Re: [PATCH v4 08/15] mm: Add unsafe_follow_pfn

[Daniel Vetter]

On Thu, Oct 29, 2020 at 9:56 AM Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
>
> > +int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
> > +     unsigned long *pfn)
>
> The one tab indent here looks weird, normally tis would be two tabs
> or aligned aftetthe opening brace.
>
> > +{
> > +#ifdef CONFIG_STRICT_FOLLOW_PFN
> > +     pr_info("unsafe follow_pfn usage rejected, see CONFIG_STRICT_FOLLOW_PFN\n");
> > +     return -EINVAL;
> > +#else
> > +     WARN_ONCE(1, "unsafe follow_pfn usage\n");
> > +     add_taint(TAINT_USER, LOCKDEP_STILL_OK);
> > +
> > +     return follow_pfn(vma, address, pfn);
> > +#endif
>
> Woudn't this be a pretty good use case of "if (IS_ENABLED(...)))"?
>
> Also I'd expect the inverse polarity of the config option, that is
> a USAFE_FOLLOW_PFN option to enable to unsafe behavior.

Was just about to send out v5, will apply your suggestions for that
using IS_ENABLED.

Wrt negative or positive Kconfig, I was following STRICT_DEVMEM symbol
as precedence. But easy to invert if there's strong feeling the other
way round, I'm not attached to either.

> > +/**
> > + * unsafe_follow_pfn - look up PFN at a user virtual address
> > + * @vma: memory mapping
> > + * @address: user virtual address
> > + * @pfn: location to store found PFN
> > + *
> > + * Only IO mappings and raw PFN mappings are allowed.
> > + *
> > + * Returns zero and the pfn at @pfn on success, -ve otherwise.
> > + */
> > +int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
> > +     unsigned long *pfn)
> > +{
> > +     return follow_pfn(vma, address, pfn);
> > +}
> > +EXPORT_SYMBOL(unsafe_follow_pfn);
>
> Any reason this doesn't use the warn and disable logic?

I figured without an mmu there's not much guarantees anyway. But I
guess I can put it in here too for consistency.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch