linux-kernel - Re: [PATCH 0/9] tools subsystem refcounter conversions

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Date:   Thu, 23 Feb 2017 09:50:08 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     "Reshetova, Elena" <elena.reshetova@...el.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "alsa-devel@...a-project.org" <alsa-devel@...a-project.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "alexander.shishkin@...ux.intel.com" 
        <alexander.shishkin@...ux.intel.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "matija.glavinic-pecotic.ext@...ia.com" 
        <matija.glavinic-pecotic.ext@...ia.com>
Subject: Re: [PATCH 0/9] tools subsystem refcounter conversions

Em Thu, Feb 23, 2017 at 11:39:10AM +0000, Reshetova, Elena escreveu:
> > Em Wed, Feb 22, 2017 at 08:23:29PM -0300, Arnaldo Carvalho de Melo
> > escreveu:
> > > Em Tue, Feb 21, 2017 at 12:39:35PM -0300, Arnaldo Carvalho de Melo
> > escreveu:
> > > > Em Tue, Feb 21, 2017 at 05:34:54PM +0200, Elena Reshetova escreveu:
> > > > > Now when new refcount_t type and API are finally merged
> > > > > (see include/linux/refcount.h), the following
> > > > > patches convert various refcounters in the tools susystem from atomic_t
> > > > > to refcount_t. By doing this we prevent intentional or accidental
> > > > > underflows or overflows that can led to use-after-free vulnerabilities.
> > > >
> > > > Thanks for working on this! I was almost going to jump on doing this
> > > > myself!
> > > >
> > > > I'll try and get this merged ASAP.
> > >
> > > So, please take a look at my tmp.perf/refcount branch at:
> > >
> > > git://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git
> 
> I took a look on it and it looks good. Just one thing I want to double check with regards to this commit:
> https://kernel.googlesource.com/pub/scm/linux/kernel/git/acme/linux/+/58d561002587bf2572f9e6f4d222659e4068fadf%5E%21/#F0
> 
> And more specifically to this chunk:
> 
> @@ -937,7 +937,7 @@
>  		munmap(map->base, perf_mmap__mmap_len(map));
>  		map->base = NULL;
>  		map->fd = -1;
> -		atomic_set(&map->refcnt, 0);
> +		refcount_set(&map->refcnt, 0);
>  	}
>  	auxtrace_mmap__munmap(&map->auxtrace_mmap);
>  }
 
> So, when the refcount set to zero in this place, what exactly happens
> to the perf_map object after?  I just want to double check that we
> don't have  another hiding reusage case here when refcounter later on
> is simply incremented vs. set to "2." 

So, this looks fishy and I don't recall why that was done that way, this
conversion to refcnt_t and the debug associated with it provides a good
opportunity for us to try to get that to a more familiar reference
counting workflow, I'll take a look at it.

- Arnaldo
 
> > > There are multiple fixes in it to get it to build and test it, so far,
> > > with:
> > >
> > >   perf top -F 15000 -d 0
> > >
> > > while doing kernel builds and tight usleep 1 loops to create lots of
> > > short lived threads with its map_groups, maps, dsos, etc.
> > >
> > > Now running some build tests in some 36 containers with assorted distros
> > > and cross compilers.
> > 
> > Tomorrow I'll inject some refcount errors to test this all.
> 
> 
> Thank you!
> 
> Best Regards,
> Elena.