logind: check whether first drmSetMaster succeeded

The initial drmSetMaster may fail if there is an active master already. We
must not assume that all existing clients comply to logind rules. We check
for this during session-activation already but didn't during device setup.
Fix this by checking the return code.

As drmSetMaster has had horrible return codes in the past (0 for failure?
EINVAL for denied access, ..) we need to be quite pedantic. To guarantee
an open file-descriptor we need to close the device and reopen it without
master if setting master failed first.

1 files changed, 23 insertions, 7 deletions

diff --git a/src/login/logind-session-device.c b/src/login/logind-session-device.c
index 27afafa1aa..546c53710e 100644
--- a/src/login/logind-session-device.c
+++ b/src/login/logind-session-device.c
@@ -144,7 +144,7 @@ static int sd_drmdropmaster(int fd) {
 }
 
 static int session_device_open(SessionDevice *sd, bool active) {
-        int fd;
+        int fd, r;
 
         assert(sd->type != DEVICE_TYPE_UNKNOWN);
 
@@ -155,9 +155,17 @@ static int session_device_open(SessionDevice *sd, bool active) {
 
         switch (sd->type) {
         case DEVICE_TYPE_DRM:
-                if (active)
-                        sd_drmsetmaster(fd);
-                else {
+                if (active) {
+                        /* Weird legacy DRM semantics might return an error
+                         * even though we're master. No way to detect that so
+                         * fail at all times and let caller retry in inactive
+                         * state. */
+                        r = sd_drmsetmaster(fd);
+                        if (r < 0) {
+                                close(fd);
+                                return r;
+                        }
+                } else {
                         /* DRM-Master is granted to the first user who opens a
                          * device automatically (ughh, racy!). Hence, we just
                          * drop DRM-Master in case we were the first. */
@@ -384,9 +392,17 @@ int session_device_new(Session *s, dev_t dev, SessionDevice **out) {
          * revoke access and thus invalidate the fd. But this is still needed
          * to pass a valid fd back. */
         sd->active = session_is_active(s);
-        sd->fd = session_device_open(sd, sd->active);
-        if (sd->fd < 0)
-                goto error;
+        r = session_device_open(sd, sd->active);
+        if (r < 0) {
+                /* EINVAL _may_ mean a master is active; retry inactive */
+                if (sd->active && r == -EINVAL) {
+                        sd->active = false;
+                        r = session_device_open(sd, false);
+                }
+                if (r < 0)
+                        goto error;
+        }
+        sd->fd = r;
 
         LIST_PREPEND(SessionDevice, sd_by_device, sd->device->session_devices, sd);