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SubjectRe: ftrace bug
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Hi Arend!

On 08/03/2022 16:20, Steven Rostedt wrote:
>>> 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

This problem is addressed by:

commit 8113e622926ef6590771ede0f7f64821e1751b67
Author: Alex Sverdlin <alexander.sverdlin@nokia.com>
Date: Mon Sep 27 14:02:45 2021 -0700

ARM: 9079/1: ftrace: Add MODULE_PLTS support

commit 79f32b221b18c15a98507b101ef4beb52444cc6f upstream

Teach ftrace_make_call() and ftrace_make_nop() about PLTs.
Teach PLT code about FTRACE and all its callbacks.
Otherwise the following might happen:

------------[ cut here ]------------
WARNING: CPU: 14 PID: 2265 at .../arch/arm/kernel/insn.c:14 __arm_gen_branch+0x83/0x8c()
...
Hardware name: LSI Axxia AXM55XX
[<c0314a49>] (unwind_backtrace) from [<c03115e9>] (show_stack+0x11/0x14)
[<c03115e9>] (show_stack) from [<c0519f51>] (dump_stack+0x81/0xa8)
[<c0519f51>] (dump_stack) from [<c032185d>] (warn_slowpath_common+0x69/0x90)
[<c032185d>] (warn_slowpath_common) from [<c03218f3>] (warn_slowpath_null+0x17/0x1c)
[<c03218f3>] (warn_slowpath_null) from [<c03143cf>] (__arm_gen_branch+0x83/0x8c)
[<c03143cf>] (__arm_gen_branch) from [<c0314337>] (ftrace_make_nop+0xf/0x24)
[<c0314337>] (ftrace_make_nop) from [<c038ebcb>] (ftrace_process_locs+0x27b/0x3e8)
[<c038ebcb>] (ftrace_process_locs) from [<c0378d79>] (load_module+0x11e9/0x1a44)
[<c0378d79>] (load_module) from [<c037974d>] (SyS_finit_module+0x59/0x84)
[<c037974d>] (SyS_finit_module) from [<c030e981>] (ret_fast_syscall+0x1/0x18)
---[ end trace e1b64ced7a89adcc ]---
------------[ cut here ]------------

And if you wanna stick with 4.19.y, it's included starting from v4.19.209.

--
Best regards,
Alexander Sverdlin.

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