[Bug 287679] md(4) assigns non-unique GEOM::ident to vnode-based disks

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=287679

            Bug ID: 287679
           Summary: md(4) assigns non-unique GEOM::ident to vnode-based
                    disks
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: phil@krylov.eu
                CC: trasz@FreeBSD.org

Created attachment 261438
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=261438&action=edit
Minimal but changes struct layout with unknown impact

In review D12230, support for GEOM::ident was added to vnode-backed md(4)
devices. The message is saying, "This is useful for gmountver(8) regression
tests."

At first, the identifier was generated as random, then, in an effort to improve
uniqueness, it was changed to MD-DEV<fsid>-INO<fileid>, and merged in this
state.

It's worth to notice that the buffer for this value is only 32 bytes long.
Unfortunately, on a 64-bit system with ZFS, such identifier immediately gets
truncated around the first digit of <fileid>, making its uniqueness much worse
than random:

# ls -i /root/disk*
29580 /root/disk1
29828 /root/disk2
# mdconfig /root/disk1
md0
# mdconfig /root/disk2
md1
# geom md list | grep -E 'name|file|ident'
Geom name: md0
   file: /root/disk1
   ident: MD-DEV11968766667392717182-INO2
Geom name: md1
   file: /root/disk2
   ident: MD-DEV11968766667392717182-INO2

[Hmm, I'd like to have a look at those gmountver(8) regression tests, are they
public?]

So, I see a number of solutions here.

1) md-ident-fix-minimal-but-changes-struct-layout.patch: Updates the buffer
size to fit two UINT64_MAX decimal representations (51 bytes, rounded up to 64
bytes). Seems to work well, but I am not familiar enough with this code to
suggest changing the struct layout: 

# geom md list | grep -E 'name|file|ident'
Geom name: md0
   file: /root/disk1
   ident: MD-DEV11968766667392717182-INO29580
Geom name: md1
   file: /root/disk2
   ident: MD-DEV11968766667392717182-INO29828

2) md-ident-fix-simple.patch: A little bit ugly. Via kvprintf's %r conversion
specifier, uses base 36 to fit two 64-bit values in 26 bytes and have 5 more
bytes left for our namespacing pleasure:

# geom md list | grep -E 'name|file|ident'
Geom name: md0
   file: /root/disk1
   ident: MD-2IXL9OT1ZWGAM-MTO
Geom name: md1
   file: /root/disk2
   ident: MD-2IXL9OT1ZWGAM-N0K

3) md-ident-fix-invasive.patch: A little less ugly, but adds another
undocumented conversion specifier (%R) to kvprintf() to skip an extra
uppercasing loop.

4) md-ident-fix-fast.patch: An ugly but speed-optimized solution which also has
the benefit of touching less code parts than 2) and 3), but serializes the
numbers in little-endian order:

# geom md list | grep -E 'name|file|ident'
Geom name: md0
   file: /root/disk1
   ident: MD-MAGWZ1TO9LXI2-OTM
Geom name: md1
   file: /root/disk2
   ident: MD-MAGWZ1TO9LXI2-K0N

I hope this helps.

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