linux-kernel - Re: [PATCH net-next 1/3] bpf: introduce current->pid, tgid, uid, gid, comm accessors

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Message-ID: <CALCETrVnhCoD9=nzDpB6UvmXyGz4Th_m1fstJDyRUKzfsSaKHw@mail.gmail.com>
Date:	Fri, 12 Jun 2015 16:25:24 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Alexei Starovoitov <ast@...mgrid.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Wang Nan <wangnan0@...wei.com>, Li Zefan <lizefan@...wei.com>,
	Daniel Wagner <daniel.wagner@...-carit.de>,
	Daniel Borkmann <daniel@...earbox.net>,
	Linux API <linux-api@...r.kernel.org>,
	Network Development <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 1/3] bpf: introduce current->pid, tgid, uid, gid,
 comm accessors

On Fri, Jun 12, 2015 at 4:23 PM, Alexei Starovoitov <ast@...mgrid.com> wrote:
> On 6/12/15 3:54 PM, Andy Lutomirski wrote:
>>
>> On Fri, Jun 12, 2015 at 3:44 PM, Alexei Starovoitov <ast@...mgrid.com>
>> wrote:
>>>
>>> On 6/12/15 3:08 PM, Andy Lutomirski wrote:
>>>>
>>>>
>>>> On Fri, Jun 12, 2015 at 2:40 PM, Alexei Starovoitov <ast@...mgrid.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> eBPF programs attached to kprobes need to filter based on
>>>>> current->pid, uid and other fields, so introduce helper functions:
>>>>>
>>>>> u64 bpf_get_current_pid_tgid(void)
>>>>> Return: current->tgid << 32 | current->pid
>>>>>
>>>>> u64 bpf_get_current_uid_gid(void)
>>>>> Return: current_gid << 32 | current_uid
>>>>
>>>>
>>>>
>>>> How does this work wrt namespaces,
>>>
>>>
>>>
>>> from_kuid(current_user_ns(), uid)
>>>
>>
>> Is current_user_ns() well defined in the context of an eBPF program?
>
>
> What do you mean 'well defined'?
> Semantically same as 'current'. Depending on where particular
> kprobe is placed, 'current' is either meaningful or not. Program
> author needs to know what he's doing. It's a tool.
>

It's a dangerous tool.  Also, shouldn't the returned uid match the
namespace of the task that installed the probe, not the task that's
being probed?

--Andy
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