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

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Date:	Tue, 03 Dec 2013 11:20:44 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>
CC:	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 12/03/2013 11:00 AM, Linus Torvalds wrote:
> 
> Yes, on x86, UPROBE_SWBP_INSN_SIZE is a single byte. But quite
> frankly, on x86, exactly *because* it's a single byte, I don't
> understand why we don't just write the damn thing with a single
> "put_user()", and stop with all the idiotic games. No need to
> invalidate caches, even, because if you overwrite the first byte of an
> instruction, it all "just works". Either the instruction decoding gets
> the old one, or it gets the new one. We already rely on that for the
> kernel bp instruction replacement.
> 
> And on non-x86, UPROBE_SWBP_INSN_SIZE is not necessarily 1, so it
> could cross a page boundary. Yes, many architectures will have
> alignment constraints, but I don't see this testing it.
> 
> Whatever. I think that code is bad, and you should feel bad. But hey,
> I think it was pretty bad before too.
> 

I guess it would have to be checked, but I would be *highly* surprised
if UPROBE_SWBP_INSN_SIZE ever[1] could be anything than the fundamental
instruction quantum, which means it should never be able to wrap a page,
but *also* should mean it should be able to just be put_user()'d
followed by whatever synchronization necessary to make it globally visible.

	-hpa

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