[PATCH] riscv/ftrace: Fix the problem modules cannot find _mcount

Alan Kao alankao at andestech.com
Tue May 8 19:36:05 PDT 2018

On Tue, May 08, 2018 at 09:11:42AM -0400, Steven Rostedt wrote:
> On Tue, 8 May 2018 11:21:57 +0800
> Alan Kao <alankao at andestech.com> wrote:
> > Enabling ftrace and module support at the same time fails the kernel
> > build process, because modules cannot find the _mcount symbol.  This
> > patch fixes this issue.
> I think you have a much bigger issue.

Yes, we do, but not this one.  Let me state more detail later.
> > 
> > Signed-off-by: Alan Kao <alankao at andestech.com>
> > Cc: Greentime Hu <greentime at andestech.com>
> > Cc: Zong Li <zong at andestech.com>
> > ---
> >  arch/riscv/kernel/mcount.S | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/riscv/kernel/mcount.S b/arch/riscv/kernel/mcount.S
> > index ce9bdc57a2a1..5721624886a1 100644
> > --- a/arch/riscv/kernel/mcount.S
> > +++ b/arch/riscv/kernel/mcount.S
> > @@ -126,5 +126,5 @@ do_trace:
> >  	ret
> >  ENDPROC(_mcount)
> > -EXPORT_SYMBOL(_mcount)
> >  #endif
> > +EXPORT_SYMBOL(_mcount)
> How does this work? How do you export _mcount if _mcount isn't even
> defined?

_mcount is defined.

This EXPORT_SYMBOL and the _mcount body was inside a
"#ifndef CONFIG_DYNAMIC_FTRACE" section, so if the config has dynamic ftrace
disabled, _mcount is visible.

For the dynamic ftrace case, there is a code snippet in this file:

> ENTRY(ftrace_stub)
> #ifdef CONFIG_DYNAMIC_FTRACE         
>       .global _mcount
>       .set    _mcount, ftrace_stub
> #endif
>        ret 
> ENDPROC(ftrace_stub)

so the _mcount symbol is visible in the kernel, but not for modules.
As a result, a module build always fails, because _mcount is neither exported,
nor aliased to a defined symbol.

That's the simple purpose of this patch:  we just want to eliminate the errors
during the kernel build process.

> -- Steve

Talking about some bigger issues here, there will be twofold.

1. This patch lacks call-site collecting mechanism.

This feature requires a pattern check in recordmcount.pl.  With this,
section __mcount_loc is properly constructed, and the call-sites will be
registered during the loading stage.

However, the loading will then fail at the first try of make_nop, due to
the instruction check.  This is thus the second issue.

2. The instruction check for kernel texts is not applicable to module texts.

The check expects a call instruction to _mcount, but here it is a call to
the .plt section of the module.  After referencing the implementation of arm64,
I think it would need much more work to have make_nop's and make_call's behavior

Quick summary: This patch ensures that a kernel build will not fail because of
the _mcount-missing problem.  But ftrace cannot trace loading modules for now
due to the stated reasons.

Alan Kao

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