ARM Ftrace Function Graph Fails With UNWINDER_FRAME_POINTER

[Justin Chen]

Hello,

Ran into an odd bug that I am unsure what the solution is. Tested a few 
kernels versions and they all fail the same.

FUNCTION_GRAPH_FP_TEST was enabled with 953f534a7ed6 ("ARM: ftrace: 
enable HAVE_FUNCTION_GRAPH_FP_TEST"). This test fails when 
UNWINDER_FRAME_POINTER is enabled. Enable function_graph tracer and you 
should see a failure similar to below.

[   63.817239] ------------[ cut here ]------------
[   63.822006] WARNING: CPU: 3 PID: 1185 at kernel/trace/fgraph.c:195 
ftrace_return_to_handler+0x228/0x374
[   63.831645] Bad frame pointer: expected d1e0df40, received d1e0df48
[   63.831645]   from func packet_setsockopt return to c0b558f4
[   63.843801] Modules linked in: bdc udc_core
[   63.848246] CPU: 3 PID: 1185 Comm: udhcpc Not tainted 
6.1.53-0.1pre-gf0bc552d12f8 #33
[   63.856209] Hardware name: Broadcom STB (Flattened Device Tree)
[   63.862227] Backtrace:
[   63.864761]  dump_backtrace from show_stack+0x20/0x24
[   63.869982]  r7:c031cd8c r6:00000009 r5:00000013 r4:c11c7fac
[   63.875736]  show_stack from dump_stack_lvl+0x48/0x54
[   63.880929]  dump_stack_lvl from dump_stack+0x18/0x1c
[   63.886111]  r5:000000c3 r4:c11bd92c
[   63.889764]  dump_stack from __warn+0x88/0x130
[   63.894339]  __warn from warn_slowpath_fmt+0x140/0x198
[   63.899631]  r8:d1e0deac r7:c11bd958 r6:c031cd8c r5:c11bd92c r4:00000000
[   63.906431]  warn_slowpath_fmt from ftrace_return_to_handler+0x228/0x374
[   63.913294]  r8:c3a8d840 r7:00000002 r6:d1e0df48 r5:c2377a94 r4:c269a400
[   63.920095]  ftrace_return_to_handler from return_to_handler+0xc/0x18
[   63.926699]  r8:c0cd8ed0 r7:00000008 r6:c418c500 r5:00000004 r4:00000107
[   63.933500]  __sys_setsockopt from return_to_handler+0x0/0x18
[   63.939415]  r8:c02002bc r7:00000126 r6:00000003 r5:00000000 r4:00000004
[   63.946217]  sys_setsockopt from return_to_handler+0x0/0x18
[   63.952053] ---[ end trace 0000000000000000 ]---

Sure enough the top of the parent stack is off by 8. (Tested with 
gcc6.3/gcc8.3/gcc12.3)
00006dcc <packet_setsockopt>:
     6dcc:       e1a0c00d        mov     ip, sp
     6dd0:       e24dd008        sub     sp, sp, #8 <======
     6dd4:       e92ddff0        push    {r4, r5, r6, r7, r8, r9, sl, 
fp, ip, lr, pc}
     6dd8:       e24cb00c        sub     fp, ip, #12
     6ddc:       e24dd06c        sub     sp, sp, #108    @ 0x6c
     6de0:       e52de004        push    {lr}            @ (str lr, [sp, 
#-4]!)
     6de4:       ebfffffe        bl      0 <__gnu_mcount_nc>

I'm not quite sure why gcc is putting this extra 8 byte frame (maybe 
some optimization?), but it isn't being accounted for thus the 
FUNCTION_GRAPH_FP_TEST for arm fails. Note that only some functions do 
this. Function graph works with FUNCTION_GRAPH_FP_TEST disabled, so it 
looks the test is hitting false positives.

Thanks,
Justin
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