Since several local symbols can have same name (e.g. t_show), we need to
use the relative address from the symbol referred by kmap->ref_reloc_sym
instead of the target symbol name itself.

Because the kernel address space layout randomize (kASLR) changes the
absolute address of kernel symbols, we can't rely on the absolute
address.

Note that this works only with debuginfo.

E.g. without this change;
  ----
  # ./perf probe -a "t_show \$vars"
  Added new events:
    probe:t_show         (on t_show with $vars)
    probe:t_show_1       (on t_show with $vars)
    probe:t_show_2       (on t_show with $vars)
    probe:t_show_3       (on t_show with $vars)

  You can now use it in all perf tools, such as:

          perf record -e probe:t_show_3 -aR sleep 1
  ----
OK, we have 4 different t_show()s. All functions have
different arguments as below;
  ----
  # cat /sys/kernel/debug/tracing/kprobe_events
  p:probe/t_show t_show m=%di:u64 v=%si:u64
  p:probe/t_show_1 t_show m=%di:u64 v=%si:u64 t=%si:u64
  p:probe/t_show_2 t_show m=%di:u64 v=%si:u64 fmt=%si:u64
  p:probe/t_show_3 t_show m=%di:u64 v=%si:u64 file=%si:u64
  ----
However, all of them have been put on the *same* address.
  ----
  # cat /sys/kernel/debug/kprobes/list
  ffffffff810d9720  k  t_show+0x0    [DISABLED]
  ffffffff810d9720  k  t_show+0x0    [DISABLED]
  ffffffff810d9720  k  t_show+0x0    [DISABLED]
  ffffffff810d9720  k  t_show+0x0    [DISABLED]
  ----

With this change;
  ----
  # ./perf probe -a "t_show \$vars"
  Added new events:
    probe:t_show         (on t_show with $vars)
    probe:t_show_1       (on t_show with $vars)
    probe:t_show_2       (on t_show with $vars)
    probe:t_show_3       (on t_show with $vars)

  You can now use it in all perf tools, such as:

          perf record -e probe:t_show_3 -aR sleep 1

  # cat /sys/kernel/debug/tracing/kprobe_events
  p:probe/t_show _stext+889880 m=%di:u64 v=%si:u64
  p:probe/t_show_1 _stext+928568 m=%di:u64 v=%si:u64 t=%si:u64
  p:probe/t_show_2 _stext+969512 m=%di:u64 v=%si:u64 fmt=%si:u64
  p:probe/t_show_3 _stext+1001416 m=%di:u64 v=%si:u64 file=%si:u64

  # cat /sys/kernel/debug/kprobes/list
  ffffffffb50d95e0  k  t_show+0x0    [DISABLED]
  ffffffffb50e2d00  k  t_show+0x0    [DISABLED]
  ffffffffb50f4990  k  t_show+0x0    [DISABLED]
  ffffffffb50eccf0  k  t_show+0x0    [DISABLED]
  ----
This time, each event is put in different address
correctly.

Note that currently this doesn't support address-based
probe on modules (thus the probes on modules are symbol
based), since it requires relative address probe syntax
for kprobe-tracer, and it isn't implemented yet.

One more note, this allows us to put events on correct
address, but --list option should be updated to show
correct corresponding source code.

Changes from v2:
  - Refer kmap->ref_reloc_sym instead of "_stext".
  - Refer map->reloc to catch up the kASLR perf fix.

Changes from v1:
  - Use _stext relative address instead of actual
    absolute address recorded in debuginfo.
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: "David A. Long" <dave.long@linaro.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: yrl.pp-manager.tt@hitachi.com
Link: http://lkml.kernel.org/r/20140206053216.29635.22584.stgit@kbuild-fedora.yrl.intra.hitachi.co.jpSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>