linux-kernel - Re: [PATCH] recordmcount: Fix spurious mcount entries on powerpc

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Date:   Thu, 27 Jun 2019 15:53:58 +0530
From:   Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Steven Rostedt <rostedt@...dmis.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] recordmcount: Fix spurious mcount entries on powerpc

On Thu, Jun 27, 2019 at 12:08:01AM +0530, 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@...ux.vnet.ibm.com>
> ---
>  scripts/recordmcount.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
Hi,

linuxppc/merge branch latest commit 850f6274c5b5 was failing to boot IBM
Power8 Box, and the failure got resolved with this patch.
https://github.com/linuxppc/issues/issues/254

# git log -2 --oneline
0e0f55b31ea8 (HEAD -> issue_254) recordmcount: Fix spurious mcount entries on powerpc
850f6274c5b5 (origin/merge, merge) Automatic merge of branches 'master', 'next' and 'fixes' into merge
# uname -r
5.2.0-rc6-00123-g0e0f55b31ea8
 
Tested-by: Satheesh Rajendran <sathnaga@...ux.vnet.ibm.com>

Regards,
-Satheesh
> 
> 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
>