dest in cause of fault

On Sat, Aug 20, 2016 at 5:45 PM, Linus Torvalds
<torvalds@...ux-foundation.org> 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