[Qemu-devel] [PATCH v3 0/1] block: pass the right options for BlockDriver.bdrv_open()

[Dong Jia Shi]

Trying to restore rbd image on ceph cluster from snapshot with
qemu-img could trigger a calling to raw_open with a NULL @options,
and that will lead to a failure of the snapshot applying.

address@hidden ~]# gdb --args qemu-img snapshot -a snap1 rbd:test_pool/dj_image
... ...
Program received signal SIGSEGV, Segmentation fault.
0x00000000801395a8 in qdict_next_entry (qdict=0x0, first_bucket=0) at 
qobject/qdict.c:327
327             if (!QLIST_EMPTY(&qdict->table[i])) {
(gdb) bt
#0  0x00000000801395a8 in qdict_next_entry (qdict=0x0, first_bucket=0) at 
qobject/qdict.c:327
#1  0x0000000080139626 in qdict_first (qdict=0x0) at qobject/qdict.c:340
#2  0x000000008013a00c in qdict_extract_subqdict (src=0x0, dst=0x3ffffffec50, 
start=0x80698260 "file.")
    at qobject/qdict.c:576
#3  0x0000000080019c26 in bdrv_open_child_bs (filename=0x0, options=0x0, 
bdref_key=0x8017ab38 "file", 
    parent=0x80630300, child_role=0x80176108 <child_file>, allow_none=false, 
errp=0x0) at block.c:2018
#4  0x0000000080019dfa in bdrv_open_child (filename=0x0, options=0x0, 
bdref_key=0x8017ab38 "file", 
    parent=0x80630300, child_role=0x80176108 <child_file>, allow_none=false, 
errp=0x0) at block.c:2065
#5  0x000000008002b9a0 in raw_open (bs=0x80630300, options=0x0, flags=8194, 
errp=0x0) at block/raw-format.c:387
#6  0x0000000080087516 in bdrv_snapshot_goto (bs=0x80630300, 
snapshot_id=0x3fffffff75c "snap1")
    at block/snapshot.c:194
#7  0x0000000080010b8c in img_snapshot (argc=4, argv=0x3fffffff4c0) at 
qemu-img.c:2937
#8  0x00000000800140e4 in main (argc=4, argv=0x3fffffff4c0) at qemu-img.c:4373

The problematic code is /block/snapshot.c:194:
178 int bdrv_snapshot_goto(BlockDriverState *bs,
179                        const char *snapshot_id)
180 {
181     BlockDriver *drv = bs->drv;
182     int ret, open_ret;
183 
184     if (!drv) {
185         return -ENOMEDIUM;
186     }
187     if (drv->bdrv_snapshot_goto) {
188         return drv->bdrv_snapshot_goto(bs, snapshot_id);
189     }
190 
191     if (bs->file) {
192         drv->bdrv_close(bs);
193         ret = bdrv_snapshot_goto(bs->file->bs, snapshot_id);
194         open_ret = drv->bdrv_open(bs, NULL, bs->open_flags, NULL);
195         if (open_ret < 0) {
196             bdrv_unref(bs->file->bs);
197             bs->drv = NULL;
198             return open_ret;
199         }
200         return ret;
201     }
202 
203     return -ENOTSUP;
204 }

After a chat with Kevin, my understanding is that's because NULL
@options is not a valid parameter value for the bdrv_open callback of
the BlockDriver.
We shoule prepare a @options by adding the "file" key-value pair to
the actual options that were given for the node (i.e. bs->options),
and pass it to the callback.

Then Max pointed out that we're not guaranteed that "file" is a
nested QDict. The following commands will both result in segfaults:
$ ./qemu-img snapshot -a foo \
"json:{'driver':'raw','file':{'driver':'qcow2','file':{'driver':'file','filename':'foo.qcow2'}}}"
$ ./qemu-img snapshot -a foo --image-opts \
driver=raw,file.driver=qcow2,file.file.driver=file,file.file.filename=foo.qcow2

Max also proposed for the previous patch (v2) to:
(1) Remove every option in "options" that has a "file." prefix before
the qdict_put() call.
(2) Use bdrv_unref_child(bs, bs->file) instead of bdrv_unref(bs->file->bs).

So I adopted Max's suggestions, and here is the new patch.

Dong Jia Shi (1):
  block: pass the right options for BlockDriver.bdrv_open()

 block/snapshot.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

-- 
2.8.4