linux-kernel - Re: ftrace bug

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Date:   Tue, 8 Mar 2022 17:02:53 +0100
From:   Alexander Sverdlin <alexander.sverdlin@...ia.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Arend van Spriel <arend.vanspriel@...adcom.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Russell King <linux@...linux.org.uk>,
        Abel Vesa <abelvesa@...ux.com>, Rabin Vincent <rabin@....in>,
        Colin Cross <ccross@...roid.com>
Subject: Re: ftrace bug

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@...ia.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.