egl+libsync: Add check for valid fence-fd

Debugging fd mix-ups (ie. where, possibly via close()ing the original
fd, etc, you end up with something that is a valid fd but not a valid
*fence* fd) can be difficult.  Fortunately we can use the FILE_INFO
ioctl, which will return an error if the fd is not a fence fd.

For android, we instead use the libsync API, which does a similar thing
on modern kernels, but has a fallback path for older android kernels.

Note that the FILE_INFO ioctl has existed upstream since at least prior
to destaging of sync_file.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Emma Anholt <emma@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11202>

src/android_stub/sync_stub.cpp

@@ -17,4 +17,13 @@ int sync_merge(const char *name, int fd, int fd2)
    return 0;
 }
 
+struct sync_file_info* sync_file_info(int32_t fd)
+{
+   return NULL;
+}
+
+void sync_file_info_free(struct sync_file_info* info)
+{
+}
+
 }

src/egl/drivers/dri2/egl_dri2.c

@@ -62,6 +62,7 @@
 #include "egl_dri2.h"
 #include "GL/mesa_glinterop.h"
 #include "loader/loader.h"
+#include "util/libsync.h"
 #include "util/os_file.h"
 #include "util/u_atomic.h"
 #include "util/u_vector.h"
@@ -3495,6 +3496,8 @@ dri2_dup_native_fence_fd(_EGLDisplay *disp, _EGLSync *sync)
       return EGL_NO_NATIVE_FENCE_FD_ANDROID;
    }
 
+   assert(sync_valid_fd(sync->SyncFd));
+
    return os_dupfd_cloexec(sync->SyncFd);
 }
 

src/util/libsync.h

@@ -30,6 +30,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <stdbool.h>
 #include <stdint.h>
 #include <string.h>
 #include <sys/ioctl.h>
@@ -46,6 +47,25 @@ extern "C" {
  * Android kernels.
  */
 #include <android/sync.h>
+
+/**
+ * Check if the fd represents a valid fence-fd.
+ *
+ * The android variant of this debug helper is implemented on top of the
+ * system's libsync for compatibility with pre-4.7 android kernels.
+ */
+static inline bool
+sync_valid_fd(int fd)
+{
+	/* sync_file_info() only available in SDK 26. */
+#if ANDROID_API_LEVEL >= 26
+	struct sync_file_info *info = sync_file_info(fd);
+	if (!info)
+		return false;
+	sync_file_info_free(info);
+#endif
+	return true;
+}
 #else
 
 #ifndef SYNC_IOC_MERGE
@@ -61,8 +81,20 @@ struct sync_merge_data {
 	uint32_t	flags;
 	uint32_t	pad;
 };
+
+struct sync_file_info {
+	char	name[32];
+	int32_t	status;
+	uint32_t	flags;
+	uint32_t	num_fences;
+	uint32_t	pad;
+
+	uint64_t	sync_fence_info;
+};
+
 #define SYNC_IOC_MAGIC		'>'
 #define SYNC_IOC_MERGE		_IOWR(SYNC_IOC_MAGIC, 3, struct sync_merge_data)
+#define SYNC_IOC_FILE_INFO	_IOWR(SYNC_IOC_MAGIC, 4, struct sync_file_info)
 #endif
 
 
@@ -109,6 +141,16 @@ static inline int sync_merge(const char *name, int fd1, int fd2)
 	return data.fence;
 }
 
+/**
+ * Check if the fd represents a valid fence-fd.
+ */
+static inline bool
+sync_valid_fd(int fd)
+{
+	struct sync_file_info info = {{0}};
+	return ioctl(fd, SYNC_IOC_FILE_INFO, &info) >= 0;
+}
+
 #endif /* !ANDROID */
 
 /* accumulate fd2 into fd1.  If *fd1 is not a valid fd then dup fd2,