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

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Date:   Thu, 16 Mar 2017 11:19:44 -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 11:09 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> +
> +       /* Since we don't care about cs, move flags there to simplify return */
> +       movl    14*4(%esp), %eax
> +       movl    %eax, 13*4(%esp)
> +
> +       /* Move return ip back to its original location */
> +       movl    12*4(%esp), %eax
> +       movl    %eax, 14*4(%esp)

Could this perhaps be removed entirely?

The return code could instead do:

        ... restore all the normal registers ..

        # Now restore flags that is under the return address and our
fake __KERNEL_CS
        pushl 8(%esp)
        popfl

        # and then return, skipping __KERNEL_CS and %flasg
        ret $8

which is smaller and simpler than (again) playing games with stack entries.

                Linus