[PATCH] recordmcount: Fix spurious mcount entries on powerpc

[Michael Ellerman]

On Wed, 2019-06-26 at 18:38:01 UTC, "Naveen N. Rao" wrote:
> 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>
> Tested-by: Satheesh Rajendran <sathnaga at linux.vnet.ibm.com>
> Acked-by: Steven Rostedt (VMware) <rostedt at goodmis.org>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/80e5302e4bc85a6b685b7668c36c6487b5f90e9a

cheers