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Subject: Re: [PATCH] bpf: Replace BUG_ON when fp_old is NULL
To:     Aditya Pakki <pakki001@umn.edu>, Yonghong Song <yhs@fb.com>
Cc:     "kjlu@umn.edu" <kjlu@umn.edu>, Alexei Starovoitov <ast@kernel.org>,
        Martin Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "bpf@vger.kernel.org" <bpf@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20191215154432.22399-1-pakki001@umn.edu>
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 <566f206c-f133-6f68-c257-2c0b3ec462fa@iogearbox.net>
 <51dcca79-f819-8ebb-308e-210a0d76b1cc@umn.edu>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <d1175d95-f279-ce29-f1bd-c0984626f3a4@iogearbox.net>
Date:   Thu, 19 Dec 2019 18:48:07 +0100
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On 12/19/19 6:39 PM, Aditya Pakki wrote:
> On 12/16/19 5:17 AM, Daniel Borkmann wrote:
>> On 12/15/19 11:08 PM, Yonghong Song wrote:
>>> On 12/15/19 7:44 AM, Aditya Pakki wrote:
>>>> If fp_old is NULL in bpf_prog_realloc, the program does an assertion
>>>> and crashes. However, we can continue execution by returning NULL to
>>>> the upper callers. The patch fixes this issue.
>>>
>>> Could you share how to reproduce the assertion and crash? I would
>>> like to understand the problem first before making changes in the code.
>>> Thanks!
>>
>> Fully agree, Aditya, please elaborate if you have seen a crash!
> 
> Thanks for your responses Alexei and Daniel. We identified this issue via static analysis
> and have not seen a crash. However, by looking at the callers of bpf_prog_realloc, I do
> agree that fp_old is never NULL.
> 
> Would you recommend removing the BUG_ON assertion altogether ?

If it would ever happen, we'd already crash in fp_old->pages there, so yes,
lets remove the unneeded BUG_ON().

Thanks,
Daniel