Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}

From: Segher Boessenkool <segher@kernel.crashing.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Matz <matz@suse.de>, Josh Poimboeuf <jpoimboe@kernel.org>,
	Borislav Petkov <bp@alien8.de>,
	linux-toolchains@vger.kernel.org,
	Indu Bhagat <indu.bhagat@oracle.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org,
	"Jose E. Marchesi" <jemarch@gnu.org>,
	Miroslav Benes <mbenes@suse.cz>,
	Mark Rutland <mark.rutland@arm.com>,
	Will Deacon <will@kernel.org>,
	x86@kernel.org, linux-arm-kernel@lists.infradead.org,
	live-patching@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
	Ard Biesheuvel <ardb@kernel.org>,
	Chen Zhongjin <chenzhongjin@huawei.com>,
	Sathvika Vasireddy <sv@linux.ibm.com>,
	Christophe Leroy <christophe.leroy@csgroup.eu>,
	Mark Brown <broonie@kernel.org>
Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}
Date: Wed, 14 Sep 2022 12:34:25 -0500	[thread overview]
Message-ID: <20220914173425.GZ25951@gate.crashing.org> (raw)
In-Reply-To: <YyHrX/fTMwfv24W7@hirez.programming.kicks-ass.net>

On Wed, Sep 14, 2022 at 04:55:27PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 14, 2022 at 02:28:26PM +0000, Michael Matz wrote:
> > Don't mix DWARF debug info with DWARF-based unwinding info, the latter 
> > doesn't imply the former.  Out of interest: how does ORC get around the 
> > need for CFI annotations (or equivalents to restore registers) and what 
> 
> Objtool 'interprets' the stackops. So it follows the call-graph and is
> an interpreter for all instructions that modify the stack. Doing that it
> konws what the stackframe is at 'most' places.

To get correct backtraces on e.g. PowerPC you need to emulate many of
the integer insns.  That is why GCC enables -fasynchronous-unwind-tables
by default for us.

The same is true for s390, aarch64, and x86 (unless 32-bit w/ frame
pointer).

The problem is that you do not know how to access anything on the stack,
whether in the current frame or in a previous frame, from a random point
in the program.  GDB has many heuristics for this, and it still does not
get it right in all cases.

> > makes it fast?  I want faster unwinding for DWARF as well, when there's 
> > feature parity :-)  Maybe something can be learned for integration into 
> > dwarf-unwind.
> 
> I think we have some details here:
> 
>  https://www.kernel.org/doc/html/latest/x86/orc-unwinder.html

It is faster because it does a whole lot less.  Is that still enough?
It's not clear (to me) what exact information it wants to provide :-(

Segher