[PATCH libdrm] xf86drm: Add drmIsMaster()

[Christopher James Halse Rogers]

We can't use drmSetMaster to query whether or not a drm fd is master
because it requires CAP_SYS_ADMIN, even if the fd *is* a master fd.

Pick DRM_IOCTL_MODE_ATTACHMODE as a long-deprecated ioctl that is
DRM_MASTER but not DRM_ROOT_ONLY as the probe by which we can detect
whether or not the fd is master.

Signed-off-by: Christopher James Halse Rogers <christopher.halse.rogers at canonical.com>
---
 xf86drm.c | 20 ++++++++++++++++++++
 xf86drm.h |  2 ++
 2 files changed, 22 insertions(+)

diff --git a/xf86drm.c b/xf86drm.c
index 10df682b..bdb0439d 100644
--- a/xf86drm.c
+++ b/xf86drm.c
@@ -2741,6 +2741,26 @@ drm_public int drmDropMaster(int fd)
         return drmIoctl(fd, DRM_IOCTL_DROP_MASTER, NULL);
 }
 
+drm_public bool drmIsMaster(int fd)
+{
+    struct drm_mode_mode_cmd cmd;
+
+    memclear(cmd);
+    /* Set an invalid connector_id to ensure that ATTACHMODE errors with
+     * EINVAL in the unlikely event someone feels like calling this on a
+     * kernel prior to 3.9. */
+    cmd.connector_id = -1;
+
+    if (drmIoctl(fd, DRM_IOCTL_MODE_ATTACHMODE, &cmd) != -1)
+    {
+        /* On 3.9 ATTACHMODE was changed to drm_noop, and so will succeed
+	 * iff we've got a master fd */
+        return true;
+    }
+
+    return errno == EINVAL;
+}
+
 drm_public char *drmGetDeviceNameFromFd(int fd)
 {
     char name[128];
diff --git a/xf86drm.h b/xf86drm.h
index 7773d71a..9e920db9 100644
--- a/xf86drm.h
+++ b/xf86drm.h
@@ -37,6 +37,7 @@
 #include <stdarg.h>
 #include <sys/types.h>
 #include <stdint.h>
+#include <stdbool.h>
 #include <drm.h>
 
 #if defined(__cplusplus)
@@ -733,6 +734,7 @@ extern void drmMsg(const char *format, ...) DRM_PRINTFLIKE(1, 2);
 
 extern int drmSetMaster(int fd);
 extern int drmDropMaster(int fd);
+extern bool drmIsMaster(int fd);
 
 #define DRM_EVENT_CONTEXT_VERSION 4
 
-- 
2.19.1