Re: [RFC PATCH 01/14] ftrace: Fix updating FTRACE_FL_TRAMP

[Naveen N. Rao]

Steven Rostedt wrote:
> On Thu, 26 Nov 2020 23:38:38 +0530
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> 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