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

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Date:   Thu, 6 Aug 2020 08:32:05 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     peterz@...radead.org
Cc:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        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
Subject: Re: [PATCH 1/2] perf: Add closing sibling events' file descriptors

> > 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.

The idea was to account the events in the locked memory allocation too.
Not sure that made it into the patch though.

It has a minor issue that it might break some existing setups that rely
on the mmap fitting exactly into the mmap allocation, but that could
be solved by allowing a little slack, since the existing setups
likely don't have that many events.

There's also a secondary issue of DoS the kernel by creating very long
lists to iterate, but I suppose this is already quite possible, so probably
not a new issue.

-Andi