[PATCH] Remove kretprobe_trampoline_holder.

[Thiago Jung Bauermann]

Fixes the following testsuite failure:

$ sudo ./perf test -v kallsyms
 1: vmlinux symtab matches kallsyms                          :
--- start ---
test child forked, pid 12489
Using /proc/kcore for kernel object code
Looking at the vmlinux_path (8 entries long)
Using /boot/vmlinux for symbols
0xc00000000003d300: diff name v: .kretprobe_trampoline_holder k: kretprobe_trampoline
Maps only in vmlinux:
 c00000000086ca38-c000000000879b6c 87ca38 [kernel].text.unlikely
 c000000000879b6c-c000000000bf0000 889b6c [kernel].meminit.text
 c000000000bf0000-c000000000c53264 c00000 [kernel].init.text
 c000000000c53264-d000000004250000 c63264 [kernel].exit.text
 d000000004250000-d000000004450000 0 [libcrc32c]
 d000000004450000-d000000004620000 0 [xfs]
 d000000004620000-d000000004680000 0 [autofs4]
 d000000004680000-d0000000046e0000 0 [x_tables]
 d0000000046e0000-d000000004780000 0 [ip_tables]
 d000000004780000-d0000000047e0000 0 [rng_core]
 d0000000047e0000-ffffffffffffffff 0 [pseries_rng]
Maps in vmlinux with a different name in kallsyms:
Maps only in kallsyms:
 d000000000000000-f000000000000000 1000000000010000 [kernel.kallsyms]
 f000000000000000-ffffffffffffffff 3000000000010000 [kernel.kallsyms]
test child finished with -1
---- end ----
vmlinux symtab matches kallsyms: FAILED!

The problem is that the kretprobe_trampoline symbol looks like this:

$ eu-readelf -s /boot/vmlinux G kretprobe_trampoline
 2431: c000000001302368     24 NOTYPE  LOCAL  DEFAULT       37 kretprobe_trampoline_holder
 2432: c00000000003d300      8 FUNC    LOCAL  DEFAULT        1 .kretprobe_trampoline_holder
97543: c00000000003d300      0 NOTYPE  GLOBAL DEFAULT        1 kretprobe_trampoline

Its type is NOTYPE, and its size is 0, and this is a problem because
symbol-elf.c:dso__load_sym skips function symbols that are not STT_FUNC
or STT_GNU_IFUNC (this is determined by elf_sym__is_function). Even
if the type is changed to STT_FUNC, when dso__load_sym calls
symbols__fixup_duplicate, the kretprobe_trampoline symbol is dropped in
favour of .kretprobe_trampoline_holder because the latter has non-zero
size (as determined by choose_best_symbol).

With this patch, all vmlinux symbols match /proc/kallsyms and the
testcase passes.

Commit c1c355c gets rid of kretprobe_trampoline_holder altogether on
x86. This commit does the same on powerpc.

Signed-off-by: Thiago Jung Bauermann <bauerman at linux.vnet.ibm.com>
---

Notes:
    With this patch, all ftrace startup tests still pass.  There's one ftrace
    testcase failure:
    
    [4] Kprobe dynamic event with function tracer   [FAIL]
    execute:
    /home/bauermann/src/linux/tools/testing/selftests/ftrace/test.d/kprobe/kprobe_ftrace.tc
    + .
    /home/bauermann/src/linux/tools/testing/selftests/ftrace/test.d/kprobe/kprobe_ftrace.tc
    ++ '[' -f kprobe_events ']'
    ++ grep function available_tracers
    blk function_graph wakeup_dl wakeup_rt wakeup function nop
    ++ echo nop
    ++ echo _do_fork
    /home/bauermann/src/linux/tools/testing/selftests/ftrace/test.d/kprobe/kprobe_ftrace.tc:
    line 9: echo: write error: Invalid argument
    
    But even without my patch that testcase fails in the same way, so this
    is not a problem introduced by the patch.

 arch/powerpc/kernel/kprobes.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index 7c053f2..417c0ea 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -278,12 +278,11 @@ no_kprobe:
  * 	- When the probed function returns, this probe
  * 		causes the handlers to fire
  */
-static void __used kretprobe_trampoline_holder(void)
-{
-	asm volatile(".global kretprobe_trampoline\n"
-			"kretprobe_trampoline:\n"
-			"nop\n");
-}
+asm(".global kretprobe_trampoline\n"
+	".type kretprobe_trampoline, @function\n"
+	"kretprobe_trampoline:\n"
+	"nop\n"
+	".size kretprobe_trampoline, .-kretprobe_trampoline\n");
 
 /*
  * Called when the probe at kretprobe trampoline is hit
-- 
1.9.1