Hi Mark,
On Mon, Jul 13, 2020 at 2:21 PM Mark Rutland <mark.rutland@xxxxxxx> wrote:
> On Fri, Jul 10, 2020 at 03:57:05PM +0200, Christoph Hellwig wrote:
> > Add helpers to wraper the get_fs/set_fs magic for undoing any damange
> > done by set_fs(KERNEL_DS). There is no real functional benefit, but this
> > documents the intent of these calls better, and will allow stubbing the
> > functions out easily for kernels builds that do not allow address space
> > overrides in the future.
>
> > diff --git a/arch/m68k/include/asm/tlbflush.h b/arch/m68k/include/asm/tlbflush.h
> > index 191e75a6bb249e..30471549e1e224 100644
> > --- a/arch/m68k/include/asm/tlbflush.h
> > +++ b/arch/m68k/include/asm/tlbflush.h
> > @@ -13,13 +13,13 @@ static inline void flush_tlb_kernel_page(void *addr)
> > if (CPU_IS_COLDFIRE) {
> > mmu_write(MMUOR, MMUOR_CNL);
> > } else if (CPU_IS_040_OR_060) {
> > - mm_segment_t old_fs = get_fs();
> > - set_fs(KERNEL_DS);
> > + mm_segment_t old_fs = force_uaccess_begin();
> > +
>
> This used to set KERNEL_DS, and now it sets USER_DS, which looks wrong
> superficially.
Thanks for noticing, and sorry for missing that myself.
The same issue is present for SuperH:
- set_fs(KERNEL_DS);
+ oldfs = force_uaccess_begin();
So the patch description should be:
"Add helpers to wraper the get_fs/set_fs magic for undoing any damage
done by set_fs(USER_DS)."
and leave alone users setting KERNEL_DS?
> If the new behaviour is fine it suggests that the old behaviour was
> wrong, or that this is superfluous and could go entirely.
>
> Geert?
Nope, on m68k, TLB cache operations operate on the current address space.
Hence to flush a kernel TLB entry, you have to switch to KERNEL_DS first.
If we're guaranteed to be already using KERNEL_DS, I guess the
address space handling can be removed. But can we be sure?
> > __asm__ __volatile__(".chip 68040\n\t"
> > "pflush (%0)\n\t"
> > ".chip 68k"
> > : : "a" (addr));
> > - set_fs(old_fs);
> > + force_uaccess_end(old_fs);
> > } else if (CPU_IS_020_OR_030)
> > __asm__ __volatile__("pflush #4,#4,(%0)" : : "a" (addr));
>
> > +/*
> > + * Force the uaccess routines to be wired up for actual userspace access,
> > + * overriding any possible set_fs(KERNEL_DS) still lingering around. Undone
> > + * using force_uaccess_end below.
> > + */
> > +static inline mm_segment_t force_uaccess_begin(void)
> > +{
> > + mm_segment_t fs = get_fs();
> > +
> > + set_fs(USER_DS);
> > + return fs;
> > +}
> > +
> > +static inline void force_uaccess_end(mm_segment_t oldfs)
> > +{
> > + set_fs(oldfs);
> > +}
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
