[PATCH] recordmcount: Fix spurious mcount entries on powerpc

[Naveen N. Rao]

The recent change enabling HAVE_C_RECORDMCOUNT on powerpc started
showing the following issue:

  # modprobe kprobe_example
   ftrace-powerpc: Not expected bl: opcode is 3c4c0001
   WARNING: CPU: 0 PID: 227 at kernel/trace/ftrace.c:2001 ftrace_bug+0x90/0x318
   Modules linked in:
   CPU: 0 PID: 227 Comm: modprobe Not tainted 5.2.0-rc6-00678-g1c329100b942 #2
   NIP:  c000000000264318 LR: c00000000025d694 CTR: c000000000f5cd30
   REGS: c000000001f2b7b0 TRAP: 0700   Not tainted  (5.2.0-rc6-00678-g1c329100b942)
   MSR:  900000010282b033 <SF,HV,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 28228222  XER: 00000000
   CFAR: c0000000002642fc IRQMASK: 0
   <snip>
   NIP [c000000000264318] ftrace_bug+0x90/0x318
   LR [c00000000025d694] ftrace_process_locs+0x4f4/0x5e0
   Call Trace:
   [c000000001f2ba40] [0000000000000004] 0x4 (unreliable)
   [c000000001f2bad0] [c00000000025d694] ftrace_process_locs+0x4f4/0x5e0
   [c000000001f2bb90] [c00000000020ff10] load_module+0x25b0/0x30c0
   [c000000001f2bd00] [c000000000210cb0] sys_finit_module+0xc0/0x130
   [c000000001f2be20] [c00000000000bda4] system_call+0x5c/0x70
   Instruction dump:
   419e0018 2f83ffff 419e00bc 2f83ffea 409e00cc 4800001c 0fe00000 3c62ff96
   39000001 39400000 386386d0 480000c4 <0fe00000> 3ce20003 39000001 3c62ff96
   ---[ end trace 4c438d5cebf78381 ]---
   ftrace failed to modify
   [<c0080000012a0008>] 0xc0080000012a0008
    actual:   01:00:4c:3c
   Initializing ftrace call sites
   ftrace record flags: 2000000
    (0)
    expected tramp: c00000000006af4c

Looking at the relocation records in __mcount_loc showed a few spurious
entries:
  RELOCATION RECORDS FOR [__mcount_loc]:
  OFFSET           TYPE              VALUE
  0000000000000000 R_PPC64_ADDR64    .text.unlikely+0x0000000000000008
  0000000000000008 R_PPC64_ADDR64    .text.unlikely+0x0000000000000014
  0000000000000010 R_PPC64_ADDR64    .text.unlikely+0x0000000000000060
  0000000000000018 R_PPC64_ADDR64    .text.unlikely+0x00000000000000b4
  0000000000000020 R_PPC64_ADDR64    .init.text+0x0000000000000008
  0000000000000028 R_PPC64_ADDR64    .init.text+0x0000000000000014

The first entry in each section is incorrect. Looking at the relocation
records, the spurious entries correspond to the R_PPC64_ENTRY records:
  RELOCATION RECORDS FOR [.text.unlikely]:
  OFFSET           TYPE              VALUE
  0000000000000000 R_PPC64_REL64     .TOC.-0x0000000000000008
  0000000000000008 R_PPC64_ENTRY     *ABS*
  0000000000000014 R_PPC64_REL24     _mcount
  <snip>

The problem is that we are not validating the return value from
get_mcountsym() in sift_rel_mcount(). With this entry, mcountsym is 0,
but Elf_r_sym(relp) also ends up being 0. Fix this by ensuring mcountsym
is valid before processing the entry.

Fixes: c7d64b560ce80 ("powerpc/ftrace: Enable C Version of recordmcount")
Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
---
 scripts/recordmcount.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/scripts/recordmcount.h b/scripts/recordmcount.h
index 13c5e6c8829c..47fca2c69a73 100644
--- a/scripts/recordmcount.h
+++ b/scripts/recordmcount.h
@@ -325,7 +325,8 @@ static uint_t *sift_rel_mcount(uint_t *mlocp,
 		if (!mcountsym)
 			mcountsym = get_mcountsym(sym0, relp, str0);
 
-		if (mcountsym == Elf_r_sym(relp) && !is_fake_mcount(relp)) {
+		if (mcountsym && mcountsym == Elf_r_sym(relp) &&
+				!is_fake_mcount(relp)) {
 			uint_t const addend =
 				_w(_w(relp->r_offset) - recval + mcount_adjust);
 			mrelp->r_offset = _w(offbase
-- 
2.22.0