linux-kernel - Re: [PATCH?] uprobes: change uprobe_write

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Message-ID: <CA+55aFyMx74jiKgS-o0axuuTBw7295UEMSf_vYDQCAghk3Vr=w@mail.gmail.com>
Date:	Wed, 4 Dec 2013 09:15:04 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Oleg Nesterov <oleg@...hat.com>, Hugh Dickins <hughd@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Jiri Kosina <jkosina@...e.cz>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH?] uprobes: change uprobe_write_opcode() to modify the page directly

On Wed, Dec 4, 2013 at 8:54 AM, H. Peter Anvin <hpa@...or.com> wrote:
>
> That is why I talk about the atomic instruction word... most (but not
> *all*) architectures have a fundamental minimum unit of instructions
> which is aligned and can be atomically written.  Typically this is 1, 2,
> or 4 bytes.

Note that it's not just about the "atomically written", it's also
about the guarantee that it's atomically *read*.

x86 can certainly atomically write a 4-byte instruction too, it's just
that there's no guarantee - even if the instruction is aligned etc -
that the actual instruction decoding always ends up reading it that
way. It might re-read an instruction after encountering a prefix byte
etc etc. So even if it's all properly aligned, the reading side might
do something odd.

            Linus
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