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

From: Michael Matz <matz@suse.de>
To: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Segher Boessenkool <segher@kernel.crashing.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	linuxppc-dev@lists.ozlabs.org,
	Chen Zhongjin <chenzhongjin@huawei.com>,
	x86@kernel.org, Nick Desaulniers <ndesaulniers@google.com>,
	linux-kernel@vger.kernel.org, Mark Brown <broonie@kernel.org>,
	Sathvika Vasireddy <sv@linux.ibm.com>,
	linux-toolchains@vger.kernel.org,
	Indu Bhagat <indu.bhagat@oracle.com>,
	live-patching@vger.kernel.org, Miroslav Benes <mbenes@suse.cz>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org,
	"Jose E. Marchesi" <jemarch@gnu.org>
Subject: Re: [RFC] Objtool toolchain proposal: -fannotate-{jump-table,noreturn}
Date: Wed, 14 Sep 2022 12:08:37 +0000 (UTC)	[thread overview]
Message-ID: <alpine.LSU.2.20.2209141201400.8265@wotan.suse.de> (raw)
In-Reply-To: <20220914102100.thl5ad35plvazark@treble>

Hello,

On Wed, 14 Sep 2022, Josh Poimboeuf wrote:

> > >    This information is needed because the
> > >    code after the call to such a function is optimized out as
> > >    unreachable and objtool has no way of knowing that.
> > 
> > Since June we (GCC) have -funreachable-traps.  This creates a trap insn
> > wherever control flow would otherwise go into limbo.
> 
> Ah, that's interesting, though I'm not sure if we'd be able to
> distinguish between "call doesn't return" traps and other traps or
> reasons for UD2.

There are two reasons (which will turn out to be the same) for a trap (say 
'UD2' on x86-64) directly after a call insn:
1) "the call shall not have returned"
2) something else jumps to that trap because it was __builtin_unreachable 
   (or equivalent), and the compiler happened to put that ud2 directly 
   after the call.  It could have done that only when the call itself was 
   noreturn:
     cmp $foo, %rax
     jne do_trap
     call noret
    do_trap:
     ud2

So, it's all the same.  If there's an ud2 (or whatever the trap maker is) 
after a call then it was because it's noreturn.

(But, of course this costs (little) code size, unlike the non-alloc 
checker sections)

Ciao,
Michael.