linux-kernel - Re: [RFC PATCH 01/17] perf: Allow mmapping only user page

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Date:   Wed, 13 Sep 2017 14:35:42 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        acme@...hat.com, kirill.shutemov@...ux.intel.com, rric@...nel.org
Subject: Re: [RFC PATCH 01/17] perf: Allow mmapping only user page

Borislav Petkov <bp@...en8.de> writes:

> On Tue, Sep 05, 2017 at 04:30:10PM +0300, Alexander Shishkin wrote:
>> The 'user page' contains offsets and sizes of data and aux areas of the
>> ring buffer. If a user wants to mmap a pre-existing buffer, they need to
>> know these in order to issue mmap()s with correct offsets and sizes.
>
> Ok, stupid question: shouldn't this be a properly defined interface
> instead of allowing userspace to poke inside the user page? Or are we
> prepared to handle any changes in the layout of that user page and there
> won't be any userspace crying because of it?

Well, it is a 'defined' interface: there's the 'struct
perf_event_mmap_page' with versioning and whatnot, which is used for all
the ring buffer metainformation. Or am I misunderstanding your question?

Thanks,
--
Alex