From 0b2d3e5052b74d55a09a133e960d2730ed488e1b Mon Sep 17 00:00:00 2001
From: Vladimir Rutsky <vladimir@rutsky.org>
Date: Mon, 1 Feb 2016 00:08:43 +0300
Subject: [PATCH 1/2] handle typical permission denied error in user friendly
 way

This is the first (and the only) error that will occur if user will
try to access /dev/nbd0 without proper permissions.
---
 buse.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/buse.c b/buse.c
index 69039b7..24c8916 100644
--- a/buse.c
+++ b/buse.c
@@ -94,7 +94,13 @@ int buse_main(const char* dev_file, const struct buse_operations *aop, void *use
   assert(!socketpair(AF_UNIX, SOCK_STREAM, 0, sp));
 
   nbd = open(dev_file, O_RDWR);
-  assert(nbd != -1);
+  if (nbd == -1) {
+    fprintf(stderr, 
+        "Failed to open `%s': %s\n"
+        "Is kernel module `nbd' is loaded and you have permissions "
+        "to access the device?\n", dev_file, strerror(errno));
+    return 1;
+  }
 
   assert(ioctl(nbd, NBD_SET_SIZE, aop->size) != -1);
   assert(ioctl(nbd, NBD_CLEAR_SOCK) != -1);

From 198ffee1cb7670b3d3ef1abde8c17d7968a682a2 Mon Sep 17 00:00:00 2001
From: Vladimir Rutsky <vladimir@rutsky.org>
Date: Mon, 1 Feb 2016 00:17:11 +0300
Subject: [PATCH 2/2] add help message to busexmp example

---
 busexmp.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/busexmp.c b/busexmp.c
index 5464902..d342a6e 100644
--- a/busexmp.c
+++ b/busexmp.c
@@ -74,7 +74,16 @@ static struct buse_operations aop = {
 
 int main(int argc, char *argv[])
 {
-  (void)(argc);
+  if (argc != 2)
+  {
+    fprintf(stderr, 
+        "Usage:\n"
+        "  %s /dev/nbd0\n"
+        "Don't forget to load nbd kernel module (`modprobe nbd`) and\n"
+        "run example from root.\n", argv[0]);
+    return 1;
+  }
+
   data = malloc(aop.size);
 
   return buse_main(argv[1], &aop, (void *)&xmpl_debug);