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 18:09:15 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] ARC: uaccess: get_user to zero out dest in cause of fault

On August 20, 2016 6:00:17 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>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

I think the specific name setjmp() is magic in gcc.
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