ufs/ffs: detect endian mismatch between machine and filesystem

[alfredo]

Summary

Mount on a LE machine a filesystem formatted for BE is not supported currently. This adds a check for the superblock magic number using swapped bytes to guess and warn the user that it may be a valid superblock but endian is incompatible.

MFC after: 2 weeks

The following kernel output is seen when endian mismatch is detected:

Trying to mount root from ufs:/dev/vtbd0s3 []...
UFS2 superblock failed: possible endian mismatch between machine and filesystem
Attempted recovery for standard superblock: failed
Attempted extraction of recovery data from standard superblock: failed
Attempt to find boot zone recovery data.
panic: ffs_use_bread: non-NULL *bufp 0xc00800001b2868b0
...
...

Test Plan

• test powerpc64 kernel with UFS2 created for BE and LE
• test powerpc64le kernel with UFS2 created for BE and LE

[alfredo]

I have some questions :)
Should we define a custom (negative) error number for endian mismatch?
Should we check for the returned error number and make the caller print the message instead?

[mckusick]

Thanks for tracking down this issue. Your patch is a good start, but needs some followup in other areas of the code. I hope to come up with a proposal soon.

[mckusick]

Here is my proposed fix:

{F53027994}

Let me know if this works for you.

[alfredo]

In D37675#856834, @mckusick wrote:

Here is my proposed fix:

{F53027994}

Let me know if this works for you.

Wow, that was fast, thanks!
How can I access {F53027994}?

[mckusick]

You should just need to click on the "Download" link in the diffs box. If that does not work let me know and I will "commandeer" this report and replace your diff with mine.

[jhibbits]

@mckusick it looks like you need to somehow give others read permission on the diff, only you have permission.

[mckusick]

I cannot figure out how to share the diffs download, so here they are inline.

diff --git a/sys/ufs/ffs/ffs_subr.c b/sys/ufs/ffs/ffs_subr.c
index 705f8c9c961d..0b462add0d05 100644
--- a/sys/ufs/ffs/ffs_subr.c
+++ b/sys/ufs/ffs/ffs_subr.c
@@ -35,6 +35,7 @@
 __FBSDID("$FreeBSD$");

 #include <sys/param.h>
+#include <sys/endian.h>
 #include <sys/limits.h>

 #ifndef _KERNEL
@@ -144,6 +145,7 @@ static int validate_sblock(struct fs *, int);
  *     EIO: non-existent or truncated superblock.
  *     EIO: error reading summary information.
  *     ENOENT: no usable known superblock found.
+ *     EILSEQ: filesystem with wrong byte order found.
  *     ENOMEM: failed to allocate space for the superblock.
  *     EINVAL: The previous newfs operation on this volume did not complete.
  *         The administrator must complete newfs before using this volume.
@@ -382,6 +384,17 @@ validate_sblock(struct fs *fs, int flags)
        prtmsg = ((flags & UFS_NOMSG) == 0);
        warnerr = (flags & UFS_NOWARNFAIL) == UFS_NOWARNFAIL ? 0 : ENOENT;
        wmsg = warnerr ? "" : " (Ignored)";
+       /*
+        * Check for endian mismatch between machine and filesystem.
+        */
+       if (((fs->fs_magic != FS_UFS2_MAGIC) &&
+           (bswap32(fs->fs_magic) == FS_UFS2_MAGIC)) ||
+           ((fs->fs_magic != FS_UFS1_MAGIC) &&
+           (bswap32(fs->fs_magic) == FS_UFS1_MAGIC))) {
+               MPRINT("UFS superblock failed due to endian mismatch "
+                   "between machine and filesystem\n");
+               return(EILSEQ);
+       }
        /*
         * If just validating for recovery, then do just the minimal
         * checks needed for the superblock fields needed to find
@@ -627,8 +640,16 @@ ffs_sbsearch(void *devfd, struct fs **fsp, int reqflags,
         * failure can be avoided.
         */
        flags = UFS_NOMSG | nocsum;
-       if (ffs_sbget(devfd, fsp, UFS_STDSB, flags, filltype, readfunc) == 0)
-               return (0);
+       error = ffs_sbget(devfd, fsp, UFS_STDSB, flags, filltype, readfunc);
+       /*
+        * If successful or endian error, no need to try further.
+        */
+       if (error == 0 || error == EILSEQ) {
+               if (msg && error == EILSEQ)
+                       printf("UFS superblock failed due to endian mismatch "
+                           "between machine and filesystem\n");
+               return (error);
+       }
        /*
         * First try: ignoring hash failures.
         */
@@ -677,6 +698,7 @@ ffs_sbsearch(void *devfd, struct fs **fsp, int reqflags,
                 * but some devices lie. So we just try a plausible range.
                 */
                error = ENOENT;
+               fsrbuf = NULL;
                for (secsize = dbtob(1); secsize <= SBLOCKSIZE; secsize *= 2)
                        if ((error = (*readfunc)(devfd, (SBLOCK_UFS2 - secsize),
                            &fsrbuf, secsize)) == 0)

[alfredo]

Update with code from mckusick

[alfredo]

remove empty line

[alfredo]

@mckusick , thank you for the improved patch!
During test I figured out that kernel retries to mount the filesystem several times. I prevented this changing sys/kern/vfs_mountroot.c

With the patch we have the following behavior during boot:

Trying to mount root from ufs:vtbd0s3 []...
UFS superblock failed due to endian mismatch between machine and filesystem
Mounting from ufs:vtbd0s3 failed with error 86.

..
..
mountroot>

And when mounting in the userland we get:

root@:~ # mount /dev/vtbd1s3 /mnt/
UFS superblock failed due to endian mismatch between machine and filesystem
mount: /dev/vtbd1s3: Illegal byte sequence

[mckusick]

Your addition to vfs_mountroot.c (parse_mount()) looks correct. Thanks for pushing this through.