RE: [PATCH 3/9] tools: convert comm_str.refcnt from atomic_t to refcount_t

["Reshetova, Elena" <elena.reshetova@intel.com>]

> Em Wed, Feb 22, 2017 at 07:20:45PM -0300, Arnaldo Carvalho de Melo
> escreveu:
> > Em Wed, Feb 22, 2017 at 05:33:50PM -0300, Arnaldo Carvalho de Melo
> escreveu:
> > > Em Tue, Feb 21, 2017 at 05:34:57PM +0200, Elena Reshetova escreveu:
> > > > refcount_t type and corresponding API should be
> > > > used instead of atomic_t when the variable is used as
> > > > a reference counter. This allows to avoid accidental
> > > > refcounter overflows that might lead to use-after-free
> > > > situations.
> > > >
> > > > Signed-off-by: Elena Reshetova <elena.reshetova@intel.com>
> > > > Signed-off-by: Hans Liljestrand <ishkamiel@gmail.com>
> > > > Signed-off-by: Kees Cook <keescook@chromium.org>
> > > > Signed-off-by: David Windsor <dwindsor@gmail.com>
> > >
> > > You are doing two things (well three) things here:
> > >
> > > 1. converting to refcnt.h
> > >
> > > 2. Initiationg the refcount to 1, which makes this take place:
> > >
> > > [acme@jouet linux]$ m
> > > make: Entering directory '/home/acme/git/linux/tools/perf'
> > >   BUILD:   Doing 'make -j4' parallel build
> > > Warning: arch/x86/include/asm/cpufeatures.h differs from kernel
> > >   CC       /tmp/build/perf/util/comm.o
> > >   INSTALL  trace_plugins
> > > util/comm.c:16:25: error: ‘comm_str__get’ defined but not used [-
> Werror=unused-function]
> > >  static struct comm_str *comm_str__get(struct comm_str *cs)
> > >                          ^~~~~~~~~~~~~
> > > cc1: all warnings being treated as errors
> > > mv: cannot stat '/tmp/build/perf/util/.comm.o.tmp': No such file or
> directory
> > > /home/acme/git/linux/tools/build/Makefile.build:101: recipe for target
> '/tmp/build/perf/util/comm.o' failed
> > > make[4]: *** [/tmp/build/perf/util/comm.o] Error 1
> > > /home/acme/git/linux/tools/build/Makefile.build:144: recipe for target
> 'util' failed
> > > make[3]: *** [util] Error 2
> > > Makefile.perf:523: recipe for target '/tmp/build/perf/libperf-in.o' failed
> > > make[2]: *** [/tmp/build/perf/libperf-in.o] Error 2
> > > Makefile.perf:204: recipe for target 'sub-make' failed
> > > make[1]: *** [sub-make] Error 2
> > > Makefile:108: recipe for target 'install-bin' failed
> > > make: *** [install-bin] Error 2
> > > make: Leaving directory '/home/acme/git/linux/tools/perf'
> > > [acme@jouet linux]$
> > >
> > > 3) not test building your patches :-\

Sorry about compilation errors: I totally forgot that tools is not getting compiled automatically when you build the whole tree with all configs on, so these patches 
really slipped through untested.

> > >
> > > I'll let this pass this time, minor, I am removing the now unused
> > > comm_str__get() function.
> >
> > But it can't get unused, because the comm_str__findnew() may return an
> > existing entry, that _needs_ to get its refcount bumped, that is the
> > reason for this refcount to be there... reinstating it:

True, we missed that it was reused behind the scenes. Your fix below does it correctly. 
The object resuse seems to be one of the main issues of this atomic_t to refcount_t conversions through the kernel. 
We have sooo many places where this happens (obvious and not so obvious ones) and every single of them would fail in run-time, unless we can modify the code not to do increments on zero. 

> >
> > #0  0x00007ffff522491f in raise () from /lib64/libc.so.6
> > #1  0x00007ffff522651a in abort () from /lib64/libc.so.6
> > #2  0x00007ffff5268200 in __libc_message () from /lib64/libc.so.6
> > #3  0x00007ffff527188a in _int_free () from /lib64/libc.so.6
> > #4  0x00007ffff52752bc in free () from /lib64/libc.so.6
> > #5  0x000000000051125f in comm_str__put (cs=0x35038e0) at util/comm.c:20
> > #6  0x00000000005115b3 in comm__free (comm=0x6f4ee90) at
> util/comm.c:113
> > #7  0x0000000000511e10 in thread__delete (thread=0x6f4ee10) at
> util/thread.c:81
> > #8  0x0000000000511f0e in thread__put (thread=0x6f4ee10) at
> util/thread.c:103
> > #9  0x0000000000504ea6 in machine__process_fork_event
> (machine=0x21f4bf8, event=0x7fffed6b54a0, sample=0x7fffffff8420) at
> util/machine.c:1496
> > #10 0x0000000000505092 in machine__process_event (machine=0x21f4bf8,
> event=0x7fffed6b54a0, sample=0x7fffffff8420) at util/machine.c:1544
> > #11 0x0000000000451ae9 in perf_top__mmap_read_idx (top=0x7fffffffa7c0,
> idx=3) at builtin-top.c:844
> > #12 0x0000000000451bb6 in perf_top__mmap_read (top=0x7fffffffa7c0) at
> builtin-top.c:857
> > #13 0x0000000000452229 in __cmd_top (top=0x7fffffffa7c0) at builtin-
> top.c:1002
> > #14 0x00000000004536a3 in cmd_top (argc=0, argv=0x7fffffffe150,
> prefix=0x0) at builtin-top.c:1332
> > #15 0x00000000004b82a8 in run_builtin (p=0xa17cd0 <commands+336>,
> argc=4, argv=0x7fffffffe150) at perf.c:359
> > #16 0x00000000004b8515 in handle_internal_command (argc=4,
> argv=0x7fffffffe150) at perf.c:421
> > #17 0x00000000004b865a in run_argv (argcp=0x7fffffffdf9c,
> argv=0x7fffffffdf90) at perf.c:467
> > #18 0x00000000004b8a5d in main (argc=4, argv=0x7fffffffe150) at perf.c:614
> >
> > And this brings us to my learning experience, i.e. this should've been caught
> > by this machinery, right? But that only if I leaked this object, right?
> >
> > I need to read more on this, that is for sure ;-)

The way how current refcount_t implemented it would refuse to do any increments/decrements on zero, or increments/decrements on max values. 
Also, it should WARN about this cases so that people can trace the issue.

> 
> For reference, this is the patch on top of this:
> 
> +++ b/tools/perf/util/comm.c
> @@ -13,6 +13,13 @@ struct comm_str {
>  /* Should perhaps be moved to struct machine */
>  static struct rb_root comm_str_root;
> 
> +static struct comm_str *comm_str__get(struct comm_str *cs)
> +{
> +       if (cs)
> +               refcount_inc(&cs->refcnt);
> +       return cs;
> +}
> +
>  static void comm_str__put(struct comm_str *cs)
>  {
>         if (cs && refcount_dec_and_test(&cs->refcnt)) {
> @@ -54,7 +61,7 @@ static struct comm_str *comm_str__findnew(const char
> *str, struct rb_root *root)
> 
>                 cmp = strcmp(str, iter->str);
>                 if (!cmp)
> -                       return iter;
> +                       return comm_str__get(iter);
> 
>                 if (cmp < 0)
>                         p = &(*p)->rb_left;
> [acme@jouet linux]$

This looks correct now.