Normally when an attempt is made to mount a UFS/FFS filesystem whose

superblock has a check-hash error, an error message noting the
superblock check-hash failure is printed and the mount fails. The
administrator then runs fsck to repair the filesystem and when
successful, the filesystem can once again be mounted.

This approach fails if the filesystem in question is a root filesystem
from which you are trying to boot. Here, the loader fails when trying
to access the filesystem to get the kernel to boot. So it is necessary
to allow the loader to ignore the superblock check-hash error and make
a best effort to read the kernel. The filesystem may be suffiently
corrupted that the read attempt fails, but there is no harm in trying
since the loader makes no attempt to write to the filesystem.

Once the kernel is loaded and starts to run, it attempts to mount its
root filesystem. Once again, failure means that it breaks to its prompt
to ask where to get its root filesystem. Unless you have an alternate
root filesystem, you are stuck.

Since the root filesystem is initially mounted read-only, it is
safe to make an attempt to mount the root filesystem with the failed
superblock check-hash. Thus, when asked to mount a root filesystem
with a failed superblock check-hash, the kernel prints a warning
message that the root filesystem superblock check-hash needs repair,
but notes that it is ignoring the error and proceeding. It does
mark the filesystem as needing an fsck which prevents it from being
enabled for writing until fsck has been run on it. The net effect
is that the reboot fails to single user, but at least at that point
the administrator has the tools at hand to fix the problem.

Reported by:    Rick Macklem (rmacklem@)
Discussed with: Warner Losh (imp@)
Sponsored by:   Netflix

lib/libufs/libufs.h

@@ -117,6 +117,12 @@ int	ffs_sbget(void *, struct fs **, off_t, char *,
 int	ffs_sbput(void *, struct fs *, off_t,
 	    int (*)(void *, off_t, void *, int));
 
+/*
+ * Request standard superblock location in ffs_sbget
+ */
+#define	STDSB			-1	/* Fail if check-hash is bad */
+#define	STDSB_NOHASHFAIL	-2	/* Ignore check-hash failure */
+
 /*
  * block.c
  */

lib/libufs/sblock.c

@@ -54,7 +54,7 @@ sbread(struct uufsd *disk)
 
 	ERROR(disk, NULL);
 
-	if ((errno = sbget(disk->d_fd, &fs, -1)) != 0) {
+	if ((errno = sbget(disk->d_fd, &fs, STDSB)) != 0) {
 		switch (errno) {
 		case EIO:
 			ERROR(disk, "non-existent or truncated superblock");

sbin/dump/main.c

@@ -433,7 +433,7 @@ main(int argc, char *argv[])
 		msgtail("to %s\n", tape);
 
 	sync();
-	if ((ret = sbget(diskfd, &sblock, -1)) != 0) {
+	if ((ret = sbget(diskfd, &sblock, STDSB)) != 0) {
 		switch (ret) {
 		case ENOENT:
 			warn("Cannot find file system superblock");

sbin/fsck_ffs/setup.c

@@ -327,7 +327,7 @@ readsb(int listerr)
 	int bad, ret;
 	struct fs *fs;
 
-	super = bflag ? bflag * dev_bsize : -1;
+	super = bflag ? bflag * dev_bsize : STDSB;
 	readcnt[sblk.b_type]++;
 	if ((ret = sbget(fsreadfd, &fs, super)) != 0) {
 		switch (ret) {

sbin/fsirand/fsirand.c

@@ -126,7 +126,7 @@ fsirand(char *device)
 	dp2 = NULL;
 
 	/* Read in master superblock */
-	if ((ret = sbget(devfd, &sblock, -1)) != 0) {
+	if ((ret = sbget(devfd, &sblock, STDSB)) != 0) {
 		switch (ret) {
 		case ENOENT:
 			warn("Cannot find file system superblock");

sbin/growfs/growfs.c

@@ -1449,7 +1449,7 @@ main(int argc, char **argv)
 	/*
 	 * Read the current superblock, and take a backup.
 	 */
-	if ((ret = sbget(fsi, &fs, -1)) != 0) {
+	if ((ret = sbget(fsi, &fs, STDSB)) != 0) {
 		switch (ret) {
 		case ENOENT:
 			errx(1, "superblock not recognized");

sbin/quotacheck/quotacheck.c

@@ -321,7 +321,7 @@ chkquota(char *specname, struct quotafile *qfu, struct quotafile *qfg)
 		}
 	}
 	sync();
-	if ((ret = sbget(fi, &fs, -1)) != 0) {
+	if ((ret = sbget(fi, &fs, STDSB)) != 0) {
 		switch (ret) {
 		case ENOENT:
 			warn("Cannot find file system superblock");

stand/libsa/ufs.c

@@ -140,6 +140,11 @@ static int	ufs_use_sa_read(void *, off_t, void **, int);
 /* from ffs_subr.c */
 int	ffs_sbget(void *, struct fs **, off_t, char *,
 	    int (*)(void *, off_t, void **, int));
+/*
+ * Request standard superblock location in ffs_sbget
+ */
+#define	STDSB			-1	/* Fail if check-hash is bad */
+#define	STDSB_NOHASHFAIL	-2	/* Ignore check-hash failure */
 
