block/vvfat: fix vvfat_child_perm crash

block/vvfat: fix vvfat_child_perm crash

It's wrong to rely on s->qcow in vvfat_child_perm, as on permission
update during bdrv_open_child() call this field is not set yet.

Still prior to aa5a04c7db27eea6b36de32f241b155f0d9ce34d, it didn't
crash, as bdrv_open_child passed NULL as child to bdrv_child_perm(),
and NULL was equal to NULL in assertion (still, it was bad guarantee
for child being s->qcow, not backing :).

Since aa5a04c7db27eea6b36de32f241b155f0d9ce34d
"add bdrv_attach_child_noperm" bdrv_refresh_perms called on parent node
when attaching child, and new correct child pointer is passed to
.bdrv_child_perm. Still, s->qcow is NULL at the moment. Let's rely only
on role instead.

Without that fix,
./build/qemu-system-x86_64 -usb -device usb-storage,drive=fat16 \
    -drive \
    file=fat:rw:fat-type=16:"<path of a host folder>",id=fat16,format=raw,if=none

crashes:
(gdb) bt
0  raise () at /lib64/libc.so.6
1  abort () at /lib64/libc.so.6
2  _nl_load_domain.cold () at /lib64/libc.so.6
3  annobin_assert.c_end () at /lib64/libc.so.6
4  vvfat_child_perm (bs=0x559186f3d690, c=0x559186f1ed20, role=3,
                     reopen_queue=0x0, perm=0, shared=31,
                     nperm=0x7ffe56f28298, nshared=0x7ffe56f282a0) at
    ../block/vvfat.c:3214
5  bdrv_child_perm (bs=0x559186f3d690, child_bs=0x559186f60190,
                    c=0x559186f1ed20, role=3, reopen_queue=0x0,
                    parent_perm=0, parent_shared=31,
                    nperm=0x7ffe56f28298, nshared=0x7ffe56f282a0)
    at ../block.c:2094
6  bdrv_node_refresh_perm (bs=0x559186f3d690, q=0x0,
                           tran=0x559186f65850, errp=0x7ffe56f28530) at
    ../block.c:2336
7  bdrv_list_refresh_perms (list=0x559186db5b90 = {...}, q=0x0,
                            tran=0x559186f65850, errp=0x7ffe56f28530)
    at ../block.c:2358
8  bdrv_refresh_perms (bs=0x559186f3d690, errp=0x7ffe56f28530) at
    ../block.c:2419
9  bdrv_attach_child
    (parent_bs=0x559186f3d690, child_bs=0x559186f60190,
     child_name=0x559184d83e3d "write-target",
     child_class=0x5591852f3b00 <child_vvfat_qcow>, child_role=3,
     errp=0x7ffe56f28530) at ../block.c:2959
10 bdrv_open_child
    (filename=0x559186f5cb80 "/var/tmp/vl.7WYmFU",
     options=0x559186f66c20, bdref_key=0x559184d83e3d "write-target",
     parent=0x559186f3d690, child_class=0x5591852f3b00
     <child_vvfat_qcow>, child_role=3, allow_none=false,
     errp=0x7ffe56f28530) at ../block.c:3351
11 enable_write_target (bs=0x559186f3d690, errp=0x7ffe56f28530) at
    ../block/vvfat.c:3177
12 vvfat_open (bs=0x559186f3d690, options=0x559186f42db0, flags=155650,
               errp=0x7ffe56f28530) at ../block/vvfat.c:1236
13 bdrv_open_driver (bs=0x559186f3d690, drv=0x5591853d97e0
                     <bdrv_vvfat>, node_name=0x0,
                     options=0x559186f42db0, open_flags=155650,
                     errp=0x7ffe56f28640) at ../block.c:1557
14 bdrv_open_common (bs=0x559186f3d690, file=0x0,
                     options=0x559186f42db0, errp=0x7ffe56f28640) at
    ../block.c:1833
...

(gdb) fr 4
 #4  vvfat_child_perm (bs=0x559186f3d690, c=0x559186f1ed20, role=3,
                      reopen_queue=0x0, perm=0, shared=31,
                      nperm=0x7ffe56f28298, nshared=0x7ffe56f282a0) at
    ../block/vvfat.c:3214
3214        assert(c == s->qcow || (role & BDRV_CHILD_COW));
(gdb) p role
 $1 = 3   # BDRV_CHILD_DATA | BDRV_CHILD_METADATA
(gdb) p *c
 $2 = {bs = 0x559186f60190, name = 0x559186f669d0 "write-target", klass
     = 0x5591852f3b00 <child_vvfat_qcow>, role = 3, opaque =
         0x559186f3d690, perm = 3, shared_perm = 4, frozen = false,
         parent_quiesce_counter = 0, next = {le_next = 0x0, le_prev =
             0x559186f41818}, next_parent = {le_next = 0x0, le_prev =
                 0x559186f64320}}
(gdb) p s->qcow
 $3 = (BdrvChild *) 0x0

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20210524101257.119377-3-vsementsov@virtuozzo.com>
Tested-by: John Arbuckle <programmingkidx@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>