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    SubjectRe: [BUG] perf: bogus correlation of kernel symbols

    * David Miller <davem@davemloft.net> wrote:

    > From: Stephane Eranian <eranian@google.com>
    > Date: Thu, 12 May 2011 16:48:46 +0200
    >
    > > I think there is a serious problem with kernel symbol correlation
    > > with the latest perf in 2.6.39-rc7-tip.
    >
    > The behavior seems to be intentional, so that we don't expose internal
    > kernel addresses to userspace.
    >
    > I hate this too, and I think it's absolutely rediculous.
    >
    > Also, like you, I lost an entire afternoon trying to figure out why
    > this started happening.

    I lost about an hour with Arnaldo on IRC to help me until we figured out that
    /proc/kallsyms started having zero value entries ... I'm too running perf as an
    unprivileged user.

    Zero is a valid symbol address so nothing within perf tripped up explicitly,
    but perf report and perf top results were nonsensical.

    There was another problem with it: perf is caching and storing known kernel
    buildid addresses in ~/.debug, under the (previously correct) assumption that
    kernel symbols do not change for one given kernel build. But with kptr_restrict
    it would cache the zero values - which were cached even after kptr_restrict was
    set back to 0.

    Thanks,

    Ingo


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