linux-kernel - Re: [PATCH v4 bpf 1/2] bpf: fix skb_do

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Message-ID: <CAO3-Pbpx8vmC_-o59s61mU=TzYLb+VpZ2qk+QhTMjVM6jf=71g@mail.gmail.com>
Date:   Tue, 1 Aug 2023 17:18:36 -0500
From:   Yan Zhai <yan@...udflare.com>
To:     Dan Carpenter <dan.carpenter@...aro.org>
Cc:     bpf@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org, kernel-team@...udflare.com,
        Jordan Griege <jgriege@...udflare.com>,
        Markus Elfring <Markus.Elfring@....de>,
        Jakub Sitnicki <jakub@...udflare.com>
Subject: Re: [PATCH v4 bpf 1/2] bpf: fix skb_do_redirect return values

On Mon, Jul 31, 2023 at 9:26 AM Dan Carpenter <dan.carpenter@...aro.org> wrote:
>
> I'm not a networking person, but I was looking at some use after free
> static checker warnings.

Did you refer to the gist I posted or something new?

>
> Apparently the rule with xmit functions is that if they return a value
> > 15 then that means the skb was not freed.  Otherwise it's supposed to
> be freed.  So like NETDEV_TX_BUSY is 0x10 so it's not freed.
>
> This is checked with using the dev_xmit_complete() function.  So I feel
> like it would make sense for LWTUNNEL_XMIT_CONTINUE to return higher
> than 15.

Yes I am adopting your suggestion in v5. Dealing with NETDEV_TX_BUSY
would be left as another item (potentially more suited for netdev
rather than bpf). Would be great to find a reproduction of memleak.

>
> Because that's the bug right?  The original code was assuming that
> everything besides LWTUNNEL_XMIT_DONE was freed.
>
> regards,
> dan carpenter
>


-- 

Yan