Christoph Biedl <[email protected]> adds the feature of

offset and size command-line options.

In the quote below are his stated motivations:

Hello,

the patch below implements a new option -o for vbladed, denoting the
number of sectors that should be skipped at the beginning of the
exported file. Default is 0 (no offset, export from beginning, current
behaviour).

Rationale:

If the exported files are actually raw block devices on the target,
something that might happen when using logical volumes for exports,
several tools running on the target that assume block devices are
for exclusive local access only might create confusion or even
havoc. Most prominently:

* os-prober of grub (might be disabled, though)
 Might mount any block device found that contain a file system.
* (Linux) logical volume management
 Will report any physical volume if the export is used as such.
* (Linux) blkid
 Reports any block device (minor impact, mostly information leakage)

The overall problem is applications are not aware some block devices
might be used from a remote client while there's no locking and that
might be hard to implement

Solutions tried but not considered helpful:

* Filter out exported block devices
 Problems: The tools need the ability to filter; new tools might come
 into existance, requiring new action. Filter languages might be hard
 to use (Linux LVM is quite a nightmare here).
* Optionally open block devices with O_EXCL
 Had no effect. Still exposes block devices where no vbladed is running
 yet.

So the goal was to hide the actual content from these tools. One
approach was to obscure the data using XOR, however it seemed easier
to sacrifice one or more sectors (8 recommended for 4k sector hard
drives) for a simple workaround.

This is based on vblade-20, tests on Debian Linux included a simple
write of each sector with a unique ID and later re-read in order to
detect any re-ordering bugs, however everything worked as expected.

   Christoph

Author: ecashin

aoe.c

@@ -462,11 +462,13 @@ int
 main(int argc, char **argv)
 {
 	int ch, omode = 0, readonly = 0;
+	vlong length = 0;
 
 	bufcnt = Bufcount;
+	offset = 0;
 	setbuf(stdin, NULL);
 	progname = *argv;
-	while ((ch = getopt(argc, argv, "b:dsrm:")) != -1) {
+	while ((ch = getopt(argc, argv, "b:dsrm:o:l:")) != -1) {
 		switch (ch) {
 		case 'b':
 			bufcnt = atoi(optarg);
@@ -485,6 +487,12 @@ main(int argc, char **argv)
 		case 'm':
 			setmask(optarg);
 			break;
+		case 'o':
+			offset = atoi(optarg);
+			break;
+		case 'l':
+			length = atoi(optarg);
+			break;
 		case '?':
 		default:
 			usage();
@@ -505,6 +513,18 @@ main(int argc, char **argv)
 	setserial(argv[3], shelf, slot);
 	size = getsize(bfd);
 	size /= 512;
+	if (size < offset) {
+		fprintf(stderr, "Offset %llu too big - remaining size is negative!\n", offset);
+		exit(1);
+	}
+	size -= offset;
+	if (length) {
+		if (length > size) {
+			fprintf(stderr, "Length %llu too big - exceeds size of file!\n", offset);
+			exit(1);
+		}
+		size = length;
+	}
 	ifname = argv[2];
 	sfd = dial(ifname, bufcnt);
 	getea(sfd, ifname, mac);

ata.c

@@ -164,12 +164,12 @@ atacmd(Ataregs *p, uchar *dp, int ndp, int payload) // do the ata cmd
 		return 0;
 	}
 	if (p->cmd == 0x20 || p->cmd == 0x24)
-		n = getsec(bfd, dp, lba, p->sectors);
+		n = getsec(bfd, dp, lba+offset, p->sectors);
 	else {
 		// packet should be big enough to contain the data
 		if (payload < 512 * p->sectors)
 			return -1;
-		n = putsec(bfd, dp, lba, p->sectors);
+		n = putsec(bfd, dp, lba+offset, p->sectors);
 	}
 	n /= 512;
 	if (n != p->sectors) {

dat.h

@@ -169,6 +169,7 @@ uchar	mac[6];
 int	bfd;		// block file descriptor
 int	sfd;		// socket file descriptor
 vlong	size;		// size of vblade
+vlong	offset;
 char	*progname;
 char	serial[Nserial+1];
 

vblade.8

@@ -55,6 +55,14 @@ The -r flag restricts the export of the device to be read-only.
 The -m flag takes an argument, a comma separated list of MAC addresses
 permitted access to the vblade.  A MAC address can be specified in upper
 or lower case, with or without colons.
+.TP
+\fB-o\fP
+The -o flag takes an argument, the number of sectors that should be
+skipped at the beginning of filename.
+.TP
+\fB-l\fP
+The -l flag takes an argument, the number of sectors to exports.
+Defaults to the end of file.
 .SH EXAMPLE
 In this example, the root user on a host named
 .I nai