[Bug 191908] New: [zfs] newer kernel and old userland binary compatibility broken

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=191908

            Bug ID: 191908
           Summary: [zfs] newer kernel and old userland binary
                    compatibility broken
           Product: Base System
           Version: 10.0-STABLE
          Hardware: amd64
                OS: Any
            Status: Needs Triage
          Severity: Affects Only Me
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: ykasap at gmail.com

I tried to update my 10-STABLE machine (full ZFS including root)
from r268480 (built on Jul 10) to r268734 (built on Jul 16), but after
make installkernel and reboot into single user mode, I couldn't proceed
further because "zfs mount -a" crashed immediately with segfault.  

Because only read-only root fs was available, I couldn't do any
debugging (/usr resides in a separate fs).  Actually any zfs or 
zpool commands seemed to crash immediately.  Of course I couldn't 
make installworld.

I guess this incompatibility had been introduced by recent several ZFS 
related MFC/MFV between two builds.  I believe old userland should be 
usable with a newer kernel (or I cannot follow the standard install
procedure).  If it is an expected behavior, it should be mentioned in 
/usr/src/UPDATING.

% uname -a
FreeBSD elvenbow.cc.kyushu-u.ac.jp 10.0-STABLE FreeBSD 10.0-STABLE #0 r268480:
Thu Jul 10 15:25:15 JST 2014    
root at elvenbow.cc.kyushu-u.ac.jp:/usr/obj/usr/src/sys/ELVENBOW  amd64

% what /boot/kernel.new/kernel
/boot/kernel.new/kernel:
        FreeBSD 10.0-STABLE #0 r268734: Wed Jul 16 12:36:48 JST 2014

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