Re: [PATCH v2 0/4] Reference count checker and related fixes

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Fri, Jan 28, 2022 at 10:26:20AM -0800, Ian Rogers escreveu:
> On Fri, Jan 28, 2022 at 7:35 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Thu, 27 Jan 2022 22:24:59 -0800
> > Ian Rogers <irogers@google.com> wrote:
> >
> > > On Thu, Jan 27, 2022 at 9:24 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > >
> > > > On Thu, 27 Jan 2022 13:33:23 -0800
> > > > Ian Rogers <irogers@google.com> wrote:
> > > >
> > > > > On Tue, Jan 25, 2022 at 12:46 PM Ian Rogers <irogers@google.com> wrote:
> > > > > >
> > > > > > This v2 patch set has the main reference count patch for cpu map from
> > > > > > the first set and then adds reference count checking to nsinfo. The
> > > > > > reference count checking on nsinfo helped diagnose a data race bug
> > > > > > which is fixed in the independent patches 2 and 3.
> > > > > >
> > > > > > The perf tool has a class of memory problems where reference counts
> > > > > > are used incorrectly. Memory/address sanitizers and valgrind don't
> > > > > > provide useful ways to debug these problems, you see a memory leak
> > > > > > where the only pertinent information is the original allocation
> > > > > > site. What would be more useful is knowing where a get fails to have a
> > > > > > corresponding put, where there are double puts, etc.
> > > > > >
> > > > > > This work was motivated by the roll-back of:
> > > > > > https://lore.kernel.org/linux-perf-users/20211118193714.2293728-1-irogers@google.com/
> > > > > > where fixing a missed put resulted in a use-after-free in a different
> > > > > > context. There was a sense in fixing the issue that a game of
> > > > > > wac-a-mole had been embarked upon in adding missed gets and puts.
> > > > > >
> > > > > > The basic approach of the change is to add a level of indirection at
> > > > > > the get and put calls. Get allocates a level of indirection that, if
> > > > > > no corresponding put is called, becomes a memory leak (and associated
> > > > > > stack trace) that leak sanitizer can report. Similarly if two puts are
> > > > > > called for the same get, then a double free can be detected by address
> > > > > > sanitizer. This can also detect the use after put, which should also
> > > > > > yield a segv without a sanitizer.
> > > > > >
> > > > > > Adding reference count checking to cpu map was done as a proof of
> > > > > > concept, it yielded little other than a location where the use of get
> > > > > > could be cleaner by using its result. Reference count checking on
> > > > > > nsinfo identified a double free of the indirection layer and the
> > > > > > related threads, thereby identifying a data race as discussed here:
> > > > > > https://lore.kernel.org/linux-perf-users/CAP-5=fWZH20L4kv-BwVtGLwR=Em3AOOT+Q4QGivvQuYn5AsPRg@mail.gmail.com/
> > > > > > Accordingly the dso->lock was extended and use to cover the race.
> > > > > >
> > > > > > An alternative that was considered was ref_tracker:
> > > > > >  https://lwn.net/Articles/877603/
> > > > > > ref_tracker requires use of a reference counted struct to also use a
> > > > > > cookie/tracker. The cookie is combined with data in a ref_tracker_dir
> > > > > > to spot double puts. When an object is finished with leaks can be
> > > > > > detected, as with this approach when leak analysis happens. This
> > > > > > approach was preferred as it doesn't introduce cookies, spots use
> > > > > > after put and appears moderately more neutral to the API. Weaknesses
> > > > > > of the implemented approcah are not being able to do adhoc leak
> > > > > > detection and a preference for adding an accessor API to structs. I
> > > > > > believe there are other issues and welcome suggestions.
> > > > >
> > > > > And so we've been here before (Dec 2015 to be exact). Namhyung pointed me to:
> > > > > https://lore.kernel.org/all/20151209021047.10245.8918.stgit@localhost.localdomain/
> > > > > by Masami Hiramatsu. In this work he adds a leak sanitizer style
> > > > > reference count checker that will describe locations of puts and gets
> > > > > for diagnosis. Firstly that's an awesome achievement! This work is
> > > > > different in that it isn't trying to invent a leak sanitizer, it is
> > > > > just using the existing one. By adding a level of indirection this
> > > > > work can catch use after put and pairs gets with puts to make lifetime
> > > > > analysis more automatic. An advantage of Masami's work is that it
> > > > > doesn't change data-structures and after the initial patch-set is
