Messages in this thread | | | From | Geert Uytterhoeven <> | Date | Mon, 13 Jul 2020 15:19:42 +0200 | Subject | Re: [PATCH 5/6] uaccess: add force_uaccess_{begin,end} helpers |
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Hi Mark,
On Mon, Jul 13, 2020 at 2:21 PM Mark Rutland <mark.rutland@arm.com> 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@linux-m68k.org
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
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