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

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Message-ID: <20131204111125.GA26552@redhat.com>
Date:	Wed, 4 Dec 2013 12:11:25 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.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 12/03, Linus Torvalds wrote:
>
> On Tue, Dec 3, 2013 at 12:54 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > So do you think the patch I sent is wrong? Why?
>
> I think the TLB shootdown should guarantee that it's ok on other
> CPU's, since that's basically what we do on mmap.

OK, thanks. I'll resend this patch.

It is still not clear to me if we can simply change a single byte on
x86 or not, but at least on powerpc we need to update 4 bytes. Perhaps
we can conditionalize these pte games later.

> But looking closer at this, I think I see why the old code did what it
> did. I think it's breaking shared mmap pages on purpose rather than
> dirtying them. Which is probably the right thing to do.

Ah, no. uprobes never writes to the shared pages. (hmm, it seems that
VM_SHARED check is buggy, but this is offtopic). Otherwise this patch
would be very wrong.

Oleg.

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