On Mon, Apr 24, 2023 at 03:49:04PM +0200, Dmitry Vyukov wrote: > On Mon, 24 Apr 2023 at 15:21, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Mon, Apr 24, 2023 at 09:38:43AM +0200, Dmitry Vyukov wrote: > > > On Mon, 24 Apr 2023 at 09:19, syzbot > > > <syzbot+606f94dfeaaa45124c90@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > If I am reading this correctly, it can lead to NULL derefs in > > > folio_mapping() if folio->mapping is read twice. I think > > > folio->mapping reads/writes need to use READ/WRITE_ONCE if racy. > > > > You aren't reading it correctly. > > > > mapping = folio->mapping; > > if ((unsigned long)mapping & PAGE_MAPPING_FLAGS) > > return NULL; > > > > return mapping; > > > > The racing write is storing NULL. So it might return NULL or it might > > return the old mapping, or it might return NULL. Either way, the caller > > has to be prepared for NULL to be returned. > > > > It's a false posiive, but probably worth silencing with a READ_ONCE(). > > Yes, but the end of the function does not limit effects of races. I I thought it did. I was under the impression that the compiler was not allowed to extract loads from within the function and move them outside. Maybe that changed since C99. > to this: > > if (!((unsigned long)folio->mapping & PAGE_MAPPING_FLAGS) && folio->mapping) > if (test_bit(AS_UNEVICTABLE, &folio->mapping->flags)) > > which does crash. Yes, if the compiler is allowed to do that, then that's a possibility.
