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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 13 Oct 2011 18:19:16 +1200
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Subject: Re: [RFC] fixing the UML failure root cause
To: Andrew Lutomirski <luto@mit.edu>
Cc: Ingo Molnar <mingo@elte.hu>,
        richard -rw- weinberger <richard.weinberger@gmail.com>,
        Adrian Bunk <bunk@stusta.de>, "H. Peter Anvin" <hpa@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        x86@kernel.org, linux-kernel@vger.kernel.org
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On Wed, Oct 12, 2011 at 5:24 AM, Andrew Lutomirski <luto@mit.edu> wrote:
>
> So here's a different approach.  It's not perfect: it always blames
> SEGV_MAPERR instead of SEGV_ACCERR.  I implemented it for vgettimeofday
> but not the other two vsyscalls.
>
> What do you think of this approach?  If it seems good, I'll finish the
> patch and submit it.

I think the approach is valid, but you should *not* do this as some
kind of crazy byte-by-byte copy_to_user() emulation.

Do the "copy tz to user mode" as individual "put_user()" calls for
tv_sec/tv_usec/timezone. IOW, there are three words being written to
user mode, not "two memcpy's".

Other than that, there doesn't seem to be anything wrong.

            Linus