hi Catalin, On Tue, Jun 15, 2021 at 11:15:15AM +0100, Catalin Marinas wrote: > Hi Rustam, > > On Mon, Jun 14, 2021 at 01:31:14PM -0700, Rustam Kovhaev wrote: > > a) kmemleak scans struct page (kmemleak.c:1462), but it does not scan > > the actual contents (page_address(page)) of the page. > > if we allocate an object with kmalloc(), then allocate page with > > alloc_page(), and if we put kmalloc pointer somewhere inside that page, > > kmemleak will report kmalloc pointer as a false positive. > > should we improve kmemleak and make it scan page contents? > > or will this bring too many false negatives? > > This is indeed on purpose otherwise (1) we'd get a lot of false > negatives and (2) the scanning would take significantly longer. There > are a lot more pages allocated for user processes than used in the > kernel, we don't need to scan them all. > > We do have a few places where we explicitly call kmemleak_alloc(): > neigh_hash_alloc(), alloc_page_ext(), blk_mq_alloc_rqs(), > early_amd_iommu_init(). makes sense, tyvm! > > b) when kmemleak object gets created (kmemleak.c:598) it gets checksum > > of 0, by the time user requests kmemleak "scan" via debugfs the pointer > > will be most likely changed to some value by the kernel and during > > first scan kmemleak won't report the object as orphan even if it did not > > find any reference to it, because it will execute update_checksum() and > > after that will proceed to updating object->count (kmemleak.c:1502). > > and so the user will have to initiate a second "scan" via debugfs and > > only then kmemleak will produce the report. > > should we document this? > > That's a mitigation against false positives. Lot's of objects that get > allocated just prior to a memory scan have a tendency to be reported as > leaks before they get referenced via e.g. a list (and the in-object > list_head structure updated). So you'd need to get the checksum > identical in two consecutive scans to report it as a leak. > > We should probably document this. thanks, i'll send a documentation patch for this
