symbol addresses are wrong when kcore is used

[David Ahern <dsahern@gmail.com>]

Hi guys:

I have been meaning to ask about this for 2 years now: perf script shows the wrong addresses when /proc/kcore is referenced. Here's an example with perf from net-next tree (4.9-rc3):

$ perf record -a -g -e fib6:* -- sleep 5

$ perf script
vhost-7489  7491 [018]  2314.004488: fib6:fib6_table_lookup: table 255 oif 0 iif 4 src 2100:1::3 dst ff02::1:ff00:64 tos 0 scope 0 flags 0 ==> dev br1 gw ::
            7fff8155f49e ip6_pol_route+0x80007f00249e ([kernel.kallsyms])
            7fff8155f77a ip6_pol_route_input+0x80007f00201a ([kernel.kallsyms])
            7fff81588515 fib6_rule_action+0x80007f0020c5 ([kernel.kallsyms])
            7fff814c2676 fib_rules_lookup+0x80007f002116 ([kernel.kallsyms])
            7fff815886b9 fib6_rule_lookup+0x80007f002059 ([kernel.kallsyms])
            7fff8155c77e ip6_route_input_lookup+0x80007f00204e ([kernel.kallsyms])
            7fff81560a98 ip6_route_input+0x80007f0020d8 ([kernel.kallsyms])
            7fff81550a92 ip6_rcv_finish+0x80007f002082 ([kernel.kallsyms])
            7fff81551344 ipv6_rcv+0x80007f002334 ([kernel.kallsyms])

The 7fff addresses are completely wrong. If I compile perf and disable use of kcore:

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index aecff69a510d..0debd7e96aa2 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -1285,7 +1285,7 @@ int __dso__load_kallsyms(struct dso *dso, const char *filename,
 int dso__load_kallsyms(struct dso *dso, const char *filename,
                       struct map *map)
 {
-       return __dso__load_kallsyms(dso, filename, map, false);
+       return __dso__load_kallsyms(dso, filename, map, true);
 }

 static int dso__load_perf_map(struct dso *dso, struct map *map)


Then the addresses are correct:

vhost-7489  7491 [018]  2314.004488: fib6:fib6_table_lookup: table 255 oif 0 iif 4 src 2100:1::3 dst ff02::1:ff00:64 tos 0 scope 0 flags 0 ==> dev br1 gw ::
        ffffffff8155d49e ip6_pol_route+0x49e ([kernel.kallsyms])
        ffffffff8155d77a ip6_pol_route_input+0x1a ([kernel.kallsyms])
        ffffffff81586515 fib6_rule_action+0xc5 ([kernel.kallsyms])
        ffffffff814c0676 fib_rules_lookup+0x116 ([kernel.kallsyms])
        ffffffff815866b9 fib6_rule_lookup+0x59 ([kernel.kallsyms])
        ffffffff8155a77e ip6_route_input_lookup+0x4e ([kernel.kallsyms])
        ffffffff8155ea98 ip6_route_input+0xd8 ([kernel.kallsyms])
        ffffffff8154ea92 ip6_rcv_finish+0x82 ([kernel.kallsyms])
        ffffffff8154f344 ipv6_rcv+0x334 ([kernel.kallsyms])


Or if I cat kallsyms into a file and pass it to perf it works:

$ cat /proc/kallsyms > /tmp/kallsyms
$ perf script --kallsyms /tmp/kallsyms

vhost-7489  7491 [018]  2314.004488: fib6:fib6_table_lookup: table 255 oif 0 iif 4 src 2100:1::3 dst ff02::1:ff00:64 tos 0 scope 0 flags 0 ==> dev br1 gw ::
        ffffffff8155d49e ip6_pol_route+0x49e ([kernel.kallsyms])
        ffffffff8155d77a ip6_pol_route_input+0x1a ([kernel.kallsyms])
        ffffffff81586515 fib6_rule_action+0xc5 ([kernel.kallsyms])
        ffffffff814c0676 fib_rules_lookup+0x116 ([kernel.kallsyms])
        ffffffff815866b9 fib6_rule_lookup+0x59 ([kernel.kallsyms])
        ffffffff8155a77e ip6_route_input_lookup+0x4e ([kernel.kallsyms])
        ffffffff8155ea98 ip6_route_input+0xd8 ([kernel.kallsyms])
        ffffffff8154ea92 ip6_rcv_finish+0x82 ([kernel.kallsyms])
        ffffffff8154f344 ipv6_rcv+0x334 ([kernel.kallsyms])

So there is something fundamentally wrong with the use of /proc/kcore.

Adrian: I seem to recall you added support for kcore. Any ideas on why the addresses are wrong?

David