Re: kmemleak memory scanning

[Catalin Marinas]

On Thu, Jun 24, 2021 at 10:36:50AM -0700, Rustam Kovhaev wrote:
> On Wed, Jun 16, 2021 at 11:25:22AM -0700, Rustam Kovhaev wrote:
> > On Tue, Jun 15, 2021 at 07:15:24AM +0200, Dmitry Vyukov wrote:
> > > On Mon, Jun 14, 2021 at 10:31 PM Rustam Kovhaev <rkovhaev@xxxxxxxxx> wrote:
> > > >
> > > > hello Catalin, Andrew!
> > > >
> > > > while troubleshooting a false positive syzbot kmemleak report i have
> > > > noticed an interesting behavior in kmemleak and i wonder whether it is
> > > > behavior by design and should be documented, or maybe something to
> > > > improve.
> > > > apologies if some of the questions do not make sense, i am still going
> > > > through kmemleak code..
> > > >
> > > > 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?
> > > 
> > > Hi Rustam,
> > > 
> > > Nice debugging!
> > > I assume lots of pages are allocated for slab and we don't want to
> > > scan the whole page if only a few slab objects are alive on the page.
> > > However alloc_pages() can be called by end kernel code as well.
> > > I grepped for any kmemleak annotations around existing calls to
> > > alloc_pages, but did not find any...
> > > Does it require an explicit kmemleak_alloc() after allocating the page
> > > and kmemleak_free () before freeing the page?
> > 
> > hi Dmitry, thank you!
> > yes, as Catalin has pointed out, there are a few places where we call
> > kmemleak_alloc()/kmemleak_free() explicitly in order for the pages to be
> > scanned, like in blk_mq_alloc_rqs()
> > 
> > > If there are more than one use case for this, I guess we could add
> > > some GFP flag for this maybe.
> > 
> > and this way kernel users won't have to use kmemleak fuctions mentioned
> > above including some or most kmemleak_not_leak() calls and basically
> > kmemleak will be kind of "transparent" to them? and they will only need
> > to use the GFP flag to instruct kmemleak to scan the page contents?
> > it sounds like a good idea to me..
> > 
> 
> i've been thinking about this and it seems like in the scenario where we
> want kmemleak to scan only some part of the page, we will have to either
> do separate alloc_page() calls with different flags or use 
> kmemleak_scan_area() to limit the memory scan area. maybe this approach
> won't simplify things and will produce more code instead of reducing it

Since page allocation is not tracked by kmemleak, you can always do an
explicit kmemleak_alloc() call with a smaller size than a full page.

-- 
Catalin