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Date:   Mon, 25 May 2020 00:45:35 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Alexander Potapenko <glider@google.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, sunhaoyl@outlook.com,
        x86@kernel.org
Subject: Re: [PATCH] fs/binfmt_elf.c: allocate initialized memory in
 fill_thread_core_info()
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On Wed, May 13, 2020 at 04:33:49AM +0100, Al Viro wrote:

> FWIW, what I'm going to do is
> 	* make all callers of copy_regset_to_user() pass 0 as pos
> (there are very few exceptions - one on arm64, three on sparc32
> and five on sparc64; I hadn't dealt with arm64 one yet, but all
> cases on sparc are handled)

[snip]

Any of that would be easy to backport, though.  Several questions
regaring XSAVE and friends:

* do we ever run on XSAVE/XSAVES-capable hardware with XFEATURE_FP
turned off?

* is it possible for x86 to have gaps between the state components
area as reported by CPUID 0x0d?  IOW, can area for feature 2
(XFEATURE_YMM) to start *not* at 0x200 and can area for N start
not right after the end of area for N-1 for some N > 2?

I think I have an easy-to-backport solution, but I'm really confused
about XFEATURE_FP situation...