linux-kernel - Re: [PATCH] arm64: uprobes: Optimize cache flushes for xol slot

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Message-ID: <ZvGRbeAtpAkZVhLt@arm.com>
Date: Mon, 23 Sep 2024 17:03:57 +0100
From: Catalin Marinas <catalin.marinas@....com>
To: Will Deacon <will@...nel.org>
Cc: "Liao, Chang" <liaochang1@...wei.com>, Oleg Nesterov <oleg@...hat.com>,
	mhiramat@...nel.org, peterz@...radead.org, mark.rutland@....com,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64: uprobes: Optimize cache flushes for xol slot

On Mon, Sep 23, 2024 at 08:18:57AM +0100, Will Deacon wrote:
> On Mon, Sep 23, 2024 at 09:57:14AM +0800, Liao, Chang wrote:
> > Unfortunately, the comparison assumes the D-cache and I-cache are already
> > in sync for the slot being copied. But this assumption is flawed if we start
> > with a page with some random bits and D-cache has not been sychronized with
> > I-cache. So, besides __GFP_ZERO, should we have a additional cache flush
> > after page allocation?
> 
> No, I think Oleg's right. The initial cache maintenance will happen when the
> executable pte is installed.

For some reason I had kprobes in mind, did not realise that this page
ends up in user-space. So yes, we have cache maintenance when the pte is
set to point to this page. Subsequent changes will need cache
maintenance.

> However, we should use __GFP_ZERO anyway because I don't think it's a
> good idea to map an uninitialised page into userspace.

Oh, that's not good.

-- 
Catalin