Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
References: <20180801182258.17834-1-ndesaulniers@google.com>
 <78c667f9-5c8b-3835-83eb-4b59e27e4f7e@bell.net> <CAKwvOdmdrA+UvAWyqe8sNJt5nxHXHsSwf5Yxih9HYgVL+kovxQ@mail.gmail.com>
 <fe7b57d8-b586-a390-31d2-2274f69db3c0@bell.net> <CAKwvOd=CLBFZRQ4HQb_+VCydDEvsbC2w2gD4hTwvp_jm1zqN9Q@mail.gmail.com>
 <54b2139b-53d2-01c5-f240-93a692cdbc59@bell.net> <CAKwvOdktvrZL8T_5K2VtQtgt_mscS7kzVXYVy0O7CSuGX53Y5A@mail.gmail.com>
 <536ef1b1-cfa2-f66d-4261-fb8405964632@bell.net> <CAKwvOdmwj76aESZNKhzfPmhfzGKvrfDut7oD5KAwA+VR-JukSg@mail.gmail.com>
 <80743c1c-2a8c-7bce-2068-24c63790eba7@bell.net> <CAKwvOdmDwO1U1sk=3-6zXFrVw6by1e_9VBx1WWMCx_rMqYSw-A@mail.gmail.com>
 <81a7e33a-0403-b6d4-fcba-c2168a608ba6@bell.net>
In-Reply-To: <81a7e33a-0403-b6d4-fcba-c2168a608ba6@bell.net>
From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 3 Aug 2018 13:33:50 -0700
Message-ID: <CAKwvOdmXi_EY5=sOO9tRqeoFrHjLsF9L988cGC2yjnFYBxac4w@mail.gmail.com>
Subject: Re: [PATCH] parisc: prefer _THIS_IP_ and _RET_IP_ statement expressions
To:     dave.anglin@bell.net
Cc:     deller@gmx.de, jejb@parisc-linux.org,
        Nathan Chancellor <natechancellor@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Pravin Shedge <pravin.shedge4linux@gmail.com>,
        Kate Stewart <kstewart@linuxfoundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        linux-parisc@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Alistair Strachan <astrachan@google.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Aug 3, 2018 at 12:09 PM John David Anglin <dave.anglin@bell.net> wrote:
>
> On 2018-08-03 2:11 PM, Nick Desaulniers wrote:
> > But the kernel uses the generic_THIS_IP_  *everywhere*, not parisc's
> > custom current_text_addr().  So if this did actually break unwinding,
> > you should have noticed by now.
> The unwind problem was noticed.

So parisc is currently broken (doesn't unwind) due to the pervasive
use of _THIS_IP_ (generic C) throughout the kernel?

If no, that implies this patch (generic C) causes no unwinding problems.
If yes, that implies that the diff I posted later in this thread
(inline assembly) is preferable, and that parisc has bigger problems
(and probably needs to do rewrite the unwinding code to handle these
extra labels everywhere).

> Patches were recently applied to gcc and binutils to try and fix it.
> The gcc patch moved
> branch tables to rodata so that the label at the head of the table
> wasn't in text.
>
> https://gcc.gnu.org/ml/gcc-patches/2018-07/msg01804.html
> https://sourceware.org/ml/binutils/2018-07/msg00474.html
>
> When I saw your suggested change, I realized there was another source of
> text labels
> that need linker relocations.

Thank you for the links.

On Fri, Aug 3, 2018 at 10:57 AM John David Anglin <dave.anglin@bell.net> wrote:
> The label breaks the unwind data, not the unwind code.  So, localizing
> the use of
> current_text_addr() to the parisc unwind code doesn't help.

Have you confirmed that applying my patch breaks *the ability to
unwind correctly*? It looks like return_address() is used in
ftrace_return_address(), so I assume you can boot a kernel with my
patch applied, and CONFIG_FTRACE=y, then run:

$ sudo trace-cmd record -p function date
$ trace-cmd report | grep date- | less

and see if the stacks aren't unwound or look messed up.
-- 
Thanks,
~Nick Desaulniers