On Wed, Jan 19, 2022 at 06:57:34PM +0000, Russell King (Oracle) wrote:
> On Wed, Jan 19, 2022 at 06:01:24PM +0000, Matthew Wilcox wrote:
> > On Wed, Jan 19, 2022 at 05:54:15PM +0000, Russell King (Oracle) wrote:
> > > On Wed, Jan 19, 2022 at 04:27:32PM +0000, Matthew Wilcox wrote:
> > > > On Wed, Jan 19, 2022 at 01:28:14PM +0000, Robin Murphy wrote:
> > > > > > + if (WARN_ON(!pfn_valid(page_to_pfn(page))))
> > > > >
> > > > > Is it page_to_pfn() guaranteed to work without blowing up if page is invalid
> > > > > in the first place? Looking at the CONFIG_SPARSEMEM case I'm not sure that's
> > > > > true...
> > > >
> > > > Even if it does blow up, at least it's blowing up here where someone
> > > > can start to debug it, rather than blowing up on first access, where
> > > > we no longer have the invlid struct page pointer.
> > > >
> > > > I don't think we have a 'page_valid' function which will tell us whether
> > > > a random pointer is actually a struct page or not.
> > >
> > > Isn't it supposed to be:
> > >
> > > if (!pfn_valid(pfn)) {
> > > handle invalid pfn;
> > > }
> > >
> > > page = pfn_to_page(pfn);
> > >
> > > Anything else - even trying to convert an invalid page back to a pfn,
> > > could well be unreliable (sparsemem or discontigmem).
> >
> > This function is passed an array of pages. We have no way of doing
> > what you propose.
>
> You can't go from a struct page to "this is valid", it's too late by the
> time you call vmap() - that's my fundamental point.
Yes, and we have debugging code in __virt_to_phys() that would have
caught this, had Yury enabled CONFIG_DEBUG_VIRTUAL. My point is that
in this instance, page_to_pfn() doesn't crash, which lets vmap() set
up a mapping to a completely bogus physical address. We're better
off checking pfn_valid() here than not.
> If the translation from a PFN to a struct page can return pointers to
> something that isn't a valid struct page, then it can also (with
> sparsemem) return a pointer to _another_ struct page that could well
> be valid depending on how the struct page arrays are laid out in
> memory.
Sure, it's not going to catch everything. But I don't think that
letting perfect be the enemy of the good here is the right approach.
