[PATCH] recordmcount: Fix spurious mcount entries on powerpc

From: Naveen N. Rao
Date: Wed Jun 26 2019 - 14:38:22 EST

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
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
expected tramp: c00000000006af4c

Looking at the relocation records in __mcount_loc showed a few spurious
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]:
0000000000000000 R_PPC64_REL64 .TOC.-0x0000000000000008
0000000000000008 R_PPC64_ENTRY *ABS*
0000000000000014 R_PPC64_REL24 _mcount

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@xxxxxxxxxxxxxxxxxx>
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