perf segfault in docker container

[Brendan Gregg <brendan.d.gregg@gmail.com>]

G'Day,

Default docker container, in Linux 4.7-rc2, with latest perf from perf/core:

docker# ./perf record -F 99 -a
Segmentation fault

The segfault is in perf_event__synthesize_kernel_mmap(). I know
symbol__read_kptr_restrict() has been updated recently to fix similar
segfaults, hence getting perf/core.

I think the problem is this:

docker# id
uid=0(root) gid=0(root) groups=0(root)
docker# cat /proc/sys/kernel/kptr_restrict
1

(I'd previously set "echo -1 > /proc/sys/kernel/perf_event_paranoid")

The current (May 24) code has, symbol__read_kptr_restrict():

                if (fgets(line, sizeof(line), fp) != NULL)
                        value = (geteuid() != 0) ?
                                        (atoi(line) != 0) :
                                        (atoi(line) == 2);

assumes that if euid is 0 && kptr_restrict isn't 2, then we're aren't
restricted. But we are. Maybe the code should check for CAP_SYS_ADMIN,
instead of euid == 0?

Brendan