Re: does gcc gives a false warning in kernel/trace/trace_events_filter.c ?

Toralf Förster
2012-09-06T16:35:59Z

On 09/05/2012 07:08 PM, Steven Rostedt wrote:
> On Sun, 2012-09-02 at 11:04 +0200, Toralf Förster wrote:
>> The current git tree of linux gave with gcc-4.6.3 :
>>
>> kernel/trace/trace_events_filter.c: In function ‘ftrace_function_set_filter_cb’:
>> kernel/trace/trace_events_filter.c:2074:8: warning: ‘ret’ may be used uninitialized in this function [-Wuninitialized]
>>
>>
>> which refers to this piece of code:
>>
>>
>> 2061 static int ftrace_function_set_filter_cb(enum move_type move,
>> 2062 struct filter_pred *pred,
>> 2063 int *err, void *data)
>> 2064 {
>> 2065 /* Checking the node is valid for function trace. */
>> 2066 if ((move != MOVE_DOWN) ||
>> 2067 (pred->left != FILTER_PRED_INVALID)) {
>> 2068 *err = ftrace_function_check_pred(pred, 0);
>> 2069 } else {
>> 2070 *err = ftrace_function_check_pred(pred, 1);
>> 2071 if (*err)
>> 2072 return WALK_PRED_ABORT;
>> 2073
>> 2074 *err = __ftrace_function_set_filter(pred->op == OP_EQ,
>> 2075 pred->regex.pattern,
>> 2076 pred->regex.len,
>> 2077 data);
>> 2078 }
>> 2079
>> 2080 return (*err) ? WALK_PRED_ABORT : WALK_PRED_DEFAULT;
>> 2081 }
>> 2082
>>
>>
>> >From a Gentoo forum user I got a hint :
>>
>> "Maybe it's some kind of a weird inlining issue? I think it's
>> referring to the ret in __ftrace_function_set_filter(), which would be
>> uninitialized if the for-loop does not run (re_cnt ≤ 0)"
>>
>> Now I'm wondering if re_cnt can become zero or if gcc is wrong here ?
>>
>
> Strange, as ret is initialized to 'ret = -EINVAL;' in
> __ftrace_function_set_filter(). I'm thinking that gcc got confused here.
> Maybe report it to the gcc maintainers?
>
> -- Steve
>

I filed a bug report

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54495
and got this answer :

--- Comment #1 from Hans-Peter Nilsson <hp at gcc dot gnu.org> 2012-09-05 22:14:00 UTC ---
But if the call to ftrace_function_filter_re sets re_cnt to 0, then ret indeed
will be used uninitialized AFAICT. What am I missing?