Re: [PATCH v1] fsnotify: Pass correct offset to fsnotify_mmap_perm()

[Kiryl Shutsemau]

On Fri, Oct 03, 2025 at 05:36:23PM +0100, Ryan Roberts wrote:
> On 03/10/2025 17:00, Kiryl Shutsemau wrote:
> > On Fri, Oct 03, 2025 at 04:52:36PM +0100, Ryan Roberts wrote:
> >> fsnotify_mmap_perm() requires a byte offset for the file about to be
> >> mmap'ed. But it is called from vm_mmap_pgoff(), which has a page offset.
> >> Previously the conversion was done incorrectly so let's fix it, being
> >> careful not to overflow on 32-bit platforms.
> >>
> >> Discovered during code review.
> > 
> > Heh. Just submitted fix for the same issue:
> > 
> > https://lore.kernel.org/all/20251003155804.1571242-1-kirill@xxxxxxxxxxxxx/T/#u
> > 
> 
> Ha... great minds...
> 
> I notice that for your version you're just doing "pgoff << PAGE_SHIFT" without
> casting pgoff.
> 
> I'm not sure if that is safe?
> 
> pgoff is unsigned long (so 32 bits on 32 bit systems). loff_t is unsigned long
> long (so always 64 bits). So is it possible that you shift off the end of 32
> bits and lose those bits without a cast to loff_t first?
> 
> TBH my knowledge of the exact rules is shaky...

I think you are right. Missing cast in my patch might be problematic on
32-bit machines.

Reviewed-by: Kiryl Shutsemau <kas@xxxxxxxxxx>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov