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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
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