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

[Xu Kuohai]

On 10/11/2022 9:34 AM, Andrii Nakryiko wrote:
> On Mon, Oct 10, 2022 at 7:08 AM Xu Kuohai <xukuohai@xxxxxxxxxx> 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@xxxxxxxxxx>
> > Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > ---
> >   .../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
> .