linux-kernel - Re: [PATCH 7/7] DWARF: add the config option

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Message-ID: <20170508144049.6x67526a677g2miw@treble>
Date:   Mon, 8 May 2017 09:40:49 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        live-patching@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jiri Kosina <jikos@...nel.org>
Subject: Re: [PATCH 7/7] DWARF: add the config option

On Sun, May 07, 2017 at 10:35:28PM -0700, Andy Lutomirski wrote:
> I think that, if the code were sufficiently robust, it would be handy
> if the unwinder displayed function arguments.  DWARF can do that to a
> limited extent.

Honestly I get the feeling that displaying function arguments wouldn't
be realistic (DWARF or no DWARF).  On x86-64, arguments are passed in
registers, so tracking down their values is a lot more involved than
just looking at the stack.

The DWARF CFI only shows you the callee-saved registers.  To figure out
the other registers you'd have to dive into the other DWARF sections and
examine previous stack frames for clues.  I think it's not a
deterministic process, based on how often I see gdb complain with
'<value optimized out>'.  I'd bet it's a lot harder than a basic stack
dump.

Also, most kernel functions rely on pointer arguments, which are pretty
much useless without dumping the contents of the structs they point to.
But then doing that properly would be a whole new level of difficulty.

-- 
Josh