Subject: [PATCH] mformat: initialize boot sector before writing it

From: Ludovic Courtès <[email-masked]>
Date: Sun, 21 Apr 2019 01:03:57 +0200

Hello,

While investigating reproducible ISO images for Guix¹, I found that
‘mformat’ would not initialize the boot sector before writing it.  This
led to non-deterministic FAT image contents.

The attached patch fixes that.

Thanks,
Ludo’.

¹ https://issues.guix.info/issue/35283

Fix a bug whereby 'mformat' could end up passing uninitialized bytes
to write(2).  This could be reproduced with:

  mformat -C -f 1440 -L 16  -N 77777777 -i  /tmp/x ::

where the output of /tmp/x would be non-deterministic.

Patch by Ludovic Courtès <[email-masked]>.

--- mtools-4.0.23/mformat.c	2019-04-21 00:12:01.496116195 +0200
+++ mtools-4.0.23/mformat.c	2019-04-21 00:12:36.675967157 +0200
@@ -927,6 +927,7 @@ void mformat(int argc, char **argv, int
 	
 	char *endptr;
 
+	memset(&boot.bytes, '\0', sizeof boot);
 	hs = hs_set = 0;
 	argtracks = 0;
 	argheads = 0;