Re: [PATCH] ARM: add BUILD_BUG_ON to check if fixmap range spans multiple pmds

[Quanyang Wang]

Hi Ard,

On 10/26/21 6:12 PM, Ard Biesheuvel wrote:
> On Tue, 26 Oct 2021 at 11:53, Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx> wrote:
> > Hi,
> > Sorry for the inconvenience.
> >
> > On 10/26/21 4:59 PM, Russell King (Oracle) wrote:
> > > On Sun, Oct 24, 2021 at 11:44:31PM +0200, Linus Walleij wrote:
> > > > On Wed, Oct 20, 2021 at 7:50 AM <quanyang.wang@xxxxxxxxxxxxx> wrote:
> > > >
> > > > > From: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> > > > >
> > > > > Not only the early fixmap range, but also the fixmap range should be
> > > > > checked if it spans multiple pmds. When enabling CONFIG_DEBUG_HIGHMEM,
> > > > > some systems which contain up to 16 CPUs will crash.
> > > > >
> > > > > Signed-off-by: Quanyang Wang <quanyang.wang@xxxxxxxxxxxxx>
> > > >
> > > > Looks reasonable to me.
> > > > Reviewed-by: Linus Walleij <linus.walleij@xxxxxxxxxx>
> > > >
> > > > Please submit this patch into Russell's patch tracker.
> > >
> > > ... and has totally broken what looks like _all_ ARM kernel builds.
> > This patch is intended to trigger build error when it check the value of
> > __end_of_fixmap_region is equal or larger than 256.
>
> Why? The fixmap region is larger than one PMD, so why do we need to cap it?
In __kmap_local_pfn_prot, arch_kmap_local_set_pte(&init_mm, vaddr, 
kmap_pte - idx, pteval) is used to set pteval.
But the ptep is calculated by "kmap_pte - idx", which means all ptes 
must be placed next to each other and no gaps. But for ARM, the ptes for 
the range "0xffe00000~0xfff00000" is not next to the ptes for the range
"0xffc80000~0xffdfffff".

When the idx is larger than 256, virtual address is in 0xffdxxxxx, 
access this address will crash since its pteval isn't set correctly.

> > In fact, it breaks the ARM kernel builds which NR_CPUS is equal or more
> > than 16. If CONFIG_DEBUG_HIGHMEM is enabled, all ARM builds which
> > NR_CPUS is more than 8 will fail.
>
> You really need to be more specific about the failure mode here.
OK, I will be more careful about this.

Thanks,
Quanyang
>