> > > > > somewhat transparent. Overall I prefer the approach in these patches,
> > > > > future patches can look to clean up the API as Masami has.
> > > >
> > > > Thanks for referring my series :-D The series aimed to solve the refcount
> > > > usage issue in the perf which lead the object leaks. At that moment,
> > > > I found that there were 2 patterns, refcount start from 0 and start from 1.
> > > > That made me confused what I should do for using a object.
> > > > But the perf uses linux/refcount.h now, I hope such issue has already gone.
> > > > (but the object leakage seems not fixed fully yet, as you found.)
> > > >
> > > > BTW, I think the introducing UNWRAP_* macro may put a burden on future
> > > > development. If it is inevitable, we should consider it as carefully as
> > > > possible. Or, it may cause another issue (it is easily missed that the new
> > > > patch does not use UNWRAP_* for object reference, because it is natual.)
> > > >
> > > > So I agree with you that you to clean up the API. :-)
> > > > I think we can make yet another refcount.h for user space debugging and
> > > > replace it with the linux/refcount.h.
> > >
> > > Thanks Masami,
> > >
> > > Agreed on the UNWRAP_ macros, hence wanting to hide them behind
> > > accessors. Making accessors could be automated with macros, for
> > > example, have a list of variables, have a macro declare the struct
> > > using the list, another macro can use the list to declare accessors. I
> > > didn't find adding the UNWRAP_ macros in this change particularly
> > > burdensome as any use of the wrapping pointer as the original type
> > > caused a compile time error telling you what and where to fix. The
> > > macro is extra stuff in the way of using just the raw object, but
> > > that's fairly typical in C++ with shared_ptr, scoped_lock, etc.
> >
> > Hi Ian,
> >
> > Hmm, but such a macro is not usual for C which perf is written in.
> > If I understand correctly, you might want to use memory leak
> > analyzer to detect refcount leak, and that analyzer will show
> > what data structure is leaked.
> 
> Firstly, thanks for the conversation - this is really useful to
> improve the code!
> 
> I think in an ideal world we'd somehow educate things like address
> sanitizer of reference counted data structures and they would do a
> better job of tracking gets and puts. The problem is pairing gets and
> puts. In C++ you use RAII types so that the destructor ensures a put -
> this can be complex when using data types like lists where you want to
> move or swap things onto the list, to keep the single pointer
> property. In the C code in Linux we use gotos, similarly to how defer
> is used in Go. Anyway, the ref_tracker that Eric Dumazet added solved
> the get/put pairing problem by adding a cookie that is passed around.
> The problem with that is that then the cookie becomes part of the API.
> To avoid that the approach here is just to change the original data
> type and add in a layer of indirection, that layer has become the
> cookie. A benefit of this approach is that once the cookie/indirection
> is freed, use of it becomes an obvious failure - we leverage address
> sanitizer for use after free.
> 
> > If so, maybe you can do the same thing by introducing a dummy
> > list node for each data structure which you want to debug.
> >
> > struct perf_cpu_map__refdebug {
> >         struct perf_cpu_map__refdebug *orig;
> > };
> >
> > And expand refcount_t as.
> >
> > typedef struct refcount_struct {
> >         atomic_t refs;
> > #ifdef REFCNT_CHECKING
> >         void *orig;
> > #endif
> > } refcount_t;
> >
> > And change the get/put as below
> >
> > struct perf_cpu_map *perf_cpu_map__get(struct perf_cpu_map *map)
> > {
> >         if (map) {
> > #ifdef REFCNT_CHECKING
> >                 struct perf_cpu_map__refdebug *new_node;
> > #endif
> >                 refcount_inc(&map->refcnt);
> > #ifdef REFCNT_CHECKING
> >                 new_node = malloc(sizeof(*new_node));
> >                 new_node->orig = map->refcnt->orig;
> >                 map->refcnt->orig = new_node;
> > #endif
> >         }
> >         return map;
> > }
> >
> > void perf_cpu_map__put(struct perf_cpu_map *map)
> > {
> >         if (map) {
> >                 if (refcount_dec_and_test(&map->refcnt))
> >                         cpu_map__delete(map);
> >                 else {
> > #ifdef REFCNT_CHECKING
> >                         struct perf_cpu_map__refdebug *node = map->refcnt->orig;
> >
> >                         map->refcnt->orig = node->orig;
> >                         free(node);
> > #endif
