Currently it is not possible to attach softraid volumes that reside on
read-only media, as the existing code assumes that it can always open the
device for read/write access.

So, for example, a usb drive with a physical write-protect switch that is used
to hold a softraid crypto volume can't be mounted if it's set to read-only.

I recently had a look at what was needed to support doing this.

A full detailed write up for anybody who is interested is here:

https://research.exoticsilicon.com/articles/read_only_softraid

But essentially we just need to modify softraid_meta_probe() to retry the
initial VOP_OPEN() as read-only, and then add support for softraid sd devices
to respond to the SCSI MODE_SENSE opcode with a read-only flag in the header.

A couple of small changes to /sbin/bioctl and /sbin/scsi make things work as
expected in userland.

Separately, the /sbin/scsi utility has some bugs in it's hex dump printing
code of raw mode pages.  I'll post details of these in a separate thread, as
they are not directly releated to the read-only code.


--- sbin/scsi/scsi.c.dist	Sun Dec  4 23:50:47 2022
+++ sbin/scsi/scsi.c		Thu May 15 15:17:51 2025
@@ -113,7 +113,8 @@
 			break;
 		case 'f':
 			if ((fd = scsi_open(optarg, O_RDWR)) < 0)
-				err(1, "unable to open device %s", optarg);
+				if ((fd = scsi_open(optarg, O_RDONLY)) < 0)
+					err(1, "unable to open device %s", optarg);
 			fflag = 1;
 			break;
 		case 'd':

--- sbin/bioctl/bioctl.c.dist	Mon Apr  7 04:25:01 2025
+++ sbin/bioctl/bioctl.c	Thu May 15 15:20:45 2025
@@ -230,7 +230,9 @@
 	if (devicename == NULL)
 		errx(1, "need device");
 
-	devh = opendev(devicename, O_RDWR, OPENDEV_PART, &realname);
+	devh = opendev(devicename, (func == BIOC_DELETERAID ||
+	    func == BIOC_INQ ? O_RDONLY : O_RDWR), OPENDEV_PART, &realname);
+
 	if (devh == -1) {
 		devh = open("/dev/bio", O_RDWR);
 		if (devh == -1)

--- sys/dev/softraid.c.dist	Mon Apr  7 04:42:29 2025
+++ sys/dev/softraid.c		Fri May  9 13:10:01 2025
@@ -334,6 +334,10 @@
 			 * and figure out the open/close dance for unwind.
 			 */
 			error = VOP_OPEN(vn, FREAD | FWRITE, NOCRED, curproc);
+			if (error == EACCES || error == EROFS) {
+				sd->sd_capabilities |= SR_CAP_READONLY;
+				error = VOP_OPEN(vn, FREAD, NOCRED, curproc);
+				}
 			if (error) {
 				DNPRINTF(SR_D_META,"%s: sr_meta_probe can't "
 				    "open %s\n", DEVNAME(sc), devname);
@@ -2321,6 +2325,7 @@
 	struct sr_softc		*sc = link->bus->sb_adapter_softc;
 	struct sr_workunit	*wu = xs->io;
 	struct sr_discipline	*sd;
+	unsigned char		mode_sense_header[4];
 
 	DNPRINTF(SR_D_CMD, "%s: sr_scsi_cmd target %d xs %p flags %#x\n",
 	    DEVNAME(sc), link->target, xs, xs->flags);
@@ -2395,6 +2400,19 @@
 		    DEVNAME(sc));
 		if (sd->sd_scsi_req_sense(wu))
 			goto stuffup;
+		goto complete;
+
+	case MODE_SENSE:
+		if ((xs->cmd.bytes[1] & 0x3f) != 0 && (xs->cmd.bytes[1] & 0x3f) != 0x3f) {
+			goto stuffup;
+			}
+		bzero (&mode_sense_header, 4);
+		mode_sense_header[0] = 0x03;
+		if (sd->sd_capabilities & SR_CAP_READONLY) {
+			mode_sense_header[2] = 0x80;
+			}
+		scsi_copy_internal_data(xs, &mode_sense_header, 4);
+		xs->datalen = 4;
 		goto complete;
 
 	default:

--- sys/dev/softraidvar.h.dist	Mon Apr  7 04:25:11 2025
+++ sys/dev/softraidvar.h	Tue May  6 09:17:54 2025
@@ -535,6 +535,7 @@
 #define SR_CAP_REBUILD		0x00000004	/* Supports rebuild. */
 #define SR_CAP_NON_COERCED	0x00000008	/* Uses non-coerced size. */
 #define SR_CAP_REDUNDANT	0x00000010	/* Redundant copies of data. */
+#define SR_CAP_READONLY		0x00000020	/* Read-only. */
 
 	union {
 	    struct sr_raid0	mdd_raid0;