mkswap: do not use uninitialized stack value

If blkdev_get_size fails, then size is not set. Exit with an error code
and indicate what went wrong instead of continuing with random stack
content.

Proof of Concept:

```
$ mkswap /dev/null
mkswap: warning: truncating swap area to 17179869180 KiB
mkswap: /dev/null: insecure permissions 0666, fix with: chmod 0600 /dev/null
mkswap: unable to assign device to libblkid probe
```

The first output line depends on stack content and sometimes does not
show up at all. Abort operation if argument is neither regular file nor
block device.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>

diff --git a/disk-utils/mkswap.c b/disk-utils/mkswap.c
index 7e21647048e861edcf3d7d63901147cc5b5993d6..bd02301774535a0084638d86120e5291906351a5 100644 (file)
--- a/disk-utils/mkswap.c
+++ b/disk-utils/mkswap.c
@@ -345,8 +345,9 @@ static unsigned long long get_size(const struct mkswap_control *ctl)
        fd = open(ctl->devname, O_RDONLY);
        if (fd < 0)
                err(EXIT_FAILURE, _("cannot open %s"), ctl->devname);
-       if (blkdev_get_size(fd, &size) == 0)
-               size /= ctl->pagesize;
+       if (blkdev_get_size(fd, &size) < 0)
+               err(EXIT_FAILURE, _("cannot determine size of %s"), ctl->devname);
+       size /= ctl->pagesize;
 
        close(fd);
        return size;