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Re: [Qemu-devel] [PATCH v2] block: fix leaks in bdrv_open_driver()

From: Manos Pitsidianakis
Subject: Re: [Qemu-devel] [PATCH v2] block: fix leaks in bdrv_open_driver()
Date: Wed, 12 Jul 2017 11:39:30 +0300
User-agent: NeoMutt/20170609-57-1e93be (1.8.3)

On Wed, Jul 12, 2017 at 10:33:37AM +0200, Kevin Wolf wrote:
Am 11.07.2017 um 20:50 hat Manos Pitsidianakis geschrieben:
On Tue, Jul 11, 2017 at 05:16:17PM +0200, Kevin Wolf wrote:
>Am 01.07.2017 um 17:39 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
>>Let's move the bs->file cleanup to bdrv_open_driver() to take care of all
>>callers and do not set bs->drv to NULL unless the driver's open function
>>failed. When bs is destroyed by removing its last reference, bdrv_close()
>>checks bs->drv to perform the needed cleanups and also call the driver's close
>>Signed-off-by: Manos Pitsidianakis <address@hidden>
>> move bdrv_unref_child(bs, bs->file) to bdrv_open_driver
>> do not set bs->drv to NULL if open succeeds
>> block.c | 21 +++++++++++++--------
>> 1 file changed, 13 insertions(+), 8 deletions(-)
>>diff --git a/block.c b/block.c
>>index 694396281b..df2a46990c 100644
>>--- a/block.c
>>+++ b/block.c
>>@@ -1091,6 +1091,7 @@ static int bdrv_open_driver(BlockDriverState *bs, 
BlockDriver *drv,
>> {
>>     Error *local_err = NULL;
>>     int ret;
>>+    bool open_failed;
>>     bdrv_assign_node_name(bs, node_name, &local_err);
>>     if (local_err) {
>>@@ -1111,7 +1112,9 @@ static int bdrv_open_driver(BlockDriverState *bs, 
BlockDriver *drv,
>>         ret = 0;
>>     }
>>-    if (ret < 0) {
>>+    open_failed = ret < 0;
>>+    if (open_failed) {
>>         if (local_err) {
>>             error_propagate(errp, local_err);
>>         } else if (bs->filename[0]) {
>>@@ -1142,10 +1145,15 @@ static int bdrv_open_driver(BlockDriverState *bs, 
BlockDriver *drv,
>>     return 0;
>> free_and_fail:
>>-    /* FIXME Close bs first if already opened*/
>>-    g_free(bs->opaque);
>>-    bs->opaque = NULL;
>>-    bs->drv = NULL;
>>+    if (open_failed) {
>>+        g_free(bs->opaque);
>>+        bs->opaque = NULL;
>>+        bs->drv = NULL;
>>+    }
>>+    if (bs->file != NULL) {
>>+        bdrv_unref_child(bs, bs->file);
>>+        bs->file = NULL;
>>+    }
>Is this bdrv_unref_child() safe if we leave bs->drv set? Format drivers
>expect that if an image is opened, it also has a valid bs->file.
>For example, if I add ret = -1 after refresh_total_sectors() (because I
>couldn't find an easier way to make it fail intentionally), I get an
>ugly heap corruption crash instead of a nice error message with this
This is triggered by bdrv_open_inherit doing
QDECREF(bs->explicit_options) and leaving the dangling pointer. Not
setting bs->drv means bdrv_close was called and tried to decref it
again, causing the heap error. Setting bs->explicit_options = NULL;
right below that fixes the heap corruption for me.

Wouldn't it be better to call drv->bdrv_close() instead and then set
bs->drv/opaque = NULL like for the other error path?

That was my first approach but I thought it wouldn't look nice since bdrv_close is called anyway on delete. I will do it in the next version.

I can send a seperate fix for this.

No, this doesn't fail before this patch, so it's a regression and we
can't merge the patch without a fix. You need to respin this one.
Yes, I meant to first fix this and then apply this patch. It's a dangling pointer anyway.

I also saw that there's no reason to use a boolean, a label would do
just fine so I can change that and finalize the patch in the next
version if everything is okay with it.

Yes, that sounds better.


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