linux-kernel - Re: [RFC v6 27/62] powerpc: helper to validate key-access permissions of a pte

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Message-Id: <20170730003924.GJ5664@ram.oc3035372033.ibm.com>
Date:   Sat, 29 Jul 2017 17:39:24 -0700
From:   Ram Pai <linuxram@...ibm.com>
To:     Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        arnd@...db.de, corbet@....net, mhocko@...nel.org,
        dave.hansen@...el.com, mingo@...hat.com, paulus@...ba.org,
        aneesh.kumar@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        khandual@...ux.vnet.ibm.com
Subject: Re: [RFC v6 27/62] powerpc: helper to validate key-access
 permissions of a pte

On Fri, Jul 28, 2017 at 06:00:02PM -0300, Thiago Jung Bauermann wrote:
> 
> Ram Pai <linuxram@...ibm.com> writes:
> > --- a/arch/powerpc/mm/pkeys.c
> > +++ b/arch/powerpc/mm/pkeys.c
> > @@ -201,3 +201,36 @@ int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot,
> >  	 */
> >  	return vma_pkey(vma);
> >  }
> > +
> > +static bool pkey_access_permitted(int pkey, bool write, bool execute)
> > +{
> > +	int pkey_shift;
> > +	u64 amr;
> > +
> > +	if (!pkey)
> > +		return true;
> > +
> > +	pkey_shift = pkeyshift(pkey);
> > +	if (!(read_uamor() & (0x3UL << pkey_shift)))
> > +		return true;
> > +
> > +	if (execute && !(read_iamr() & (IAMR_EX_BIT << pkey_shift)))
> > +		return true;
> > +
> > +	if (!write) {
> > +		amr = read_amr();
> > +		if (!(amr & (AMR_RD_BIT << pkey_shift)))
> > +			return true;
> > +	}
> > +
> > +	amr = read_amr(); /* delay reading amr uptil absolutely needed */
> 
> Actually, this is causing amr to be read twice in case control enters
> the "if (!write)" block above but doesn't enter the other if block nested
> in it.
> 
> read_amr should be called only once, right before "if (!write)".

the code can be simplified without having to read amr twice.
will fix it.

thanks,
RP

> 
> -- 
> Thiago Jung Bauermann
> IBM Linux Technology Center

-- 
Ram Pai