 /*
  * Read a new inode into a file structure.
@@ -519,7 +524,8 @@ ufs_open(upath, f)
 
 	/* read super block */
 	twiddle(1);
-	if ((rc = ffs_sbget(f, &fs, -1, "stand", ufs_use_sa_read)) != 0)
+	if ((rc = ffs_sbget(f, &fs, STDSB_NOHASHFAIL, "stand",
+	     ufs_use_sa_read)) != 0)
 		goto out;
 	fp->f_fs = fs;
 	/*

sys/geom/journal/g_journal_ufs.c

@@ -72,7 +72,7 @@ g_journal_ufs_dirty(struct g_consumer *cp)
 
 	fs = NULL;
 	if (SBLOCKSIZE % cp->provider->sectorsize != 0 ||
-	    ffs_sbget(cp, &fs, -1, M_GEOM, g_use_g_read_data) != 0) {
+	    ffs_sbget(cp, &fs, STDSB, M_GEOM, g_use_g_read_data) != 0) {
 		GJ_DEBUG(0, "Cannot find superblock to mark file system %s "
 		    "as dirty.", cp->provider->name);
 		KASSERT(fs == NULL,

sys/geom/label/g_label_ufs.c

@@ -77,7 +77,7 @@ g_label_ufs_taste_common(struct g_consumer *cp, char *label, size_t size, int wh
 
 	fs = NULL;
 	if (SBLOCKSIZE % pp->sectorsize != 0 ||
-	    ffs_sbget(cp, &fs, -1, M_GEOM, g_use_g_read_data) != 0) {
+	    ffs_sbget(cp, &fs, STDSB, M_GEOM, g_use_g_read_data) != 0) {
 		KASSERT(fs == NULL,
 		    ("g_label_ufs_taste_common: non-NULL fs %p\n", fs));
 		return;

sys/ufs/ffs/ffs_extern.h

@@ -125,6 +125,12 @@ void	process_deferred_inactive(struct mount *mp);
 #define	FFSR_FORCE	0x0001
 #define	FFSR_UNSUSPEND	0x0002
 
+/*
+ * Request standard superblock location in ffs_sbget
+ */
+#define	STDSB			-1	/* Fail if check-hash is bad */
+#define	STDSB_NOHASHFAIL	-2	/* Ignore check-hash failure */
+
 /*
  * Definitions for TRIM interface
  *

sys/ufs/ffs/ffs_subr.c

@@ -51,6 +51,11 @@ struct malloc_type;
 #define UFS_MALLOC(size, type, flags) malloc(size)
 #define UFS_FREE(ptr, type) free(ptr)
 #define UFS_TIME time(NULL)
+/*
+ * Request standard superblock location in ffs_sbget
+ */
+#define	STDSB			-1	/* Fail if check-hash is bad */
+#define	STDSB_NOHASHFAIL	-2	/* Ignore check-hash failure */
 
 #else /* _KERNEL */
 #include <sys/systm.h>
@@ -139,14 +144,14 @@ ffs_load_inode(struct buf *bp, struct inode *ip, struct fs *fs, ino_t ino)
 	ip->i_gid = dip2->di_gid;
 	return (0);
 }
-#endif /* KERNEL */
+#endif /* _KERNEL */
 
 /*
  * These are the low-level functions that actually read and write
  * the superblock and its associated data.
  */
 static off_t sblock_try[] = SBLOCKSEARCH;
-static int readsuper(void *, struct fs **, off_t, int,
+static int readsuper(void *, struct fs **, off_t, int, int,
 	int (*)(void *, off_t, void **, int));
 
