perf tests bpf: Use SyS_epoll_wait alias

Something made the sys_epoll_wait() function alias not to be found in
the vmlinux DWARF info, being found only in /proc/kallsyms, which made
the BPF perf tests to fail:

  [root@jouet ~]# perf test BPF
  37: Test BPF filter                                          :
  37.1: Test basic BPF filtering                               : FAILED!
  37.2: Test BPF prologue generation                           : Skip
  37.3: Test BPF relocation checker                            : Skip
  [root@jouet ~]#

Using -v we can see it is failing to find DWARF info for the probed function,
sys_epoll_wait, which we can find in /proc/kallsyms but not in vmlinux with
CONFIG_DEBUG_INFO:

  [root@jouet ~]# grep -w sys_epoll_wait /proc/kallsyms
  ffffffffbd295b50 T sys_epoll_wait
  [root@jouet ~]#

  [root@jouet ~]# readelf -wi /lib/modules/4.7.0+/build/vmlinux | grep -w sys_epoll_wait
  [root@jouet ~]#

If we try to use perf probe:

[root@jouet ~]# perf probe sys_epoll_wait
Failed to find debug information for address ffffffffbd295b50
Probe point 'sys_epoll_wait' not found.
  Error: Failed to add events.
[root@jouet ~]#

It all works if we use SyS_epoll_wait, that is just an alias to the probed
function:

  [root@jouet ~]# grep -i sys_epoll_wait /proc/kallsyms
  ffffffffbd295b50 T SyS_epoll_wait
  ffffffffbd295b50 T sys_epoll_wait
  [root@jouet ~]#

So use it:

  [root@jouet ~]# perf test BPF
  37: Test BPF filter                                          :
  37.1: Test basic BPF filtering                               : Ok
  37.2: Test BPF prologue generation                           : Ok
  37.3: Test BPF relocation checker                            : Ok
  [root@jouet ~]#

Further info:

  [root@jouet ~]# gcc --version
  gcc (GCC) 6.1.1 20160621 (Red Hat 6.1.1-3)
  [acme@jouet linux]$ cat /etc/fedora-release
  Fedora release 24 (Twenty Four)

Investigation as to why it fails is still underway, but it was always
going from sys_epoll_wait to SyS_epoll_wait when looking up the DWARF
info in vmlinux, and this is what is breaking now.

Switching to use SyS_epoll_wait allows this test to proceed and test the
BPF code it was designed for, so lets have this in to allow passing this
test while we fix the root cause.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-7hekjp0bodwjbb419sl2b55h@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/tests/bpf-script-example.c

@@ -31,8 +31,8 @@ struct bpf_map_def SEC("maps") flip_table = {
 	.max_entries = 1,
 };
 
-SEC("func=sys_epoll_wait")
-int bpf_func__sys_epoll_wait(void *ctx)
+SEC("func=SyS_epoll_wait")
+int bpf_func__SyS_epoll_wait(void *ctx)
 {
 	int ind =0;
 	int *flag = bpf_map_lookup_elem(&flip_table, &ind);