linux-kernel - Re: ftrace bug

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Date:   Tue, 8 Mar 2022 11:53:35 +0100
From:   Arend van Spriel <arend.vanspriel@...adcom.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: ftrace bug

On 3/7/2022 2:24 PM, Steven Rostedt wrote:
> On Mon, 7 Mar 2022 12:26:45 +0100
> Arend van Spriel <arend.vanspriel@...adcom.com> wrote:
> 
>> Hi Steven,
>>
>> I wanted to use FTRACE on an ARM platform and I hit the following
>> warning which results in ftrace bug. This happens upon loading a module.
>> Looking up the warning I suspect the branch target is too far off. The
>> module is quite large and therefor not loaded in the modules section. Is
>> there a way to exclude a module. In ftrace_module_init I see a check for
>> !mod->num_ftrace_callsites. Is there a way to avoid creating ftrace
>> callsites in a module?
> 
> Are you sure it's the size. Just want to make sure that's the reason before
> going with different solutions.

Interpreting the warning statement in insn.c and given the fact that the 
module is loaded at 0xe3xxxxxx instead of kernel module space at 
0xbfxxxxxx I made this assumption, but ....

>>
>> Regards,
>> Arend
>>
>> ------------[ cut here ]------------
>>
>> WARNING: CPU: 2 PID: 1525 at arch/arm/kernel/insn.c:47
>> __arm_gen_branch+0x70/0x78
>>
>> CPU: 2 PID: 1525 Comm: insmod Tainted: P                  4.19.183 #2
>>
>> Hardware name: Generic DT based system
>>
>> [<c02139e4>] (unwind_backtrace) from [<c020d4f8>] (show_stack+0x20/0x24)
>>
>> [<c020d4f8>] (show_stack) from [<c0922aec>] (dump_stack+0x98/0xac)
>>
>> [<c0922aec>] (dump_stack) from [<c0919b98>] (__warn.part.0+0xcc/0xe8)
>>
>> [<c0919b98>] (__warn.part.0) from [<c0919d4c>]
>> (warn_slowpath_null+0x54/0x74)
>> [<c0919d4c>] (warn_slowpath_null) from [<c021225c>]
>> (__arm_gen_branch+0x70/0x78)
>> [<c021225c>] (__arm_gen_branch) from [<c02120e4>]
>> (ftrace_make_nop+0x64/0xec)
>> [<c02120e4>] (ftrace_make_nop) from [<c02ac2b0>]
>> (ftrace_process_locs+0x370/0x4b4)
>>
>> [<c02ac2b0>] (ftrace_process_locs) from [<c02af538>]
>> (ftrace_module_init+0x38/0x3c)
>>
>> [<c02af538>] (ftrace_module_init) from [<c02a47d8>]
>> (load_module+0x18d0/0x2570)
>> [<c02a47d8>] (load_module) from [<c02a56f8>]
>> (sys_finit_module+0xe0/0xf8)
>> [<c02a56f8>] (sys_finit_module) from [<c0201000>]
>> (ret_fast_syscall+0x0/0x58)
>> Exception stack(0xd9b1bfa8 to 0xd9b1bff0)
>>
>> bfa0:                   01170228 00000000 00000003 01170228 00000000
>> beb7ceb3
>> bfc0: 01170228 00000000 beb7cdc4 0000017b 00127010 00000000 00000000
>> 00000000
>> bfe0: beb7cc38 beb7cc28 0001a0dc 00012890
>>
>> ---[ end trace f48808a851a4544a ]---
>>
>> ------------[ cut here ]------------
>>
>> WARNING: CPU: 2 PID: 1525 at kernel/trace/ftrace.c:2034
>> ftrace_bug+0xfc/0x394
> 
> Three should be more content after the "cut here" that is very relevant (I
> hate that cut here, because I constantly need to tell people to show me
> more :-p I need to add a "ftrace bug cut here" line.)
> 
> 
> Could you show me the output right after that.

... here is the output I think you were looking for:

ftrace failed to modify 

[<a82ca82d>] (suspected corrupt symbol) 

  actual:   63:17:16:eb 

Initializing ftrace call sites 

ftrace record flags: 2000000 

  (0) 

  expected tramp: c0211b88 

ftrace failed to modify 

[<6c88ec64>] (suspected corrupt symbol) 

  actual:   72:ab:08:eb 

Initializing ftrace call sites 

ftrace record flags: 2000000 

  (0) 

  expected tramp: c0211b88 

ftrace failed to modify 

[<b1ed303a>] (suspected corrupt symbol) 

  actual:   04:12:04:eb 

Initializing ftrace call sites 

ftrace record flags: 2000000 

  (0) 

  expected tramp: c0211b88 

ftrace failed to modify 

[<eb2fee66>] (suspected corrupt symbol) 

  actual:   d6:7e:00:eb 

Initializing ftrace call sites 

ftrace record flags: 2000000 

  (0) 

  expected tramp: c0211b88

Regards,
Arend

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