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.

1 files changed, 30 insertions, 6 deletions

diff --git a/src/shared/util.c b/src/shared/util.c
index 872f6f7371..020b75d0f2 100644
--- a/src/shared/util.c
+++ b/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;