Re: [Qemu-devel] [PATCH] block: fix bs->file leak in bdrv_new_open_driver()

[Manos Pitsidianakis <el13635@mail.ntua.gr>]

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On Thu, Jun 29, 2017 at 01:18:24PM +0200, Kevin Wolf wrote:
>Am 29.06.2017 um 08:03 hat Manos Pitsidianakis geschrieben:
>> bdrv_open_driver() is called in two places, bdrv_new_open_driver() and
>> bdrv_open_common(). In the latter, failure cleanup in is in its caller,
>> bdrv_open_inherit(), which unrefs the bs->file of the failed driver open
>> if it exists. Let's check for this in bdrv_new_open_driver() as well.
>>
>> Signed-off-by: Manos Pitsidianakis <el13635@mail.ntua.gr>
>> ---
>>  block.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/block.c b/block.c
>> index 694396281b..aeacd520e0 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -1165,6 +1165,9 @@ BlockDriverState *bdrv_new_open_driver(BlockDriver *drv, const char *node_name,
>>
>>      ret = bdrv_open_driver(bs, drv, node_name, bs->options, flags, errp);
>>      if (ret < 0) {
>> +        if (bs->file != NULL) {
>> +            bdrv_unref_child(bs, bs->file);
>> +        }
>>          QDECREF(bs->explicit_options);
>>          QDECREF(bs->options);
>>          bdrv_unref(bs);
>
>I think we should set bs->file = NULL here to remove the dangling
>pointer. I think it is never accessed anyway because of the
>bs->drv = NULL in the error path of bdrv_open_driver(), but better safe
>than sorry.

You can't see it in the diff but after bdrv_unref(bs), 
bdrv_new_open_driver returns NULL so there won't be any access to bs 
anyway. And since bs is destroyed by bdrv_unref (its refcount is 1), 
there's not really a point in setting bs->file = NULL.

>But what would you think about avoiding the code duplication and just
>moving the bdrv_unref_child() call from bdrv_open_inherit() down to
>bdrv_open_driver(), so that bdrv_new_open_driver() is automatically
>covered?

The result would be the same, but this will cover future callers of 
bdrv_open_driver. Should I submit a v2?
>
>And later we can maybe move it into the individual .bdrv_open
>implementations where it really belongs (whoever creates something is
>responsible for cleaning it up in error cases).

freeing bs->file was recently removed from individual '.bdrv_open's 
since bdrv_open_inherit takes care of it 
(de234897b60e034ba94b307fc289e2dc692c9251). I think this is simpler 
since a driver could neglect to free their bs->file whereas this is a 
catchall solution.

>Kevin
>

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