Re: [PATCH bpf v3 4/6] selftest/bpf: Fix memory leak in kprobe_multi_test

[Xu Kuohai <xukuohai@huawei.com>]

On 10/11/2022 9:34 AM, Andrii Nakryiko wrote:
> On Mon, Oct 10, 2022 at 7:08 AM Xu Kuohai <xukuohai@huawei.com> wrote:
>>
>> The get_syms() function in kprobe_multi_test.c does not free the string
>> memory allocated by sscanf correctly. Fix it.
>>
>> Fixes: 5b6c7e5c4434 ("selftests/bpf: Add attach bench test")
>> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
>> Acked-by: Jiri Olsa <jolsa@kernel.org>
>> ---
>>   .../bpf/prog_tests/kprobe_multi_test.c          | 17 ++++++++---------
>>   1 file changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
>> index d457a55ff408..07dd2c5b7f98 100644
>> --- a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
>> +++ b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
>> @@ -360,15 +360,14 @@ static int get_syms(char ***symsp, size_t *cntp)
>>                   * to them. Filter out the current culprits - arch_cpu_idle
>>                   * and rcu_* functions.
>>                   */
>> -               if (!strcmp(name, "arch_cpu_idle"))
>> -                       continue;
>> -               if (!strncmp(name, "rcu_", 4))
>> -                       continue;
>> -               if (!strcmp(name, "bpf_dispatcher_xdp_func"))
>> -                       continue;
>> -               if (!strncmp(name, "__ftrace_invalid_address__",
>> -                            sizeof("__ftrace_invalid_address__") - 1))
>> +               if (!strcmp(name, "arch_cpu_idle") ||
>> +                       !strncmp(name, "rcu_", 4) ||
>> +                       !strcmp(name, "bpf_dispatcher_xdp_func") ||
>> +                       !strncmp(name, "__ftrace_invalid_address__",
>> +                                sizeof("__ftrace_invalid_address__") - 1)) {
>> +                       free(name);
>>                          continue;
>> +               }
> 
> it seems cleaner if we add if (name) free(name) under error: goto
> label. And in the success case when we assign name to syms[cnt] we can
> reset name to NULL to avoid double-free. WDYT?
> 

Fine, but since free(NULL) works perfectly, will call free(name) unconditionally,
and also initialize name to NULL, and call free(name) before sscanf.

> 
>>                  err = hashmap__add(map, name, NULL);
>>                  if (err) {
>>                          free(name);
>> @@ -394,7 +393,7 @@ static int get_syms(char ***symsp, size_t *cntp)
>>          hashmap__free(map);
>>          if (err) {
>>                  for (i = 0; i < cnt; i++)
>> -                       free(syms[cnt]);
>> +                       free(syms[i]);
>>                  free(syms);
>>          }
>>          return err;
>> --
>> 2.30.2
>>
> .