linux-kernel - Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get

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Message-ID: <5aa15b12-d193-b505-5786-3e187751323e@fb.com>
Date:   Wed, 10 Aug 2022 08:09:09 -0700
From:   Yonghong Song <yhs@...com>
To:     Lee Jones <lee@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Song Liu <song@...nel.org>, KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of
 find_get_pid()



On 8/10/22 4:03 AM, Lee Jones wrote:
> On Tue, 09 Aug 2022, Alexei Starovoitov wrote:
> 
>> On Mon, Aug 8, 2022 at 11:50 PM Lee Jones <lee@...nel.org> wrote:
>>>
>>> On Thu, 04 Aug 2022, Alexei Starovoitov wrote:
>>>
>>>> On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <lee@...nel.org> wrote:
>>>>>
>>>>> The documentation for find_pid() clearly states:
>>>>>
>>>>>    "Must be called with the tasklist_lock or rcu_read_lock() held."
>>>>>
>>>>> Presently we do neither.
>>>>>
>>>>> Let's use find_get_pid() which searches for the vpid, then takes a
>>>>> reference to it preventing early free, all within the safety of
>>>>> rcu_read_lock().  Once we have our reference we can safely make use of
>>>>> it up until the point it is put.
>>>>>
>>>>> Cc: Alexei Starovoitov <ast@...nel.org>
>>>>> Cc: Daniel Borkmann <daniel@...earbox.net>
>>>>> Cc: John Fastabend <john.fastabend@...il.com>
>>>>> Cc: Andrii Nakryiko <andrii@...nel.org>
>>>>> Cc: Martin KaFai Lau <martin.lau@...ux.dev>
>>>>> Cc: Song Liu <song@...nel.org>
>>>>> Cc: Yonghong Song <yhs@...com>
>>>>> Cc: KP Singh <kpsingh@...nel.org>
>>>>> Cc: Stanislav Fomichev <sdf@...gle.com>
>>>>> Cc: Hao Luo <haoluo@...gle.com>
>>>>> Cc: Jiri Olsa <jolsa@...nel.org>
>>>>> Cc: bpf@...r.kernel.org
>>>>> Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
>>>>> Signed-off-by: Lee Jones <lee@...nel.org>
>>>>> ---
>>>>>
>>>>> v1 => v2:
>>>>>    * Commit log update - no code differences
>>>>>
>>>>>   kernel/bpf/syscall.c | 5 ++++-
>>>>>   1 file changed, 4 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
>>>>> index 83c7136c5788d..c20cff30581c4 100644
>>>>> --- a/kernel/bpf/syscall.c
>>>>> +++ b/kernel/bpf/syscall.c
>>>>> @@ -4385,6 +4385,7 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
>>>>>          const struct perf_event *event;
>>>>>          struct task_struct *task;
>>>>>          struct file *file;
>>>>> +       struct pid *ppid;
>>>>>          int err;
>>>>>
>>>>>          if (CHECK_ATTR(BPF_TASK_FD_QUERY))
>>>>> @@ -4396,7 +4397,9 @@ static int bpf_task_fd_query(const union bpf_attr *attr,
>>>>>          if (attr->task_fd_query.flags != 0)
>>>>>                  return -EINVAL;
>>>>>
>>>>> -       task = get_pid_task(find_vpid(pid), PIDTYPE_PID);
>>>>> +       ppid = find_get_pid(pid);
>>>>> +       task = get_pid_task(ppid, PIDTYPE_PID);
>>>>> +       put_pid(ppid);
>>>>
>>>> rcu_read_lock/unlock around this line
>>>> would be a cheaper and faster alternative than pid's
>>>> refcount inc/dec.
>>>
>>> This was already discussed here:
>>>
>>> https://lore.kernel.org/all/YtsFT1yFtb7UW2Xu@krava/
>>
>> Since several people thought about rcu_read_lock instead of your
>> approach it means that it's preferred.
>> Sooner or later somebody will send a patch to optimize
>> refcnt into rcu_read_lock.
>> So let's avoid the churn and do it now.
> 
> I'm not wed to either approach.  Please discuss it with Yonghong and
> Jiri and I'll do whatever is agreed upon.

Hi, Lee, Let us just do rcu_read_lock() approach then. I am okay with 
that. Thanks!

>