file-posix: Clear out first sector in hdev_create

People get surprised when, after "qemu-img create -f raw /dev/sdX", they
still see qcow2 with "qemu-img info", if previously the bdev had a qcow2
header. While this is natural because raw doesn't need to write any
magic bytes during creation, hdev_create is free to clear out the first
sector to make sure the stale qcow2 header doesn't cause such confusion.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block/file-posix.c b/block/file-posix.c
index ab12a2b591acd6db29ccd64a4530d3a955de3544..36ee89e9402277360821baf679d3c82da9724917 100644 (file)
--- a/block/file-posix.c
+++ b/block/file-posix.c
@@ -2730,6 +2730,16 @@ static int hdev_create(const char *filename, QemuOpts *opts,
         ret = -ENOSPC;
     }
 
+    if (!ret && total_size) {
+        uint8_t buf[BDRV_SECTOR_SIZE] = { 0 };
+        int64_t zero_size = MIN(BDRV_SECTOR_SIZE, total_size);
+        if (lseek(fd, 0, SEEK_SET) == -1) {
+            ret = -errno;
+        } else {
+            ret = qemu_write_full(fd, buf, zero_size);
+            ret = ret == zero_size ? 0 : -errno;
+        }
+    }
     qemu_close(fd);
     return ret;
 }