Messages in this thread | | | Date | Sat, 1 Nov 2014 21:39:17 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: [PATCH v2 5/8] perf: Add pmu callbacks to track event mapping and unmapping |
| |
On Sat, 1 Nov 2014, Andy Lutomirski wrote: > On Sat, Nov 1, 2014 at 12:59 PM, Thomas Gleixner <tglx@linutronix.de> wrote: > > On Fri, 24 Oct 2014, Andy Lutomirski wrote: > > > > I'm probably missing something. > > > > Why is this tied to the mmap? If I just open a counter then I should > > be able to read the counter from user space w/o mmapping something in > > the first place. > > You can read it with read(2), which this patch shouldn't affect. If > you want to read it with rdpmc, then you need to know what rdpmc index > to use, and the API for that is to mmap the event, check the > userpage's cap_user_rdpmc, and then rdpmc on ->idx - 1 (assuming that > ->idx != 0). You can't safely make any assumptions about which rdpmc > index it will be without explicitly checking, because perf reserves > the right to change the index whenever it wants.
Got it. As I expected: I was missing something :)
> There's plenty of room to tighten up the restrictions further, but > this is, I think, a decent first step, and it solves the problem of > information leaking into seccomp sandboxes.
In which way?
Thanks,
tglx
| |