[Qemu-block] [PATCH] block: Don't lock /dev/null and /dev/zero automatically

[Fam Zheng]

On my Fedora 28, /dev/null is locked by some other process (couldn't
inspect it due to the current lslocks limitation), so iotests 226 fails
with some unexpected image locking errors because it uses qemu-io to
open it.

Actually it's safe to not use any lock on /dev/null or /dev/zero.

Signed-off-by: Fam Zheng <address@hidden>
---
 block/file-posix.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/file-posix.c b/block/file-posix.c
index 60af4b3d51..8bf034108a 100644
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -503,7 +503,12 @@ static int raw_open_common(BlockDriverState *bs, QDict 
*options,
         s->use_lock = false;
         break;
     case ON_OFF_AUTO_AUTO:
-        s->use_lock = qemu_has_ofd_lock();
+        if (!strcmp(filename, "/dev/null") ||
+                   !strcmp(filename, "/dev/zero")) {
+            s->use_lock = false;
+        } else {
+            s->use_lock = qemu_has_ofd_lock();
+        }
         break;
     default:
         abort();
-- 
2.17.1