linux-kernel - Re: [PATCH v2 4/4] arm: replace vector mem type with read-only type

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Message-ID: <20201023101257.GZ1551@shell.armlinux.org.uk>
Date:   Fri, 23 Oct 2020 11:12:58 +0100
From:   Russell King - ARM Linux admin <linux@...linux.org.uk>
To:     Miles Chen <miles.chen@...iatek.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Minchan Kim <minchan@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org, wsd_upstream@...iatek.com,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v2 4/4] arm: replace vector mem type with read-only type

On Fri, Oct 23, 2020 at 05:14:37PM +0800, Miles Chen wrote:
> Since kernel no longer writes to the vector, try to replace
> the vector mem type with read-only type and remove L_PTE_MT_VECTORS.
> 
> from Catalin in [1]:
> "
> > I don't think this matters since the kernel no longer writes to the
> > vectors page at run-time but it needs cleaning up a bit (and testing in
> > case I missed something). IOW, do we still need a dedicated mapping type
> > for the vectors or we can simply use the read-only user page attributes?
> "

Catalin is incorrect. If CONFIG_KUSER_HELPERS is enabled, then the
vectors page is definitely written to - it's a user interface, so
it's not going to change:

#ifdef CONFIG_KUSER_HELPERS
                        /*
                         * User space must never try to access this
                         * directly.  Expect your app to break
                         * eventually if you do so.  The user helper
                         * at 0xffff0fe0 must be used instead.  (see
                         * entry-armv.S for details)
                         */
                        *((unsigned int *)0xffff0ff0) = val;
#endif

-- 
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