block/mirror: check backing in bdrv_mirror_top_refresh_filename

Backing may be zero after failed bdrv_attach_child in bdrv_set_backing_hd, which leads to SIGSEGV.

---

Hi all.

We have faced into this SIGSEGV because of image locking:
trying to call qemu-img commit on locked image leads to the following:

    Program terminated with signal 11, Segmentation fault.
    #0  bdrv_mirror_top_refresh_filename (bs=0x5616df9a7400, opts=0x5616df268400)
        at block/mirror.c:1203
    1203        bdrv_refresh_filename(bs->backing->bs);
    
    (gdb) bt
    #0  bdrv_mirror_top_refresh_filename (bs=0x5616df9a7400, opts=0x5616df268400)
        at block/mirror.c:1203
    #1  0x00005616ddc3d35f in bdrv_refresh_filename (bs=0x5616df9a7400) at block.c:4739
    #2  0x00005616ddc3d672 in bdrv_set_backing_hd (bs=bs@entry=0x5616df9a7400, 
        backing_hd=backing_hd@entry=0x5616df25c000, errp=errp@entry=0x7ffff7896a20)
        at block.c:2035
    #3  0x00005616ddc3dee3 in bdrv_append (bs_new=bs_new@entry=0x5616df9a7400, 
        bs_top=bs_top@entry=0x5616df25c000, errp=errp@entry=0x7ffff7896ad8)
        at block.c:3168
    #4  0x00005616ddc84e5f in mirror_start_job (
        job_id=job_id@entry=0x5616ddd16a31 "commit", bs=bs@entry=0x5616df25c000, 
        creation_flags=creation_flags@entry=0, target=target@entry=0x5616df262800, 
        replaces=replaces@entry=0x0, speed=speed@entry=0, granularity=65536, 
        granularity@entry=0, buf_size=16777216, buf_size@entry=0, 
        backing_mode=backing_mode@entry=MIRROR_LEAVE_BACKING_CHAIN, 
        on_source_error=on_source_error@entry=BLOCKDEV_ON_ERROR_REPORT, 
        on_target_error=on_target_error@entry=BLOCKDEV_ON_ERROR_REPORT, 
        unmap=unmap@entry=true, cb=cb@entry=0x5616ddc35470 <common_block_job_cb>, 
        opaque=opaque@entry=0x7ffff7896c80, errp=errp@entry=0x7ffff7896bd0, 
        driver=driver@entry=0x5616ddf8d100 <commit_active_job_driver>, 
        is_none_mode=is_none_mode@entry=false, base=base@entry=0x5616df262800, 
        auto_complete=auto_complete@entry=false, 
        filter_node_name=filter_node_name@entry=0x0) at block/mirror.c:1314
    #5  0x00005616ddc87580 in commit_active_start (
        job_id=job_id@entry=0x5616ddd16a31 "commit", bs=bs@entry=0x5616df25c000, 
        base=base@entry=0x5616df262800, creation_flags=creation_flags@entry=0, 
        speed=speed@entry=0, on_error=on_error@entry=BLOCKDEV_ON_ERROR_REPORT, 
        filter_node_name=filter_node_name@entry=0x0, 
        cb=cb@entry=0x5616ddc35470 <common_block_job_cb>, 
        opaque=opaque@entry=0x7ffff7896c80, errp=errp@entry=0x7ffff7896c78, 
        auto_complete=auto_complete@entry=false) at block/mirror.c:1463
    #6  0x00005616ddc33a68 in img_commit (argc=<optimized out>, argv=<optimized out>)
        at qemu-img.c:1013
    #7  0x00005616ddc2fa79 in main (argc=4, argv=0x7ffff7896e00) at qemu-img.c:4548


    (gdb) p bs->backing
    $2 = (BdrvChild *) 0x0
    (gdb) fr 2
    #2  0x00005616ddc3d672 in bdrv_set_backing_hd (bs=bs@entry=0x5616df9a7400, 
        backing_hd=backing_hd@entry=0x5616df25c000, errp=errp@entry=0x7ffff7896a20)
        at block.c:2035
    2035        bdrv_refresh_filename(bs);
    (gdb) p *errp
    $4 = (Error *) 0x5616df1c2660
    (gdb) p **errp
    $5 = {msg = 0x5616df2554e0 "Failed to get \"write\" lock", 
      err_class = ERROR_CLASS_GENERIC_ERROR, 
      src = 0x5616ddd267fe "block/file-posix.c", 
      func = 0x5616ddd26fe0 <__func__.27999> "raw_check_lock_bytes", line = 682, 
      hint = 0x5616df1fe520}

 block/mirror.c | 5 +++++
 1 file changed, 5 insertions(+)

index 6f5cb9f26c..351b80ca2c 100644
@@ -1073,6 +1073,11 @@ static int coroutine_fn bdrv_mirror_top_pdiscard(BlockDriverState *bs,
 
 static void bdrv_mirror_top_refresh_filename(BlockDriverState *bs, QDict *opts)
 {
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_attach_child in
+         * bdrv_set_backing_hd */
+        return;
+    }
     bdrv_refresh_filename(bs->backing->bs);
     pstrcpy(bs->exact_filename, sizeof(bs->exact_filename),
             bs->backing->bs->filename);

2.11.1

Thanks, applied to my block branch:

https://github.com/XanClic/qemu/commits/block

Max

Backing may be zero after failed bdrv_append in mirror_start_job, which leads to SIGSEGV.

---

similar SIGSEGV.
looks like (I guess by code, don't have full back-trace because of coroutine switch on bdrv_flush):
mirror_start_job,
bdrv_append failed, backing is not set
bdrv_unref

    bdrv_delete
      bdrv_close
        bdrv_flush
         ...
         bdrv_mirror_top_flush 
           Segmentation fault on
           return bdrv_co_flush(bs->backing->bs);
           as bs->backing = 0

 block/mirror.c | 4 ++++
 1 file changed, 4 insertions(+)

index 6f5cb9f26c..f17c0d8726 100644
@@ -1056,6 +1056,10 @@ static int coroutine_fn bdrv_mirror_top_pwritev(BlockDriverState *bs,
 
 static int coroutine_fn bdrv_mirror_top_flush(BlockDriverState *bs)
 {
+    if (bs->backing == NULL) {
+        /* we can be here after failed bdrv_append in mirror_start_job */
+        return 0;
+    }
     return bdrv_co_flush(bs->backing->bs);
 }
 

2.11.1

Thanks, applied to my block branch:

https://github.com/XanClic/qemu/commits/block

Max

I guess in this case we trust bdrv_set_backing_hd to carry the errp parameter back to the caller indicating that not only did we fail to set the new backing_hd, but we've also lost the old one, as evidenced by the nop-style return in .bdrv_refresh_filename...

so the error case should already be "handled", and this just cleans up a segfault.