sd-bus: derive uid from cgroup if possible

Whenever we run in a user context, sd_bus_{default_user,open_user}() and
friends should always connect to the user-bus of the current context,
instead of deriving the uid from getuid(). This allows us running
programs via sudo/su, without the nasty side-effect of accidentally
connecting to the root user-bus.

This patch enforces the idea of making su/sudo *not* opening sessions by
default. That is, all they do is raising privileges, but keeping
everything set as before. You can still use su/sudo to open real sessions
by requesting a login-session (or loading pam_systemd otherwise).
However, in this case XDG_RUNTIME_DIR= will not be set (as usual in these
cases), hence, you will not be able to connect to *any* user-bus.

Long story short: With this patch applied, both:
        - ./busctl --user
        - sudo ./busctl --user
..will successfully connect to the user-bus of the local user.

Fixes #390.

1 files changed, 8 insertions, 2 deletions

diff --git a/src/libsystemd/sd-bus/sd-bus.c b/src/libsystemd/sd-bus/sd-bus.c
index 4ed508427e..4a9181613a 100644
--- a/src/libsystemd/sd-bus/sd-bus.c
+++ b/src/libsystemd/sd-bus/sd-bus.c
@@ -1241,6 +1241,8 @@ fail:
 
 int bus_set_address_user(sd_bus *b) {
         const char *e;
+        uid_t uid;
+        int r;
 
         assert(b);
 
@@ -1248,6 +1250,10 @@ int bus_set_address_user(sd_bus *b) {
         if (e)
                 return sd_bus_set_address(b, e);
 
+        r = cg_pid_get_owner_uid(0, &uid);
+        if (r < 0)
+                uid = getuid();
+
         e = secure_getenv("XDG_RUNTIME_DIR");
         if (e) {
                 _cleanup_free_ char *ee = NULL;
@@ -1256,9 +1262,9 @@ int bus_set_address_user(sd_bus *b) {
                 if (!ee)
                         return -ENOMEM;
 
-                (void) asprintf(&b->address, KERNEL_USER_BUS_ADDRESS_FMT ";" UNIX_USER_BUS_ADDRESS_FMT, getuid(), ee);
+                (void) asprintf(&b->address, KERNEL_USER_BUS_ADDRESS_FMT ";" UNIX_USER_BUS_ADDRESS_FMT, uid, ee);
         } else
-                (void) asprintf(&b->address, KERNEL_USER_BUS_ADDRESS_FMT, getuid());
+                (void) asprintf(&b->address, KERNEL_USER_BUS_ADDRESS_FMT, uid);
 
         if (!b->address)
                 return -ENOMEM;