dos: improve MBR signature generation

Using tv_usec in struct timeval from gettimeofday() doesn't provide
enough precision to fill an unsigned 32-bit integer and isn't really
random. It it always less than one million when using the GNU C library
while an unsigned 32-bit integer ranges between 0 and 4294967295.

In FAT filesystem creation, parted already uses a better random
generator, so move that code into a common function and use it
for MS-DOS MBR signature generation.

* libparted/fs/r/fat/fat.c (_gen_new_serial_number): Remove.
(fat_create): Use generate_random_uint32 instead of
_gen_new_serial_number.
* libparted/labels/dos.c (generate_random_id): Remove.
(msdos_write): Use generate_random_uint32 instead of
generate_random_id.
* libparted/labels/misc.h (generate_random_uint32): New function.
Created from _gen_new_serial_number in libparted/fs/r/fat/fat.c with
additional check to avoid returning zero, which may be interpreted
as no FAT serial number or no MBR signature.

1 files changed, 4 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 98f7c6e..50faf4d 100644
--- a/NEWS
+++ b/NEWS
@@ -23,6 +23,10 @@ GNU parted NEWS                                    -*- outline -*-
   partprobe now tells the kernel to forget about any partitions
   on a device that has no recognizable partition table.
 
+  dos: the range of random MBR signature values was artificially limited
+  to 0..999999, which mistakenly included 0.  Now, we use the full 32-bit
+  range, but exclude 0.
+
 ** Changes in behavior
 
   parted -l no longer lists device-mapper devices other than