Re: [PATCH 1/2] block: Fix Transaction leak in bdrv_root_attach_child()

[Vladimir Sementsov-Ogievskiy]

03.05.2021 15:14, Vladimir Sementsov-Ogievskiy wrote:
> 03.05.2021 14:53, Max Reitz wrote:
> > On 03.05.21 13:51, Vladimir Sementsov-Ogievskiy wrote:
> > > 03.05.2021 14:49, Max Reitz wrote:
> > > > On 03.05.21 13:05, Kevin Wolf wrote:
> > > > > The error path needs to call tran_finalize(), too.
> > > > >
> > > > > Fixes: CID 1452773
> > > > > Fixes: 548a74c0dbc858edd1a7ee3045b5f2fe710bd8b1
> > > > > Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> > > > > ---
> > > > >   block.c | 7 ++++---
> > > > >   1 file changed, 4 insertions(+), 3 deletions(-)
> > > > >
> > > > > diff --git a/block.c b/block.c
> > > > > index 874c22c43e..5c0ced6238 100644
> > > > > --- a/block.c
> > > > > +++ b/block.c
> > > > > @@ -2918,13 +2918,14 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState 
> > > > > *child_bs,
> > > > >                                      child_role, perm, shared_perm, opaque,
> > > > >                                      &child, tran, errp);
> > > > >       if (ret < 0) {
> > > > > -        bdrv_unref(child_bs);
> > > > > -        return NULL;
> > > > > +        assert(child == NULL);
> > > > > +        goto out;
> > > > >       }
> > > > >       ret = bdrv_refresh_perms(child_bs, errp);
> > > > > -    tran_finalize(tran, ret);
> > > > > +out:
> > > > > +    tran_finalize(tran, ret);
> > > > >       bdrv_unref(child_bs);
> > > > >       return child;
> > > >
> > > > Looks OK, so:
> > > >
> > > > Reviewed-by: Max Reitz <mreitz@redhat.com>
> > > >
> > > > However, the function’s description says that it will return NULL on error.  
> > > > But if bdrv_refresh_perms() fails, it will still return a non-NULL child.  Is 
> > > > that right?
> > >
> > > No, it's reset to NULL on transaction abort, so code is correct. It's not obvious, and 
> > > I've added a comment and assertion in my version of this fix "[PATCH 1/6] block: fix 
> > > leak of tran in bdrv_root_attach_child"
> >
> > The fact that the transaction keeps the pointer to this local variable around 
> > is a bit horrifying, but well.
>
> Yes this looks overcomplicated here. But it's useful for bdrv_set_backing_noperm 
> where we would have to add a separate entry to @tran to rollback bs->backing 
> change. Probably, it's better to refactor this thing. Or at least document that 
> out-pointer argument goes into transaction, and should be valid up to transaction 
> finalization.

documentation done:

  [PATCH 7/6] block: document child argument of bdrv_attach_child_common()

--
Best regards,
Vladimir