util.c: [ref] move group find/copy into new functions

Move the logic from clean_supplementary_groups into the following new
functions:

* find_group
* copy_group_ifcont

These will be reused later.

Misc: The latter function's signature is based on getgrouplist(2), which
is used on clean_supplementary_groups.

src/firejail/util.c

@@ -103,6 +103,30 @@ void errLogExit(char* fmt, ...) {
  exit(1);
 }
 
+static int find_group(gid_t group, const gid_t *groups, int ngroups) {
+ int i;
+ for (i = 0; i < ngroups; i++) {
+ if (group == groups[i])
+ return i;
+ }
+
+ return -1;
+}
+
+// Gets group from "groupname" and adds it to "new_groups" if it exists on
+// "groups". Always returns the current value of new_ngroups.
+static int copy_group_ifcont(const char *groupname,
+ const gid_t *groups, int ngroups,
+ gid_t *new_groups, int *new_ngroups) {
+ gid_t g = get_group_id(groupname);
+ if (g && find_group(g, groups, ngroups) >= 0) {
+ new_groups[*new_ngroups] = g;
+ (*new_ngroups)++;
+ }
+
+ return *new_ngroups;
+}
+
 static void clean_supplementary_groups(gid_t gid) {
  assert(cfg.username);
  gid_t groups[MAX_GROUPS];
@@ -126,17 +150,8 @@ static void clean_supplementary_groups(gid_t gid) {
 
  int i = 0;
  while (allowed[i]) {
- gid_t g = get_group_id(allowed[i]);
- if (g) {
- int j;
- for (j = 0; j < ngroups; j++) {
- if (g == groups[j]) {
- new_groups[new_ngroups] = g;
- new_ngroups++;
- break;
- }
- }
- }
+ copy_group_ifcont(allowed[i], groups, ngroups,
+ new_groups, &new_ngroups);
  i++;
  }