netdev - Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

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Message-ID: <CAD=hENc8CmxdPeWbQ=4GFdtQCoCqUc87xS5sq+VePZEGC2-Z6g@mail.gmail.com>
Date:   Wed, 9 Dec 2020 12:30:55 +0800
From:   Zhu Yanjun <zyjzyj2000@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Zhu Yanjun <yanjun.zhu@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        netdev <netdev@...r.kernel.org>, jonathan.lemon@...il.com
Subject: Re: [PATCH v2 1/1] xdp: avoid calling kfree twice

On Wed, Dec 9, 2020 at 1:12 AM Daniel Borkmann <daniel@...earbox.net> wrote:
>
> On 12/9/20 6:03 AM, Zhu Yanjun wrote:
> > In the function xdp_umem_pin_pages, if npgs != umem->npgs and
> > npgs >= 0, the function xdp_umem_unpin_pages is called. In this
> > function, kfree is called to handle umem->pgs, and then in the
> > function xdp_umem_pin_pages, kfree is called again to handle
> > umem->pgs. Eventually, umem->pgs is freed twice.
> >
> > Acked-by: Björn Töpel <bjorn.topel@...el.com>
> > Signed-off-by: Zhu Yanjun <yanjun.zhu@...el.com>
>
> Please also fix up the commit log according to Bjorn's prior feedback [0].
> If it's just a cleanup, it should state so, the commit message right now
> makes it sound like an actual double free bug.

The umem->pgs is actually freed twice. Since umem->pgs is set to NULL
after the first kfree,
the second kfree would not trigger call trace.
IMO, the commit log is very clear about this.

Zhu Yanjun
>
>    [0] https://lore.kernel.org/netdev/0fef898d-cf5e-ef1b-6c35-c98669e9e0ed@intel.com/