Description of problem:
When attempting to launch a KVM instance with a disk using the vvfat protocol, QEMU exits with apparently random error messages reported back

So far I've seen 

qemu: could not open disk image fat:floppy:/home/berrange/src/virt/libvirt-tck/fat: Inappropriate ioctl for device

qemu: could not open disk image fat:floppy:/tmp: No such file or directory

qemu: could not open disk image fat:floppy:/tmp: Invalid argument

This appears to be a flaw in vl.c handling of bdrv_open() return codes. It currently does


         fprintf(stderr, "qemu: could not open disk image %s: %s\n",
                         file, strerror(errno));

However, the bdrv_open() API contract does *not* actually set errno, so this is just reporting the last errno that happened to be set by some other random piece of QEMU code. The actual error is in the bdrv_open() return value.

Something like this appears to be neccessary:

diff --git a/vl.c b/vl.c
index 6d08ec8..4a4c818 100644
--- a/vl.c
+++ b/vl.c
@@ -792,6 +792,7 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
     const char *devaddr;
     DriveInfo *dinfo;
     int snapshot = 0;
+    int err;
 
     *fatal_error = 1;
 
@@ -1122,9 +1123,9 @@ DriveInfo *drive_init(QemuOpts *opts, void *opaque,
 
     bdrv_flags |= ro ? 0 : BDRV_O_RDWR;
 
-    if (bdrv_open(dinfo->bdrv, file, bdrv_flags, drv) < 0) {
+    if ((err = bdrv_open(dinfo->bdrv, file, bdrv_flags, drv)) < 0) {
         fprintf(stderr, "qemu: could not open disk image %s: %s\n",
-                        file, strerror(errno));
+                        file, strerror(-err));
         return NULL;
     }
 

There is still a problem with this though. When bdrv_open() fails to open a disk due to its protocol/format not being in the whitelist, it set  ret=-ENOENT.

The ENOENT error code is a very commonly seen error in user configuration errors due to typos in disk paths. Thus it is rather mis-leading as a way of reporting a block driver is not allowed by the whitelist.

Ideally this will all use qerror() eventually, but in the meantime, it would be more helpful to use a different errno for this case


diff --git a/block.c b/block.c
index 39724c1..36b7e3f 100644
--- a/block.c
+++ b/block.c
@@ -565,7 +565,7 @@ int bdrv_open(BlockDriverState *bs, const char *filename, int flags,
     }
 
     if (!drv) {
-        ret = -ENOENT;
+        ret = -ENOSYS;
         goto unlink_and_fail;
     }
 


This makes it report:

qemu: could not open disk image fat:floppy:/tmp: Function not implemented


which gives a better indication that the fat capability has been disabled by the whitelist.

Version-Release number of selected component (if applicable):
qemu-kvm-0.12.1.2-2.78.el6.x86_64

How reproducible:
Sometimes, occassionally the previously set errno matches the currently ENOENT, but often it doesn't.  Regardless ENOENT is a very misleading code.

Steps to Reproduce:
1. qemu -drive file=fat:floppy:/tmp
2.
3.
  
Actual results:
qemu: could not open disk image fat:floppy:/tmp: No such file or directory


Expected results:
qemu: could not open disk image fat:floppy:/tmp: Function not implemented


Additional info: