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

[Miles Chen]

On Fri, 2020-10-23 at 11:12 +0100, Russell King - ARM Linux admin wrote:
> 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
> 

Thanks for the comment. 
We have to keep L_PTE_MT_VECTORS for KUSER_HELPERS.


Miles