[Qemu-devel] [PATCH v3 0/4] Fix subsection ambiguity in the migration format

[Paolo Bonzini]

With the current migration format, VMS_STRUCTs with subsections
are ambiguous.  The protocol cannot tell whether a 0x5 byte after
the VMS_STRUCT is a subsection or part of the parent data stream.
In the past QEMU assumed it was always a part of a subsection; after
commit eb60260 (savevm: fix corruption in vmstate_subsection_load(),
2011-02-03) the choice depends on whether the VMS_STRUCT has subsections
defined.

Unfortunately, this means that if a destination has no subsections
defined for the struct, it will happily read subsection data into
its own fields.  And if you are "lucky" enough to stumble on a
zero byte at the right time, it will be interpreted as QEMU_VM_EOF
and migration will be interrupted with half-loaded state.

There is no way out of this except defining an incompatible migration
protocol; a sentinel at the end of embedded structs is enough to remove
the ambiguity, but is already incompatible of course.  So this can be
restricted to new machine models, and this is what the patch series does.
(Only patch 3 is specific to the short-term solution, everything else
is entirely generic).

v2->v3
        resolve conflicts, move savevm.o to libhw

v1->v2
        coding style

Paolo Bonzini (4):
  add support for machine models to specify their migration format
  add pc-0.14/pc-0.15 machine
  savevm: define new unambiguous migration format
  Partially revert "savevm: fix corruption in vmstate_subsection_load()."

 Makefile.objs |    4 ++--
 hw/boards.h   |    4 ++++
 hw/pc_piix.c  |   16 +++++++++++++++-
 savevm.c      |   44 ++++++++++++++++++++++++++++++--------------
 4 files changed, 51 insertions(+), 17 deletions(-)

-- 
1.7.6