root@juju-80cd62-0:~/src# ceph-mon --version
ceph version 12.2.8 (ae699615bac534ea496ee965ac6192cb7e0e07c0) luminous (stable)

Attach to ceph-mon
------------------
# while [ true ]; do pid=$(pgrep "ceph-mon"); if [ "$pid" != "" ]; then gdb -p $pid; fi; done

(gdb) dir /root/src/ceph-12.2.8
(gdb) set print pretty on


The problem seems to occur in decode_32_or_64_string_map()
----------------------------------------------------------
(gdb) b decode_32_or_64_string_map
(gdb) c
Thread 15 "ms_dispatch" hit Breakpoint 1, decode_32_or_64_string_map (m=std::map with 0 elements, blp=...)
    at ./src/crush/CrushWrapper.cc:2451
2451    {


(gdb) b ::decode_nohead
(gdb) c
Thread 15 "ms_dispatch" hit Breakpoint 2, decode_32_or_64_string_map (m=std::map with 1 element = {...}, blp=...)
    at ./src/crush/CrushWrapper.cc:2465
2465        ::decode_nohead(strlen, m[key], blp);
(gdb) print strlen
$1 = 50331648 (48 MiB)
(gdb) print m 
$2 = std::map with 1 element = {[0] = ""}
(gdb) print blp
$3 = (ceph::buffer::list::iterator &) @0x3ff86ff8a78: {
  <ceph::buffer::list::iterator_impl<false>> = {
    <std::iterator<std::forward_iterator_tag, char, long, char*, char&>> = {<No data fields>},.
    members of ceph::buffer::list::iterator_impl<false>:.
    bl = 0x3ff86ff8c70,.
    ls = 0x3ff86ff8c70,.
    off = 124,.
    p = {
  _raw = 0x3ff600187a8,.
  _off = 0,.
  _len = 340
},. 
    p_off = 124
  }, <No data fields>}

(gdb) s
(gdb) s
(gdb) s
(gdb) s
(gdb) s
decode_nohead (p=..., s=..., len=50331648) at ./src/include/encoding.h:208
208       p.copy(len, s);
(gdb) print len
$5 = 50331648
(gdb) print s
$6 = <optimized out>

(gdb) s
(gdb) s
Thread 15 "ms_dispatch" received signal SIGABRT, Aborted.
__GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
51      ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.