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

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Date:   Mon, 22 Aug 2016 15:23:06 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     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>,
        "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 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

View attachment "setjmp.patch" of type "text/x-patch" (5950 bytes)