journalctl: give a nice hint about group membership based on ACLs of /var/log/journal

If we notice that we unprivileged and not in any of the groups which
have access to /var/log/journal, print a nice message about which groups
do.

This checks and prints all groups that are in the default ACL for
/var/log/journal, which is not necessarily correct for all journal
files, but pretty close.

src/shared/strv.c

@@ -387,11 +387,29 @@ fail:
         return NULL;
 }
 
-int strv_extend(char ***l, const char *value) {
+int strv_push(char ***l, char *value) {
         char **c;
-        char *v;
         unsigned n;
 
+        if (!value)
+                return 0;
+
+        n = strv_length(*l);
+        c = realloc(*l, sizeof(char*) * (n + 2));
+        if (!c)
+                return -ENOMEM;
+
+        c[n] = value;
+        c[n+1] = NULL;
+
+        *l = c;
+        return 0;
+}
+
+int strv_extend(char ***l, const char *value) {
+        char *v;
+        int r;
+
         if (!value)
                 return 0;
 
@@ -399,18 +417,11 @@ int strv_extend(char ***l, const char *value) {
         if (!v)
                 return -ENOMEM;
 
-        n = strv_length(*l);
-        c = realloc(*l, sizeof(char*) * (n + 2));
-        if (!c) {
+        r = strv_push(l, v);
+        if (r < 0)
                 free(v);
-                return -ENOMEM;
-        }
 
-        c[n] = v;
-        c[n+1] = NULL;
-
-        *l = c;
-        return 0;
+        return r;
 }
 
 char **strv_uniq(char **l) {

src/shared/strv.h

@@ -41,6 +41,7 @@ char **strv_merge(char **a, char **b);
 char **strv_merge_concat(char **a, char **b, const char *suffix);
 char **strv_append(char **l, const char *s);
 int strv_extend(char ***l, const char *value);
+int strv_push(char ***l, char *value);
 
 char **strv_remove(char **l, const char *s);
 char **strv_remove_prefix(char **l, const char *s);

src/shared/util.c

@@ -4190,6 +4190,23 @@ char* uid_to_name(uid_t uid) {
         return r;
 }
 
+char* gid_to_name(gid_t gid) {
+        struct group *p;
+        char *r;
+
+        if (gid == 0)
+                return strdup("root");
+
+        p = getgrgid(gid);
+        if (p)
+                return strdup(p->gr_name);
+
+        if (asprintf(&r, "%lu", (unsigned long) gid) < 0)
+                return NULL;
+
+        return r;
+}
+
 int get_group_creds(const char **groupname, gid_t *gid) {
         struct group *g;
         gid_t id;
@@ -4228,14 +4245,10 @@ int get_group_creds(const char **groupname, gid_t *gid) {
         return 0;
 }
 
-int in_group(const char *name) {
-        gid_t gid, *gids;
+int in_gid(gid_t gid) {
+        gid_t *gids;
         int ngroups_max, r, i;
 
-        r = get_group_creds(&name, &gid);
-        if (r < 0)
-                return r;
-
         if (getgid() == gid)
                 return 1;
 
@@ -4258,6 +4271,17 @@ int in_group(const char *name) {
         return 0;
 }
 
+int in_group(const char *name) {
+        int r;
+        gid_t gid;
+
+        r = get_group_creds(&name, &gid);
+        if (r < 0)
+                return r;
+
+        return in_gid(gid);
+}
+
 int glob_exists(const char *path) {
         glob_t g;
         int r, k;

src/shared/util.h

@@ -429,9 +429,11 @@ int socket_from_display(const char *display, char **path);
 int get_user_creds(const char **username, uid_t *uid, gid_t *gid, const char **home, const char **shell);
 int get_group_creds(const char **groupname, gid_t *gid);
 
+int in_gid(gid_t gid);
 int in_group(const char *name);
 
 char* uid_to_name(uid_t uid);
+char* gid_to_name(gid_t gid);
 
 int glob_exists(const char *path);