Re: [PATCH] vmap(): don't allow invalid pages

[Matthew Wilcox]

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.