linux-kernel - Re: [RFC PATCH 01/14] ftrace: Fix updating FTRACE_FL

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Date:   Tue, 01 Dec 2020 19:16:21 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [RFC PATCH 01/14] ftrace: Fix updating FTRACE_FL_TRAMP

Steven Rostedt wrote:
> On Thu, 26 Nov 2020 23:38:38 +0530
> "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com> wrote:
> 
>> On powerpc, kprobe-direct.tc triggered FTRACE_WARN_ON() in
>> ftrace_get_addr_new() followed by the below message:
>>   Bad trampoline accounting at: 000000004222522f (wake_up_process+0xc/0x20) (f0000001)
>> 
>> The set of steps leading to this involved:
>> - modprobe ftrace-direct-too
>> - enable_probe
>> - modprobe ftrace-direct
>> - rmmod ftrace-direct <-- trigger
>> 
>> The problem turned out to be that we were not updating flags in the
>> ftrace record properly. From the above message about the trampoline
>> accounting being bad, it can be seen that the ftrace record still has
>> FTRACE_FL_TRAMP set though ftrace-direct module is going away. This
>> happens because we are checking if any ftrace_ops has the
>> FTRACE_FL_TRAMP flag set _before_ updating the filter hash.
>> 
>> The fix for this is to look for any _other_ ftrace_ops that also needs
>> FTRACE_FL_TRAMP.
> 
> I'm applying this now and sending this for -rc and stable.
> 
> The code worked on x86 because x86 has a way to make all users use
> trampolines, so this was never an issue (everything has a trampoline).
> I modified the kernel so that x86 would not create its own trampoline
> (see the weak function arch_ftrace_update_trampoline(), and I was able
> to reproduce the bug.

Good to know that you were able to reproduce this.

> 
> I'm adding:
> 
> Cc: stable@...r.kernel.org
> Fixes: a124692b698b0 ("ftrace: Enable trampoline when rec count returns back to one")

That looks good to me. Thanks for picking the two patches and for your 
review on the others!


- Naveen