On (07/04/16 14:29), Joonsoo Kim wrote: > > > On Sun, Jul 03, 2016 at 01:16:56AM +0900, Sergey Senozhatsky wrote: > > > > Introduce PAGE_OWNER_TRACK_FREE config option to extend page owner with > > > > free_pages() tracking functionality. This adds to the dump_page_owner() > > > > output an additional backtrace, that tells us what path has freed the > > > > page. > > > > > > Hmm... Do you have other ideas to use this feature? Following example is > > > just to detect use-after-free and we have other good tools for it > > > (KASAN or DEBUG_PAGEALLOC) so I'm not sure whether it's useful or not. > > > > there is no kasan for ARM32, for example (apart from the fact that > > it's really hard to use kasan sometimes due to its cpu cycles and > > memory requirements). > > Hmm... for debugging purpose, KASAN provides many more things so IMO it's > better to implement/support KASAN in ARM32 rather than expand > PAGE_OWNER for free. >
hm, the last time I checked kasan didn't catch that extra put_page() on x86_64. AFAIK, kasan on ARM32 is a bit hard to do properly http://www.serverphorums.com/read.php?12,1206479,1281087#msg-1281087 I've played with kasan on arm32 (an internal custom version)... and extended page_owner turned out to be *incomparably* easier and faster to use (especially paired with stackdepot). > > educate me, will DEBUG_PAGEALLOC tell us what path has triggered the > > extra put_page()? hm... does ARM32 provide ARCH_SUPPORTS_DEBUG_PAGEALLOC? > > Hmm... Now, I notice that PAGE_OWNER_TRACK_FREE will detect > double-free rather than use-after-free. well, yes. current hits bad_page(), page_owner helps to find out who stole and spoiled it from under current. CPU a CPU b alloc_page() put_page() << legitimate alloc_page() err: put_page() << legitimate, again. << but is actually buggy. put_page() << double free. but we need << to report put_page() from << CPU a. -ss > DEBUG_PAGEALLOC doesn't catch double-free but it can be implemented > easily. In this case, we can show call path for second free. > > AFAIK, ARM32 doesn't support ARCH_SUPPORTS_DEBUG_PAGEALLOC. > > Thanks. >
