Re: [Qemu-block] [PATCH v2 1/1] qemu/migration: fix the double free problem on from_src_file

[QingFeng Hao]

在 2017/6/6 20:49, Kevin Wolf 写道:
> Am 06.06.2017 um 07:24 hat QingFeng Hao geschrieben:
> > In load_snapshot, mis->from_src_file is freed twice, the first free is by
> > qemu_fclose, the second is by migration_incoming_state_destroy and
> > it causes Illegal instruction exception. The fix is just to remove the
> > first free.
> >
> > This problem is found by qemu-iotests case 068 since commit
> > "660819b migration: shut src return path unconditionally". The error is:
> > 068 1s ... - output mismatch (see 068.out.bad)
> >      --- tests/qemu-iotests/068.out     2017-05-06 01:00:26.417270437 +0200
> >      +++ 068.out.bad    2017-06-03 13:59:55.360274640 +0200
> >      @@ -6,6 +6,8 @@
> >       QEMU X.Y.Z monitor - type 'help' for more information
> >       (qemu) savevm 0
> >       (qemu) quit
> >      +./common.config: line 107: 242472 Illegal instruction     (core dumped) ( if [ -n 
> > "${QEMU_NEED_PID}" ]; then
> >      +    echo $BASHPID > "${QEMU_TEST_DIR}/qemu-${_QEMU_HANDLE}.pid";
> >      +fi; exec "$QEMU_PROG" $QEMU_OPTIONS "$@" )
> >       QEMU X.Y.Z monitor - type 'help' for more information
> >      -(qemu) quit
> >      -*** done
> >      +(qemu) *** done
> >
> > Signed-off-by: QingFeng Hao <address@hidden>
> > Reviewed-by: Dr. David Alan Gilbert <address@hidden>
> > Reviewed-by: Peter Xu <address@hidden>
> Dave, as you only gave R-b rather than merging the patch, should this be
> merged through the block tree?
>
> > diff --git a/migration/savevm.c b/migration/savevm.c
> > index 9c320f59d0..853e14e34e 100644
> > --- a/migration/savevm.c
> > +++ b/migration/savevm.c
> > @@ -2290,7 +2290,6 @@ int load_snapshot(const char *name, Error **errp)
> >   
> >       aio_context_acquire(aio_context);
> >       ret = qemu_loadvm_state(f);
> > -    qemu_fclose(f);
> >       aio_context_release(aio_context);
> >   
> >       migration_incoming_state_destroy();
> Did we check other callers of migration_incoming_state_destroy()?
>
> For example, qmp_xen_load_devices_state() looks suspicious, too.
Good reminder! Yes, I checked it and there is no assignment of 
from_src_file there and f
is opened locally, so I think that qemu_fclose doesn't impact 
migration_incoming_state_destroy.
migration_incoming_state_destroy is called in 4 places:
process_incoming_migration_bh, postcopy_ram_listen_thread, 
qmp_xen_load_devices_state
and load_snapshot. process_incoming_migration_bh is launched by 
process_incoming_migration_co
whose qemu_fclose is removed by commit 660819b.
For postcopy_ram_listen_thread, I didn't see where it calls qemu_fclose.
Actually to simplify the check for the problem, I just searched where 
from_src_file
is assigned to and got 2 places: process_incoming_migration_co and 
load_snapshot.
qemu_fclose in the first function is removed by commit 660819b, and 
qemu_fclose in the
second is removed by this one. I think a potential risk might be opaque 
is closed
by anywhere else than process_incoming_migration_co, but there is legacy 
qemu_close
before commit 660819b, so the risk might be low? thanks :)
> I can't tell for postcopy_ram_listen_thread() - commit 660819b didn't
> seem to remove a qemu_fclose() call there, but I can't see one left
> behind either. Was the file leaked before commit 660819b or am I
> missing something?
I don't think so because loadvm_postcopy_handle_listen creates thread 
postcopy_ram_listen_thread
and passes mis->from_src_file as its arg, which will be closed by 
migration_incoming_state_destroy.
What confuses me is in the series function calls of 
qemu_loadvm_state_main etc, argument f looks
to be redundant as mis already contains from_src_file which equals to f. 
Furthermore, mis may be
also redundant as it can be got via migration_incoming_get_current. Thanks!
> Kevin

--
Regards
QingFeng Hao