linux-kernel - Re: [PATCH] parisc: prefer _THIS_IP_ and _RET_IP

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Date:   Fri, 3 Aug 2018 13:33:50 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     dave.anglin@...l.net
Cc:     deller@....de, jejb@...isc-linux.org,
        Nathan Chancellor <natechancellor@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Pravin Shedge <pravin.shedge4linux@...il.com>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        linux-parisc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Alistair Strachan <astrachan@...gle.com>
Subject: Re: [PATCH] parisc: prefer _THIS_IP_ and _RET_IP_ statement expressions

On Fri, Aug 3, 2018 at 12:09 PM John David Anglin <dave.anglin@...l.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@...l.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