linux-kernel - Re: [Question] mprotect() can't clear PROT

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Message-ID: <aQUEk6a_3OWapRbE@arm.com>
Date: Fri, 31 Oct 2025 18:48:51 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Yang Shi <yang@...amperecomputing.com>
Cc: LAK <linux-arm-kernel@...ts.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [Question] mprotect() can't clear PROT_MTE

Hi Yang,

On Wed, Oct 29, 2025 at 03:41:17PM -0700, Yang Shi wrote:
> Our customers have usecase to untag memory w/o unmapping it, but mprotect
> can't do it. It seems like an intended behavior because I saw MTE doc
> explicitly says PROT_MTE flags can't be cleared by mprotect().
> But I don't see why mprotect() can't do it if I don't miss anything. So I'd
> like to know why it behaves in this way.

It would be interesting to know more about the use-case. At the time,
clearing PROT_MTE got in the way. The theory was that an allocator
controls the tags and the PROT_MTE property but if that range is used by
something like a JIT, toggling between PROT_WRITE and PROT_EXEC would
inadvertently clear PROT_MTE. I'm not sure whether this would happen in
practice though but it's ABI already, so we can't change it.

I'm happy to add support for this if there's a concrete use-case but it
will need to be gated by a prctl() flag to keep the current ABI. A
weirder approach would be to add a PROT_MTE_CLEAR flag (I think I prefer
the prctl).

-- 
Catalin