 /*
@@ -177,21 +182,25 @@ ffs_sbget(void *devfd, struct fs **fsp, off_t altsblock,
 	int i, error, size, blks;
 	uint8_t *space;
 	int32_t *lp;
+	int chkhash;
 	char *buf;
 
 	fs = NULL;
 	*fsp = NULL;
-	if (altsblock != -1) {
-		if ((error = readsuper(devfd, &fs, altsblock, 1,
+	chkhash = 1;
+	if (altsblock >= 0) {
+		if ((error = readsuper(devfd, &fs, altsblock, 1, chkhash,
 		     readfunc)) != 0) {
 			if (fs != NULL)
 				UFS_FREE(fs, filltype);
 			return (error);
 		}
 	} else {
+		if (altsblock == STDSB_NOHASHFAIL)
+			chkhash = 0;
 		for (i = 0; sblock_try[i] != -1; i++) {
 			if ((error = readsuper(devfd, &fs, sblock_try[i], 0,
-			     readfunc)) == 0)
+			     chkhash, readfunc)) == 0)
 				break;
 			if (fs != NULL) {
 				UFS_FREE(fs, filltype);
@@ -255,7 +264,7 @@ ffs_sbget(void *devfd, struct fs **fsp, off_t altsblock,
  */
 static int
 readsuper(void *devfd, struct fs **fsp, off_t sblockloc, int isaltsblk,
-    int (*readfunc)(void *devfd, off_t loc, void **bufp, int size))
+    int chkhash, int (*readfunc)(void *devfd, off_t loc, void **bufp, int size))
 {
 	struct fs *fs;
 	int error, res;
@@ -285,8 +294,9 @@ readsuper(void *devfd, struct fs **fsp, off_t sblockloc, int isaltsblk,
 		if (fs->fs_ckhash != (ckhash = ffs_calc_sbhash(fs))) {
 #ifdef _KERNEL
 			res = uprintf("Superblock check-hash failed: recorded "
-			    "check-hash 0x%x != computed check-hash 0x%x\n",
-			    fs->fs_ckhash, ckhash);
+			    "check-hash 0x%x != computed check-hash 0x%x%s\n",
+			    fs->fs_ckhash, ckhash,
+			    chkhash == 0 ? " (Ignored)" : "");
 #else
 			res = 0;
 #endif
@@ -297,7 +307,14 @@ readsuper(void *devfd, struct fs **fsp, off_t sblockloc, int isaltsblk,
 			if (res == 0)
 				printf("Superblock check-hash failed: recorded "
 				    "check-hash 0x%x != computed check-hash "
-				    "0x%x\n", fs->fs_ckhash, ckhash);
+				    "0x%x%s\n", fs->fs_ckhash, ckhash,
+				    chkhash == 0 ? " (Ignored)" : "");
+			if (chkhash == 0) {
+				fs->fs_flags |= FS_NEEDSFSCK;
+				fs->fs_fmod = 1;
+				return (0);
+			}
+			fs->fs_fmod = 0;
 			return (EINVAL);
 		}
 		/* Have to set for old filesystems that predate this field */

sys/ufs/ffs/ffs_vfsops.c

@@ -774,6 +774,7 @@ ffs_mountfs(devvp, mp, td)
 	struct g_consumer *cp;
 	struct mount *nmp;
 	int candelete;
+	off_t loc;
 
 	fs = NULL;
 	ump = NULL;
@@ -810,9 +811,11 @@ ffs_mountfs(devvp, mp, td)
 		goto out;
 	}
 	/* fetch the superblock and summary information */
-	if ((error = ffs_sbget(devvp, &fs, -1, M_UFSMNT, ffs_use_bread)) != 0)
+	loc = STDSB;
+	if ((mp->mnt_flag & MNT_ROOTFS) != 0)
+		loc = STDSB_NOHASHFAIL;
+	if ((error = ffs_sbget(devvp, &fs, loc, M_UFSMNT, ffs_use_bread)) != 0)
 		goto out;
-	fs->fs_fmod = 0;
 	/* none of these types of check-hashes are maintained by this kernel */
 	fs->fs_metackhash &= ~(CK_INODE | CK_INDIR | CK_DIR);
 	/* no support for any undefined flags */

usr.sbin/fstyp/ufs.c

@@ -50,7 +50,7 @@ fstyp_ufs(FILE *fp, char *label, size_t labelsize)
 {
 	struct fs *fs;
 
-	switch (sbget(fileno(fp), &fs, -1)) {
+	switch (sbget(fileno(fp), &fs, STDSB)) {
 	case 0:
 		strlcpy(label, fs->fs_volname, labelsize);
 		return (0);

usr.sbin/quot/quot.c

@@ -550,7 +550,7 @@ quot(char *name, char *mp)
 		close(fd);
 		return;
 	}
-	switch (sbget(fd, &fs, -1)) {
+	switch (sbget(fd, &fs, STDSB)) {
 	case 0:
 		break;
 	case ENOENT: