wrapper: powerpc64: fix kernel crash caused by do_get_kallsyms

wrapper: powerpc64: fix kernel crash caused by do_get_kallsyms

Kernel crashes on powerpc64 ABIv2 as follow when lttng_tracer initializes,
since do_get_kallsyms in lttng_wrapper fails to return a proper address of
kallsyms_lookup_name.

root@qemuppc64:~# lttng create trace_session --live -U net://127.0.0.1
Spawning a session daemon
lttng_kretprobes: loading out-of-tree module taints kernel.
BUG: Unable to handle kernel data access on read at 0xfffffffffffffff8
Faulting instruction address: 0xc0000000001f6fd0
Oops: Kernel access of bad area, sig: 11 [#1]
<snip>
NIP [c0000000001f6fd0] module_kallsyms_lookup_name+0xf0/0x180
LR [c0000000001f6f28] module_kallsyms_lookup_name+0x48/0x180
Call Trace:
module_kallsyms_lookup_name+0x34/0x180 (unreliable)
kallsyms_lookup_name+0x258/0x2b0
wrapper_kallsyms_lookup_name+0x4c/0xd0 [lttng_wrapper]
wrapper_get_pfnblock_flags_mask_init+0x28/0x60 [lttng_wrapper]
lttng_events_init+0x40/0x344 [lttng_tracer]
do_one_initcall+0x78/0x340
do_init_module+0x6c/0x2f0
__do_sys_finit_module+0xd0/0x120
system_call_exception+0x194/0x2f0
system_call_vectored_common+0xe8/0x278
<snip>

do_get_kallsyms makes use of kprobe_register and in turn kprobe_lookup_name
to get the address of the kernel function kallsyms_lookup_name. In case of
PPC64_ELF_ABI_v2, when kprobes are placed at function entry,
kprobe_lookup_name adjusts the global entry point of the function returned
by kallsyms_lookup_name to the local entry point(at some fixed offset of
global one). This adjustment is all for kprobes to be able to work properly.
Global and local entry point are defined in powerpc64 ABIv2.

When the local entry point is given, some instructions at the beginning of
the function are skipped and thus causes the above kernel crash. We just
want to make a simple function call which needs global entry point.

This patch adds 4 bytes which is the length of one instruction to
kallsyms_lookup_name so that it will not trigger the global to local
adjustment, and then substracts 4 bytes from the returned address. See the
following kernel change for more details.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=290e3070762ac80e5fc4087d8c4de7e3f1d90aca

Signed-off-by: He Zhe <zhe.he@windriver.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I34e68e886b97e3976d0b5e25be295a8bb866c1a4