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    Subject[PATCH AUTOSEL 4.20 09/72] perf core: Fix perf_proc_update_handler() bug
    Date
    From: Stephane Eranian <eranian@google.com>

    [ Upstream commit 1a51c5da5acc6c188c917ba572eebac5f8793432 ]

    The perf_proc_update_handler() handles /proc/sys/kernel/perf_event_max_sample_rate
    syctl variable. When the PMU IRQ handler timing monitoring is disabled, i.e,
    when /proc/sys/kernel/perf_cpu_time_max_percent is equal to 0 or 100,
    then no modification to sysctl_perf_event_sample_rate is allowed to prevent
    possible hang from wrong values.

    The problem is that the test to prevent modification is made after the
    sysctl variable is modified in perf_proc_update_handler().

    You get an error:

    $ echo 10001 >/proc/sys/kernel/perf_event_max_sample_rate
    echo: write error: invalid argument

    But the value is still modified causing all sorts of inconsistencies:

    $ cat /proc/sys/kernel/perf_event_max_sample_rate
    10001

    This patch fixes the problem by moving the parsing of the value after
    the test.

    Committer testing:

    # echo 100 > /proc/sys/kernel/perf_cpu_time_max_percent
    # echo 10001 > /proc/sys/kernel/perf_event_max_sample_rate
    -bash: echo: write error: Invalid argument
    # cat /proc/sys/kernel/perf_event_max_sample_rate
    10001
    #

    Signed-off-by: Stephane Eranian <eranian@google.com>
    Reviewed-by: Andi Kleen <ak@linux.intel.com>
    Reviewed-by: Jiri Olsa <jolsa@kernel.org>
    Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
    Cc: Kan Liang <kan.liang@linux.intel.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Link: http://lkml.kernel.org/r/1547169436-6266-1-git-send-email-eranian@google.com
    Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/events/core.c | 14 +++++++-------
    1 file changed, 7 insertions(+), 7 deletions(-)

    diff --git a/kernel/events/core.c b/kernel/events/core.c
    index 84530ab358c37..f113534bf22e9 100644
    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -436,18 +436,18 @@ int perf_proc_update_handler(struct ctl_table *table, int write,
    void __user *buffer, size_t *lenp,
    loff_t *ppos)
    {
    - int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
    -
    - if (ret || !write)
    - return ret;
    -
    + int ret;
    + int perf_cpu = sysctl_perf_cpu_time_max_percent;
    /*
    * If throttling is disabled don't allow the write:
    */
    - if (sysctl_perf_cpu_time_max_percent == 100 ||
    - sysctl_perf_cpu_time_max_percent == 0)
    + if (write && (perf_cpu == 100 || perf_cpu == 0))
    return -EINVAL;

    + ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
    + if (ret || !write)
    + return ret;
    +
    max_samples_per_tick = DIV_ROUND_UP(sysctl_perf_event_sample_rate, HZ);
    perf_sample_period_ns = NSEC_PER_SEC / sysctl_perf_event_sample_rate;
    update_perf_cpu_limits();
    --
    2.19.1
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