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

From: hpa@zytor.com
To: Josh Poimboeuf <jpoimboe@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Jiri Slaby <jslaby@suse.cz>,
	Andrew Morton <akpm@linux-foundation.org>,
	live-patching@vger.kernel.org,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	the arch/x86 maintainers <x86@kernel.org>,
	Andy Lutomirski <luto@kernel.org>, Jiri Kosina <jikos@kernel.org>
Subject: Re: [PATCH 7/7] DWARF: add the config option
Date: Mon, 08 May 2017 11:57:59 -0700	[thread overview]
Message-ID: <5EAB44CD-482B-485F-A0E0-0AE7937FFF2C@zytor.com> (raw)
In-Reply-To: <20170508144049.6x67526a677g2miw@treble>

On May 8, 2017 7:40:49 AM PDT, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>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.

At some point you are just reinventing k(g)db...
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