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

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Message-ID: <20170519213556.pv5kxocfprfkloay@treble>
Date:   Fri, 19 May 2017 16:35:56 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     linux-kernel@...r.kernel.org, Jiri Slaby <jslaby@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        live-patching@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/7] DWARF: add the config option

On Fri, May 19, 2017 at 04:29:13PM -0500, Josh Poimboeuf wrote:
> > How are you handling control flow?
> 
> Control flow of what?
> 
> > > Here's the struct in its current state:
> > > 
> > > 	#define UNDWARF_REG_UNDEFINED		0
> > > 	#define UNDWARF_REG_CFA			1
> > > 	#define UNDWARF_REG_SP			2
> > > 	#define UNDWARF_REG_FP			3
> > > 	#define UNDWARF_REG_SP_INDIRECT		4
> > > 	#define UNDWARF_REG_FP_INDIRECT		5
> > > 	#define UNDWARF_REG_R10			6
> > > 	#define UNDWARF_REG_DI			7
> > > 	#define UNDWARF_REG_DX			8
> > > 
> > 
> > Why only those registers?  Also, if you have the option I would really
> > suggest using the actual x86 register numbers (ax, ex, dx, bx, sp, bp,
> > si, di, r8-r15 in that order.)
> 
> Those are the only registers which are ever needed as the base for
> finding the previous stack frame.  99% of the time it's sp or bp, the
> other registers are needed for aligned stacks and entry code.
> 
> Using the actual register numbers isn't an option because I don't need
> them all and they need to fit in a small number of bits.
> 
> This construct might be useful for other arches, which is why I called
> it "FP" instead of "BP".  But then I ruined that with the last 3 :-)

BTW, here's the link to the unwinder code if you're interested:

  https://github.com/jpoimboe/linux/blob/undwarf/arch/x86/kernel/unwind_undwarf.c

-- 
Josh