From: Daniel Borkmann <daniel@iogearbox.net>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
Richard Weinberger <richard@nod.at>
Cc: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Alexei Starovoitov <ast@kernel.org>
Subject: Re: [PATCH 3/3] bpf: Make sure that ->comm does not change under us.
Date: Tue, 17 Oct 2017 00:19:26 +0200 [thread overview]
Message-ID: <59E5306E.8050000@iogearbox.net> (raw)
In-Reply-To: <CAADnVQ+0kTYr2azBW6mDSU6JwWtjWRZVaJQ=ZFmSs3JxCFrrRg@mail.gmail.com>
On 10/17/2017 12:10 AM, Alexei Starovoitov wrote:
> On Mon, Oct 16, 2017 at 2:10 PM, Richard Weinberger <richard@nod.at> wrote:
>> Am Montag, 16. Oktober 2017, 23:02:06 CEST schrieb Daniel Borkmann:
>>> 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.
>>
>> Ah, that's true.
>> So, when someone wants to use bpf_get_current_comm() while tracing task_lock,
>> we have a problem. I agree.
>> On the other hand, without locking the function may return wrong results.
>
> it will surely race with somebody else setting task comm and it's fine.
> all of bpf tracing is read-only, so locks are only allowed inside bpf core
> bits like maps. Taking core locks like task_lock() is quite scary.
> bpf scripts rely on bpf_probe_read() of all sorts of kernel fields
> so reading comm here w/o lock is fine.
Yeah, and perf_event_comm() -> perf_event_comm_event() out of __set_task_comm()
is having same approach wrt comm read-out.
next prev parent reply other threads:[~2017-10-16 22:19 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-16 22:10 [PATCH 3/3] bpf: Make sure that ->comm does not change under us Alexei Starovoitov
2017-10-16 22:19 ` Daniel Borkmann [this message]
-- strict thread matches above, loose matches on Subject: below --
2017-10-16 18:18 [PATCH 1/3] bpf: Don't check for current being NULL Richard Weinberger
2017-10-16 18:18 ` [PATCH 3/3] bpf: Make sure that ->comm does not change under us Richard Weinberger
2017-10-16 20:50 ` Daniel Borkmann
2017-10-16 20:55 ` Richard Weinberger
2017-10-16 21:02 ` Daniel Borkmann
2017-10-16 21:10 ` Richard Weinberger
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