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

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Message-ID: <BACAAB8F-2F96-4B94-86C7-1BFA0D258E2B@zytor.com>
Date:   Mon, 22 Aug 2016 16:19:21 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>
CC:     Jakub Jelinek <jakub@...hat.com>,
        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 22, 2016 3:23:06 PM PDT, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>On Sun, Aug 21, 2016 at 10:52 AM, Linus Torvalds
><torvalds@...ux-foundation.org> wrote:
>> On Sat, Aug 20, 2016 at 11:42 PM, Al Viro <viro@...iv.linux.org.uk>
>wrote:
>>>
>>> It's not exactly setjmp/longjmp; what I had in mind was along the
>lines of
>>
>> That ends up having all the exact same issues as setjmp, and
>generally
>> you *do* want the compiler to know about it.
>
>So just in case you wanted to play around with it, here's a kernel
>implementation of 'setjmp/longjmp' for x86.
>
>It's very lightly tested (and I'll admit to editing it for some
>cleanups after that light testing), but it does look largely sane.
>
>The whole interface choice may be debatable: maybe it would be better
>to allocate the register buffer on the stack, and just hide a pointer
>to it in the task struct. Things like that could be changed fairly
>easily. But if you want to play around with this, this patch should
>get you started.
>
>Of course, you'd want to wrap things up somehow, and I would *not*
>want to see naked setjmp() calls in the kernel.
>
>And we'd need this for all other architectures too, but it's usually
>not hard to do. It needs to save all the callee-saved registers and
>the stack pointer and return address. That should generally be it.
>
>The 32-bit version has not been tested at all, but it compiled at some
>point, and the code looks mostly sane. The 64-bit code I actually had
>a stupid non-user-access test-case for.
>
>                Linus

The nice thing about using __builtin_ is that I believe gcc is aware of which registers need saving, and also know that the common path doesn't clobber registers at all.
-- 
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