makefs(8) proposed changes [repost from fs@]

[Sean Bruno]

On Wed, 2013-10-02 at 09:15 -0700, Sean Bruno wrote:
> Using makefs from an amd64 host to build a f/s in a VTOC8 partition will
> destroy the entire partition table.  I simplified the test to simple dd
> to an existing partition and got the same result, exonerating mkfs
> 
> I suspect a lack of endian knowledge in geom itself, not
> geom_part_vtoc8.  
> 
> test case:
> 	using amd64 host (10.0 current) create monolothic image
> 		truncate -s+5G /var/tmp/myfile.img
> 		mdconfig -f /var/tmp/myfile.img
>         build and load geom_part_vtoc8 kernel module
> 	use gpart to create VTOC8 part table
> 	add partition to part table to yeild the following:
> 
> =>       0  10442250  md0  VTOC8  (5.0G)
>          0  10442250    1  freebsd-ufs  (5G)  
> 
> 	dd to md0a from dev zero for just a bit
> 		dd if=/dev/zero of=/dev/md0a bs=64k count=100
> 
> 	destroy md0 via mdconfig -d -u 0
> 	recreate it with mdconfig -f /var/tmp/myfile.img
> 
> 	gpart displays no partions for the image any more:
> 		gpart: No such geom: md0.
> 
> bcc freebsd-geom


Nathan brought me some knowledge.  newfs(8) knows that the first 16
sectors for the first partition are special and not to touch it.
makefs(8) does not.

So, as a proof of concept, I modified makefs to read() the first 16
sectors from the "image" (in my case, /dev/md0a) and throw it away.
(lseek() failed on the partition, so read() was used)

This got me to a bootable SPARC64 image in qemu-system-sparc64, and will
probably allow further booting in other BE architectures.

Is this patch going to break non "device" makefs calls?  e.g. if I want
to create an image file and not use a loopback device?


Index: ffs.c
===================================================================
--- ffs.c       (revision 255871)
+++ ffs.c       (working copy)
@@ -470,6 +470,7 @@
        char    *buf;
        int     i, bufsize;
        off_t   bufrem;
+       char temp_buf[16*512];
 
        assert (image != NULL);
        assert (fsopts != NULL);
@@ -480,6 +481,7 @@
                warn("Can't open `%s' for writing", image);
                return (-1);
        }
+       read(fsopts->fd, temp_buf, 16*512);
 
                /* zero image */
 #if HAVE_STRUCT_STATVFS_F_IOSIZE && HAVE_FSTATVFS


bcc - nwhitehorn


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