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

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Date:	Tue, 03 Dec 2013 15:47:47 -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 02:01 PM, 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.
> 

I think that is true for other CPUs; however, there are definitely CPUs
out there (which Linux supports) for which you have to synchronize the I
and D sides "manually" after writing code through memory, at least
through the CPU.  That is at least one reason why MIPS has a
cacheflush() system call, for example.

> 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.
> 

In other words, treating them as MAP_PRIVATE?  Wouldn't it be better to
throw an error if we can't honor the semantics of the mapping that we
are using?

	-hpa

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