linux-kernel - Re: [PATCH 1/2] perf: Add closing sibling events' file descriptors

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Message-ID: <20200806083530.GV2674@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Aug 2020 10:35:30 +0200
From:   peterz@...radead.org
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Jiri Olsa <jolsa@...nel.org>, alexey.budankov@...ux.intel.com,
        adrian.hunter@...el.com, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/2] perf: Add closing sibling events' file descriptors

On Wed, Jul 08, 2020 at 06:16:34PM +0300, Alexander Shishkin wrote:
> Currently, perf requires one file descriptor per event. In large groups,
> this may mean running into the limit on open file descriptors. However,
> the sibling events in a group only need file descriptors for the initial
> configuration stage, after which they may not be needed any more.
> 
> This adds an opt-in flag to the perf_event_open() syscall to retain
> sibling events after their file descriptors are closed. In this case, the
> actual events will be closed with the group leader.

So having the 1:1 relation with filedesc imposes a resource limit on
userspace.

This patch breaks that and enables a user to basically DoS the system by
creating unbound events.