linux-kernel - Re: [PATCH v5 3/8] arm: fixmap: implement __set

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Date:	Thu, 4 Sep 2014 18:27:48 +0100
From:	Will Deacon <will.deacon@....com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>,
	Laura Abbott <lauraa@...eaurora.org>,
	Rob Herring <robh@...nel.org>,
	Leif Lindholm <leif.lindholm@...aro.org>,
	"msalter@...hat.com" <msalter@...hat.com>,
	Liu hua <sdu.liu@...wei.com>,
	Nikolay Borisov <Nikolay.Borisov@....com>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Doug Anderson <dianders@...gle.com>,
	Jason Wessel <jason.wessel@...driver.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v5 3/8] arm: fixmap: implement __set_fixmap()

On Thu, Sep 04, 2014 at 06:23:42PM +0100, Kees Cook wrote:
> On Thu, Sep 4, 2014 at 10:03 AM, Will Deacon <will.deacon@....com> wrote:
> > Hi Kees,
> >
> > On Wed, Sep 03, 2014 at 10:57:04PM +0100, Kees Cook wrote:
> >> This is used from set_fixmap() and clear_fixmap() via asm-generic/fixmap.h.
> >> Also makes sure that the fixmap allocation fits into the expected range.
> >>
> >> Based on patch by Rabin Vincent.
> >
> > [...]
> >
> >> +void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
> >> +{
> >> +     unsigned long vaddr = __fix_to_virt(idx);
> >> +     pte_t *pte = pte_offset_kernel(pmd_off_k(vaddr), vaddr);
> >> +
> >> +     /* Make sure fixmap region does not exceed available allocation. */
> >> +     BUILD_BUG_ON(FIXADDR_START + (__end_of_fixed_addresses * PAGE_SIZE) >
> >> +                  FIXADDR_END);
> >> +     BUG_ON(idx >= __end_of_fixed_addresses);
> >> +
> >> +     if (pgprot_val(prot))
> >> +             set_pte_at(NULL, vaddr, pte,
> >> +                     pfn_pte(phys >> PAGE_SHIFT, prot));
> >> +     else
> >> +             pte_clear(NULL, vaddr, pte);
> >> +
> >> +     /*
> >> +      * Given the potential a15 tlbi errata, we can only do tlb flushes
> >> +      * with interrupts disabled. Callers must have taken care of this.
> >> +      */
> >> +     WARN_ON_ONCE(!irqs_disabled());
> >> +     flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE);
> >
> > Aha, this explains why we were confusing each other! The issue is that
> > interrupts must be *enabled*, so this code does the exact opposite of
> > what we need.
> >
> > I think this got lost in a sea of double negatives during the last round
> > of review.
> 
> Ah! If this is the case, perhaps we can get away with
> local_flush_tlb_kernel_range() then?

That's a bit tricky, since you need to ensure that preemption is disabled
until the mapping is put back like it was.

Will
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