> >                 }
> >         }
> > }
> >
> > This need a bit complex get/put, but no need to change other parts.
> 
> Adding a list like this gives an ability to say something like of the
> current reference count of 3 what indirection objects exist. This
> could be useful for diagnosis but you probably want to pair it with a
> stack trace, and the approach here is punting that problem to the
> address/leak sanitizer. I'm also concerned that there should be a lock
> around the list. I think pursuing this ends up with something like
> ref_tracker.
> 
> If we're using indirection, as in my proposal, then adding a common
> indirection struct is problematic as anything declared to be a "struct
> cpumap" now needs to be either the indirection or the original type -
> hence using macros to hide that in the code. If we embed the
> information into the refcount_t then we end up with something like
> ref_tracker, API problems and losing use-after-put checking. Outside
> of the macros, I think there is a simplicity to the approach I've put
> forward.
> 
> > > The
> > > question is, is it worth it to make sure use of the reference counted
> > > object is correct and misuse is easier to diagnose?
> >
> > You mean the stackdump for every get/put as I did? That's a good
> > question. Let's think what may happen.
> >
> > For example, if funcA() expects its caller funcB() will put the object
> > but actually funcB() doesn't, or the funcC() which is the another
> > caller of funcA()) doesn't expect the funcA() gets the object.
> >
> > funcA() {
> >         get(obj);
> >         return obj;
> > }
> >
> > funcB() {
> >         obj = funcA();
> >         ...
> >         // wrong! it should do put(obj);
> > }
> >
> > funcC() {
> >         obj = funcA();
> >         get(obj);               // this is wrong get().
> >         ...
> >         put(obj);
> > }
> >
> > If we just list the non-released object, both logs seems same because
> > funcB()'s get/put pair will be skipped. If the analyzer shows the
> > stacktrace when the object was got, maybe we can notice the difference
> > of funcB() and funcC() path, but this is the simplest case. funcA()
> > can be called from funcB/C via several different functions.
> > But perhaps I'm too worried.
> 
> So in the logs we should see for funcB:
> 
> Memory leak of ... at:
> malloc...
> get...
> funcA
> funcB
> ...
> 
> as the put on the indirection object was missed and this is now a leak
> of the indirection object. For funcC we should see:
> 
> Memory leak of ... at:
> malloc..
> get..
> funcA
> funcC
> 
> So from the stack traces we can see that there is an unpaired get
> happening in funcA called from either funcB and funcC, which means we
> need to a put there. In the funcC case we can see the put was missed
> from a call to funcA, rather than a get it made.
> 
> As the code in perf is complex, multi-threaded and sometimes
> unintentionally racy a get may happen on 1 thread, the object is
> placed in a global, the object is put by another thread and also
> accessed by a 3rd thread. This is what was happening in the
> dso->nsinfo case. The bug there is that there was a double put
> happening by the third thread because of a race. Leak sanitizer treats
> memory visible from a global as not a leak, this can mean to get the
> most information on leaks in perf we need to aggressively
> free/delete/deconstruct when terminating so that leaks become visible.
> This feels to me like good hygiene, but it could also be argued to be
> a tax.

We can have it perfect, i.e. freeing everything but then leaving that
disabled in !DEBUG builds so that the exit time is kept fast.
 
> Anyway, I think I'm still at the same point I was when I posted these
> changes. That an indirection object is the simplest, smallest,
> cleanest way to get the most information. I think making the rest of
> the reference counted data structures have this feature would be
> great, so I'd like to merge the 4 patches here and work to add more. I
> think we can also build on that foundation for extra debug
> information.
> 
> Thanks,
> Ian
> 
> > Thank you,
> >
> > > I think it is near
> > > as least offensive as possible while providing the best information -
> > > hence being able to solve the dso->nsinfo put data race, that has been
> > > a problem to solve up to this point. I'm open to better suggestions
> > > though :-)
> > >
> > > Thanks again,
> > > Ian
> > >
> > > > Thank you,
> > > >
> > > > --
> > > > Masami Hiramatsu <mhiramat@kernel.org>
> >
> >
> > --
> > Masami Hiramatsu <mhiramat@kernel.org>

-- 

- Arnaldo