Re: [RESEND PATCH] hw/dma: fix crash caused by race condition

[Tong Zhang <ztong0001@gmail.com>]

Hi David,

On Mon, May 30, 2022 at 9:19 AM David Hildenbrand <david@redhat.com> wrote:
>
> On 27.04.22 22:51, Tong Zhang wrote:
> > assert(dbs->acb) is meant to check the return value of io_func per
> > documented in commit 6bee44ea34 ("dma: the passed io_func does not
> > return NULL"). However, there is a chance that after calling
> > aio_context_release(dbs->ctx); the dma_blk_cb function is called before
> > the assertion and dbs->acb is set to NULL again at line 121. Thus when
> > we run assert at line 181 it will fail.
> >
> >   softmmu/dma-helpers.c:181: dma_blk_cb: Assertion `dbs->acb' failed.
> >
> > Reported-by: Francisco Londono <f.londono@samsung.com>
> > Signed-off-by: Tong Zhang <t.zhang2@samsung.com>
> > ---
> >  softmmu/dma-helpers.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/softmmu/dma-helpers.c b/softmmu/dma-helpers.c
> > index 7820fec54c..cb81017928 100644
> > --- a/softmmu/dma-helpers.c
> > +++ b/softmmu/dma-helpers.c
> > @@ -177,8 +177,8 @@ static void dma_blk_cb(void *opaque, int ret)
> >      aio_context_acquire(dbs->ctx);
> >      dbs->acb = dbs->io_func(dbs->offset, &dbs->iov,
> >                              dma_blk_cb, dbs, dbs->io_func_opaque);
> > -    aio_context_release(dbs->ctx);
> >      assert(dbs->acb);
> > +    aio_context_release(dbs->ctx);
> >  }
> >
> >  static void dma_aio_cancel(BlockAIOCB *acb)
>
> I'm fairly new to that code, but I wonder what prevents dma_blk_cb() to
> run after you reshuffled the code?
>

IMO if the assert is to test whether io_func returns a non-NULL value
shouldn't it be immediately after calling io_func.
Also... as suggested by commit 6bee44ea346aed24e12d525daf10542d695508db
  >     dma: the passed io_func does not return NULL

Thanks,
- Tong

> After all, acquire/release is only around the dbs->io_func() call, so I
> don't immediately see how it interacts with re-entrance?
>
> --
> Thanks,
>
> David / dhildenb
>