Re: [PATCH] gup: return -EFAULT on access_ok failure

[Michael S. Tsirkin]

On Thu, Apr 05, 2018 at 08:40:05AM -0700, Linus Torvalds wrote:
> On Thu, Apr 5, 2018 at 7:17 AM, Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> > I wonder however whether all the following should be changed then:
> >
> > static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
> >
> > ...
> >
> >                         if (!vma || check_vma_flags(vma, gup_flags))
> >                                 return i ? : -EFAULT;
> >
> > is this a bug in __get_user_pages?
> 
> Note the difference between "get_user_pages()", and "get_user_pages_fast()".
> 
> It's the *fast* versions that just return the number of pages pinned.
> 
> The non-fast ones will return an error code for various cases.
> 
> Why?
> 
> The non-fast cases actually *have* various error cases. They can block
> and get interrupted etc.
> 
> The fast cases are basically "just get me the pages, dammit, and if
> you can't get some page, stop".
> 
> At least that's one excuse for the difference in behavior.
> 
> The real excuse is probably just "that's how it worked" - the fast
> case just walked the page tables and that was it.
> 
>                  Linus

I see, thanks for the clarification Linus.

to repeat what you are saying IIUC __get_user_pages_fast returns 0 if it can't
pin any pages and that is by design.  Returning 0 on error isn't usual I think
so I guess this behaviour should we well documented.

That part of my patch was wrong and should be replaced with a doc
update.

What about get_user_pages_fast though? That's the other part of the
patch. Right now get_user_pages_fast does:

                ret = get_user_pages_unlocked(start, nr_pages - nr, pages,
                                write ? FOLL_WRITE : 0);

                /* Have to be a bit careful with return values */
                if (nr > 0) {
                        if (ret < 0)
                                ret = nr;
                        else
                                ret += nr;
                }

so an error on the 1st page gets propagated to the caller,
and that get_user_pages_unlocked eventually calls __get_user_pages
so it does return an error sometimes.

Would it be correct to apply the second part of the patch then
(pasted below for reference) or should get_user_pages_fast
and all its callers be changed to return 0 on error instead?

@@ -1806,9 +1809,12 @@ int get_user_pages_fast(unsigned long start, int nr_pages, int write,
 	len = (unsigned long) nr_pages << PAGE_SHIFT;
 	end = start + len;
 
+	if (nr_pages <= 0)
+		return 0;
+
 	if (unlikely(!access_ok(write ? VERIFY_WRITE : VERIFY_READ,
 					(void __user *)start, len)))
-		return 0;
+		return -EFAULT;
 
 	if (gup_fast_permitted(start, nr_pages, write)) {
 		local_irq_disable();

-- 
MST