linux-kernel - Re: [PATCH] parisc/kernel/ftrace: Remove function callback casts

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Message-ID: <20200704121236.GA3143@ubuntu>
Date:   Sat, 4 Jul 2020 14:12:36 +0200
From:   Oscar Carter <oscar.carter@....com>
To:     Helge Deller <deller@....de>
Cc:     Oscar Carter <oscar.carter@....com>,
        Kees Cook <keescook@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "James E . J . Bottomley" <James.Bottomley@...senPartnership.com>,
        kernel-hardening@...ts.openwall.com, linux-parisc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] parisc/kernel/ftrace: Remove function callback casts

On Sat, Jun 27, 2020 at 10:09:40PM +0200, Helge Deller wrote:
> On 27.06.20 15:43, Oscar Carter wrote:
> > In an effort to enable -Wcast-function-type in the top-level Makefile to
> > support Control Flow Integrity builds, remove all the function callback
> > casts.
> >
> > To do this remove the cast to a function pointer type in the comparison
> > statement and add to the right and left operand a cast to unsigned long
> > type. This can be done since the comparison is against function address
> > (these operands are not function calls).
>
> On some architectures (namely ia64, ppc64 and parisc64) function pointers
> actually refer to function descriptors, which in turn point to the real
> function address. The compiler usually takes care of such comparism.
> That said, casting to "unsigned long" might break this...

Ok, thanks for the clarification. Today I have learned about function
descriptors.

> > Signed-off-by: Oscar Carter <oscar.carter@....com>
> > ---
> >  arch/parisc/kernel/ftrace.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/parisc/kernel/ftrace.c b/arch/parisc/kernel/ftrace.c
> > index 1df0f67ed667..86b49a5fc049 100644
> > --- a/arch/parisc/kernel/ftrace.c
> > +++ b/arch/parisc/kernel/ftrace.c
> > @@ -64,7 +64,7 @@ void notrace __hot ftrace_function_trampoline(unsigned long parent,
> >  				function_trace_op, regs);
> >
> >  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > -	if (ftrace_graph_return != (trace_func_graph_ret_t) ftrace_stub ||
> > +	if ((unsigned long)ftrace_graph_return != (unsigned long)ftrace_stub ||
>
> Untested, but maybe better use:
> 	dereference_function_descriptor(ftrace_graph_return) !=
> 		dereference_function_descriptor(ftrace_stub)

Thanks for point me to the right direction about this theme. I will do a
new version and I will resend.

> ?
> Helge

Regards,
Oscar Carter