Re: [Qemu-block] [PATCH] block/mirror: check backing in bdrv_mirror_top_refresh_filename

[John Snow]

On 09/28/2017 08:03 AM, Vladimir Sementsov-Ogievskiy wrote:
> Backing may be zero after failed bdrv_attach_child in
> bdrv_set_backing_hd, which leads to SIGSEGV.
> 

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.

> Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> ---
> 
> 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 (address@hidden, 
>         address@hidden, address@hidden)
>         at block.c:2035
>     #3  0x00005616ddc3dee3 in bdrv_append (address@hidden, 
>         address@hidden, address@hidden)
>         at block.c:3168
>     #4  0x00005616ddc84e5f in mirror_start_job (
>         address@hidden "commit", address@hidden, 
>         address@hidden, address@hidden, 
>         address@hidden, address@hidden, granularity=65536, 
>         address@hidden, buf_size=16777216, address@hidden, 
>         address@hidden, 
>         address@hidden, 
>         address@hidden, 
>         address@hidden, address@hidden <common_block_job_cb>, 
>         address@hidden, address@hidden, 
>         address@hidden <commit_active_job_driver>, 
>         address@hidden, address@hidden, 
>         address@hidden, 
>         address@hidden) at block/mirror.c:1314
>     #5  0x00005616ddc87580 in commit_active_start (
>         address@hidden "commit", address@hidden, 
>         address@hidden, address@hidden, 
>         address@hidden, address@hidden, 
>         address@hidden, 
>         address@hidden <common_block_job_cb>, 
>         address@hidden, address@hidden, 
>         address@hidden) 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 (address@hidden, 
>         address@hidden, address@hidden)
>         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(+)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 6f5cb9f26c..351b80ca2c 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -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);
> 

Reviewed-by: John Snow <address@hidden>