linux-kernel - Re: [patch V4 part 3 13/29] x86/traps: Prepare for using DEFINE

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Date:   Wed, 13 May 2020 21:37:10 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [patch V4 part 3 13/29] x86/traps: Prepare for using DEFINE_IDTENTRY

On Tue, May 5, 2020 at 7:15 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> From: Thomas Gleixner <tglx@...utronix.de>
>
> Prepare for using IDTENTRY to define the C exception/trap entry points. It
> would be possible to glue this into the existing macro maze, but it's
> simpler and better to read at the end to just make them distinct. Provide
> a trivial inline helper to read the trap address.
>
> The existing macros will be removed once all instances are converted.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Reviewed-by: Alexandre Chartre <alexandre.chartre@...cle.com>
>
> ---
>  arch/x86/kernel/traps.c |    5 +++++
>  1 file changed, 5 insertions(+)
>
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -205,6 +205,11 @@ static void do_error_trap(struct pt_regs
>         }
>  }
>
> +static __always_inline void __user *error_get_trap_addr(struct pt_regs *regs)
> +{
> +       return (void __user *)uprobe_get_trap_addr(regs);
> +}

My mind boggles.  WTF is this?

Perhaps this should have a comment like:

/*
 * Returns the address from which a user trap originated.  This would
be the same as regs->ip,
 * except for frhgnieawfn nvlrkvklsrvs and mfkealwf, and this lets the
thingummy pass a
 * modified value to the signal frame, but only for #DE and #UD,
because #*!&@&#@.
 */

except with the blanks filled in.

After reading a bit of uprobe code, I assume this is a fixup for when
we're running code that got moved out of line because it got replaced
by a breakpoint.  I'm still mystified by why it only seems to apply to
#DE and #UD.