Specifying root device in /etc/fstab

[Jared McNeill <jmcneill%invisible.ca@localhost>]

The ARM images currently contain an fstab entry that points to ld0. I want 
to make this more generic as we may have different root devices and/or 
root device units may not be consistent across reboots.

I came up with this simple patch to getfsspecname that allows ROOT=<part> 
syntax for fs_spec. It uses the value of the kern.root_device sysctl to 
construct a device path, so my fstab can have entries like this:

  ROOT=a          /               ffs     rw,noatime      1 1
  ROOT=b          none            swap    sw      0 0
  ROOT=e          /boot           msdos   rw      1 1

Thoughts? Suggestions?

Jared



Index: getfsspecname.c
===================================================================
RCS file: /cvsroot/src/lib/libutil/getfsspecname.c,v
retrieving revision 1.5
diff -u -p -r1.5 getfsspecname.c
--- getfsspecname.c	25 May 2014 13:46:07 -0000	1.5
+++ getfsspecname.c	30 Sep 2018 22:10:33 -0000
@@ -59,6 +59,30 @@ getfsspecname(char *buf, size_t bufsiz,
 	char *vname;

 	p = drives = vname = NULL;
+
+	if (strncasecmp(name, "ROOT=", 5) == 0) {
+		if (sysctlbyname("kern.root_device", NULL, &len, NULL, 0) == -1) {
+			savee = errno;
+			strlcpy(buf, "sysctl kern.root_device failed", bufsiz);
+			goto out;
+		}
+		dk = malloc(len);
+		if (dk == NULL) {
+			savee = errno;
+			strlcpy(buf, "malloc failed", bufsiz);
+			goto out;
+		}
+		if (sysctlbyname("kern.root_device", dk, &len, NULL, 0) == -1) {
+			savee = errno;
+			strlcpy(buf, "sysctl kern.root_device failed", bufsiz);
+			free(dk);
+			goto out;
+		}
+		snprintf(buf, bufsiz, "/dev/%s%s", dk, name + 5);
+		free(dk);
+		return buf;
+	}
+
 	if (strncasecmp(name, "NAME=", 5) != 0) {
 #ifdef COMPAT_DKWEDGE
 		/*