netdev - Re: [PATCH 3/3] bpf: Make sure that ->comm does not change under us.

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Message-ID: <59E51E4E.4060009@iogearbox.net>
Date:   Mon, 16 Oct 2017 23:02:06 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Richard Weinberger <richard@....at>
CC:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        ast@...nel.org
Subject: Re: [PATCH 3/3] bpf: Make sure that ->comm does not change under
 us.

On 10/16/2017 10:55 PM, Richard Weinberger wrote:
> Am Montag, 16. Oktober 2017, 22:50:43 CEST schrieb Daniel Borkmann:
>>>    	struct task_struct *task = current;
>>>
>>> +	task_lock(task);
>>>
>>>    	strncpy(buf, task->comm, size);
>>>
>>> +	task_unlock(task);
>>
>> Wouldn't this potentially lead to a deadlock? E.g. you attach yourself
>> to task_lock() / spin_lock() / etc, and then the BPF prog triggers the
>> bpf_get_current_comm() taking the lock again ...
>
> Yes, but doesn't the same apply to the use case when I attach to strncpy()
> and run bpf_get_current_comm()?

You mean due to recursion? In that case trace_call_bpf() would bail out
due to the bpf_prog_active counter.