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

[Manos Pitsidianakis]

On Fri, Jul 07, 2017 at 11:28:15AM +0200, Kevin Wolf wrote:

Am 29.06.2017 um 22:06 hat Manos Pitsidianakis geschrieben:

On Thu, Jun 29, 2017 at 03:57:49PM +0200, Kevin Wolf wrote:
>Am 29.06.2017 um 14:07 hat Manos Pitsidianakis geschrieben:
>>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 <el13...@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.
>
>Yes, but bdrv_unref() doesn't have to expect inconsistent BDSes. It
>doesn't access bs->file currently when bs->drv == NULL, but that's more
>by luck than by design.
>
>>>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?
>
>I would prefer this, yes.

Perhaps it would be better to destroy bs at failure in
bdrv_open_driver and not leave it to the caller which takes care of
bdrv_close and unrefing bs->file anyway (Also bs->children). Setting
bs->drv to NULL at failure in bdrv_open_driver means some things
won't be executed in bdrv_close when the bs is destroyed eventually
as well, so that fixes another mistake.

Oh, didn't I reply here yet? Your suggestion sounds good to me.

https://lists.nongnu.org/archive/html/qemu-devel/2017-07/msg00019.html

I ended up sending a v2 some days ago, and instead just not setting bs->drv to NULL unless open failed which I think is cleaner. The open's ret value is stored in a boolean, but probably would be better to goto a specific open_fail label. If you think the change is ok I will resend it.

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