linux-kernel - Re: [RFC PATCH 3/6] bpf: Save the pointer to struct perf

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Message-ID: <20150717114054.GE19282@twins.programming.kicks-ass.net>
Date:	Fri, 17 Jul 2015 13:40:54 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Wangnan (F)" <wangnan0@...wei.com>
Cc:	kaixu xia <xiakaixu@...wei.com>, ast@...mgrid.com,
	davem@...emloft.net, acme@...nel.org, mingo@...hat.com,
	masami.hiramatsu.pt@...achi.com, jolsa@...nel.org,
	linux-kernel@...r.kernel.org, pi3orama@....com, hekuang@...wei.com
Subject: Re: [RFC PATCH 3/6] bpf: Save the pointer to struct perf_event to map

On Fri, Jul 17, 2015 at 07:34:22PM +0800, Wangnan (F) wrote:
> On 2015/7/17 19:21, Wangnan (F) wrote:
> >On 2015/7/17 19:06, Peter Zijlstra wrote:
> >>On Fri, Jul 17, 2015 at 06:43:33PM +0800, kaixu xia wrote:
> >>>diff --git a/kernel/events/core.c b/kernel/events/core.c
> >>>index e965cfa..c4e34b7 100644
> >>>--- a/kernel/events/core.c
> >>>+++ b/kernel/events/core.c
> >>>@@ -8582,6 +8582,28 @@ void perf_event_delayed_put(struct task_struct
> >>>*task)
> >>>          WARN_ON_ONCE(task->perf_event_ctxp[ctxn]);
> >>>  }
> >>>  +struct perf_event *perf_event_get(unsigned int fd)
> >>>+{
> >>>+    struct perf_event *event;
> >>>+    struct fd f;
> >>>+
> >>>+    f = fdget(fd);
> >>>+
> >>>+    if (!f.file)
> >>>+        return ERR_PTR(-EBADF);
> >>>+
> >>>+    if (f.file->f_op != &perf_fops) {
> >>>+        fdput(f);
> >>>+        return ERR_PTR(-EINVAL);
> >>>+    }
> >>>+
> >>>+    event = f.file->private_data;
> >>>+
> >>>+    fdput(f);
> >>>+
> >>>+    return event;
> >>>+}
> >>And what is stopping userspace from closing those FDs while you're using
> >>them?

> Shall we put atomic_long_inc_not_zero() between fdget() and fdput()?

You pretty much _have_ to do that.
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