linux-kernel - Re: [PATCH 4/5 v3] ftrace/x86_32: Clean up ftrace_regs

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Date:   Thu, 16 Mar 2017 12:28:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 4/5 v3] ftrace/x86_32: Clean up ftrace_regs_caller

On Thu, Mar 16, 2017 at 12:19 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> The thing is we don't return, we jump to the location that may be
> modified to run the function graph tracer.

Hmm.

How about just making the stack frame a tiny bit bigger, and getting
rid of *all* the games.

IOW, just duplicate the return address, and make the entry code do

        pushfl
        pushl   $__KERNEL_CS
        pushl   8(%esp)         /* Save the return ip *again* */
        pushl   $0
        pushl   %gs
        pushl   %fs
        pushl   %es
        pushl   %ds
        pushl   %eax
        ....

and not have any silly code to modify the old stack frame at all. Just
skip the values (all the segments, ORIG_EAX, duplicated return
address, __KERNEL_CS), and you can finish off with a "popf", and all
you have left i the original return ip that you didn't touch.

Hmm?

               Linus