linux-kernel - Re: [PATCH] [RFC] bpf: tracing: new helper bpf_get_current_cgroup

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Message-ID: <ef833a5c-8307-fee7-26ec-057f623a9049@iogearbox.net>
Date:   Wed, 23 May 2018 10:57:06 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Y Song <ys114321@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alban Crequy <alban.crequy@...il.com>,
        netdev <netdev@...r.kernel.org>, linux-kernel@...r.kernel.org,
        containers@...ts.linux-foundation.org, cgroups@...r.kernel.org,
        Alban Crequy <alban@...volk.io>, tj@...nel.org
Subject: Re: [PATCH] [RFC] bpf: tracing: new helper bpf_get_current_cgroup_ino

On 05/23/2018 06:31 AM, Y Song wrote:
> On Tue, May 22, 2018 at 8:35 PM, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>> On Tue, May 22, 2018 at 08:33:24PM -0700, Y Song wrote:
>>> +       struct cgroup *cgrp = task_dfl_cgroup(current);
>>> +       if (!cgrp)
>>> +               return -EINVAL;
>>
>> why this check is needed?
> 
> No reason :-) Originally I am concerned whether it is possible cgrp
> could be NULL.
> By looking at the code, it SEEMS to me that it could not be NULL, but I am not
> 100% sure (as I am not a cgroup expert). Since you are asking,
> probably means it cannot be NULL, so will remove it in formal upstream patch.

Aside from this the cgrp->kn->id.id is also u64, so overlapping this with
error codes we'll get into a similar issue as with bpf_perf_event_read().