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

From: Xu Kuohai <xukuohai@huawei.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: <bpf@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
	<linux-kselftest@vger.kernel.org>, <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	Martin KaFai Lau <martin.lau@linux.dev>,
	Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>,
	Jiri Olsa <jolsa@kernel.org>, Mykola Lysenko <mykolal@fb.com>,
	Shuah Khan <shuah@kernel.org>,
	"David S . Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Jesper Dangaard Brouer <hawk@kernel.org>,
	Kumar Kartikeya Dwivedi <memxor@gmail.com>,
	Alan Maguire <alan.maguire@oracle.com>,
	Delyan Kratunov <delyank@fb.com>,
	Lorenzo Bianconi <lorenzo@kernel.org>
Subject: Re: [PATCH bpf v3 4/6] selftest/bpf: Fix memory leak in kprobe_multi_test
Date: Tue, 11 Oct 2022 14:26:45 +0800	[thread overview]
Message-ID: <df6ea78f-e980-9bba-d3e0-dc048b4cb9a1@huawei.com> (raw)
In-Reply-To: <CAEf4BzbOgHKfe0bq13qnuQ74TiwT6JW_4Rk3-+YvF2kthhdrcA@mail.gmail.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
>>
> .