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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 9 Aug 2022 07:38:46 -0700
Message-ID: <CAADnVQ+5eq3qQTgHH6nDdVM-n1i4TWkZ35Ou8TDMi3MqGzm63w@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] bpf: Drop unprotected find_vpid() in favour of find_get_pid()
To:     Lee Jones <lee@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        bpf <bpf@vger.kernel.org>
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On Mon, Aug 8, 2022 at 11:50 PM Lee Jones <lee@kernel.org> wrote:
>
> On Thu, 04 Aug 2022, Alexei Starovoitov wrote:
>
> > On Wed, Aug 3, 2022 at 6:48 AM Lee Jones <lee@kernel.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@kernel.org>
> > > Cc: Daniel Borkmann <daniel@iogearbox.net>
> > > Cc: John Fastabend <john.fastabend@gmail.com>
> > > Cc: Andrii Nakryiko <andrii@kernel.org>
> > > Cc: Martin KaFai Lau <martin.lau@linux.dev>
> > > Cc: Song Liu <song@kernel.org>
> > > Cc: Yonghong Song <yhs@fb.com>
> > > Cc: KP Singh <kpsingh@kernel.org>
> > > Cc: Stanislav Fomichev <sdf@google.com>
> > > Cc: Hao Luo <haoluo@google.com>
> > > Cc: Jiri Olsa <jolsa@kernel.org>
> > > Cc: bpf@vger.kernel.org
> > > Fixes: 41bdc4b40ed6f ("bpf: introduce bpf subcommand BPF_TASK_FD_QUERY")
> > > Signed-off-by: Lee Jones <lee@kernel.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.