Re: dynamic ftrace/recordmcount.c problem on ARMv5

[Uwe Kleine-König]

Hello,

On Wed, Jan 13, 2016 at 09:51:28AM +0100, Marc Kleine-Budde wrote:
> Hello,
> 
> I'm on a ARMv5 (freescale mx25) and seeing this ftrace bug during bootup:
> 
> > [    0.059235] ------------[ cut here ]------------
> > [    0.059449] WARNING: CPU: 0 PID: 0 at kernel/trace/ftrace.c:1938 ftrace_bug+0x210/0x2c8()
> > [    0.059645] Modules linked in:
> > [    0.059780] CPU: 0 PID: 0 Comm: swapper Not tainted 4.0.9-20151211-1-g45dbe7d2c077 #1
> > [    0.059966] Hardware name: Freescale i.MX25 (Device Tree Support)
> > [    0.060157] [<8000f494>] (unwind_backtrace) from [<8000ce3c>] (show_stack+0x20/0x24)
> > [    0.060396] [<8000ce3c>] (show_stack) from [<8041ea68>] (dump_stack+0x20/0x28)
> > [    0.060630] [<8041ea68>] (dump_stack) from [<8001a74c>] (warn_slowpath_common+0x88/0xc0)
> > [    0.060853] [<8001a74c>] (warn_slowpath_common) from [<8001a840>] (warn_slowpath_null+0x2c/0x34)
> > [    0.061091] [<8001a840>] (warn_slowpath_null) from [<80083748>] (ftrace_bug+0x210/0x2c8)
> > [    0.061332] [<80083748>] (ftrace_bug) from [<80083bb8>] (ftrace_process_locs+0x330/0x6f0)
> > [    0.061576] [<80083bb8>] (ftrace_process_locs) from [<805bdc84>] (ftrace_init+0x8c/0x14c)
> > [    0.061815] [<805bdc84>] (ftrace_init) from [<805b0c80>] (start_kernel+0x33c/0x3b4)
> > [    0.062039] [<805b0c80>] (start_kernel) from [<80008040>] (0x80008040)
> > [    0.062238] ---[ end trace cb88537fdc8fa200 ]---
> > [    0.062364] ftrace failed to modify [<8000c7d8>] walk_stackframe+0x24/0x44
> > [    0.062544]  actual: 16:ff:2f:e1
> > [    0.062702] ftrace record flags: 0
> > [    0.062803]  (0)   expected tramp: 8000e864
> 
> The problem is that walk_stackframe ends up in the __mcount_loc section,
> although it has a "notrace" attribute:

Not having much clue about elf and stuff, but being hit by this problem
I added some debug output to recordmcount.c and found that
sift_rel_mcount (aka sift32_rel_mcount) loops over .rel.text which looks
as follows for me:

	$ readelf -R .rel.text arch/arm/kernel/stacktrace.o 
	...
	Relocation section '.rel.text' at offset 0xb5c contains 15 entries:
	 Offset     Info    Type            Sym.Value  Sym. Name
	0000001c  00000028 R_ARM_V4BX       
	0000002c  00001d1c R_ARM_CALL        00000000   unwind_frame
	00000044  00001f1c R_ARM_CALL        00000000   __gnu_mcount_nc
	00000104  0000201c R_ARM_CALL        00000000   in_sched_functions
	00000118  00002102 R_ARM_ABS32       00000000   __exception_text_start
	0000011c  00002202 R_ARM_ABS32       00000000   __exception_text_end
	00000130  00001f1c R_ARM_CALL        00000000   __gnu_mcount_nc
	00000194  00001c1c R_ARM_CALL        00000000   walk_stackframe
	000001e0  00000902 R_ARM_ABS32       0000003c   save_trace
	000001e4  00000c02 R_ARM_ABS32       00000120   __save_stack_trace
	000001ec  00001f1c R_ARM_CALL        00000000   __gnu_mcount_nc
	00000218  00001f1c R_ARM_CALL        00000000   __gnu_mcount_nc
	00000260  00001c1c R_ARM_CALL        00000000   walk_stackframe
	0000028c  00000902 R_ARM_ABS32       0000003c   save_trace
	00000294  00001f1c R_ARM_CALL        00000000   __gnu_mcount_nc
	...

and the unwanted entry in __mcount_loc is generated from the first line
which isn't a call, but just the information that there is a bx call
that needs a fixup for ARMv4 machines (that don't support the bx
instruction).

For this entry get_mcountsym returns 0.

recordmcount.pl only operates on R_ARM_(CALL|PC24|THM_CALL) types for
ARM and so does it right.

I don't know how to fix recordmcount.[ch], but maybe this is enough info
to allow someone more knowledgeable to fix it.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |