linux-kernel - annotating jump tables (Re: [PATCH v2 5/5] LoongArch: Enable jump table with GCC for objtool)

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Message-ID: <20241113211119.lfwlxv2bjyqfqeh2@jpoimboe>
Date: Wed, 13 Nov 2024 13:11:19 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Tiezhu Yang <yangtiezhu@...ngson.cn>
Cc: Xi Ruoyao <xry111@...111.site>, Peter Zijlstra <peterz@...radead.org>,
	Huacai Chen <chenhuacai@...nel.org>, loongarch@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-toolchains@...r.kernel.org,
	Ard Biesheuvel <ardb@...nel.org>, Jan Beulich <jbeulich@...e.com>,
	"Jose E. Marchesi" <jemarch@....org>, Kees Cook <kees@...nel.org>
Subject: annotating jump tables (Re: [PATCH v2 5/5] LoongArch: Enable jump
 table with GCC for objtool)

On Tue, Nov 12, 2024 at 08:26:56PM +0800, Tiezhu Yang wrote:
> On 11/12/2024 11:15 AM, Xi Ruoyao wrote:
> > On Wed, 2024-11-06 at 13:03 +0800, Tiezhu Yang wrote:
> > > On 11/05/2024 10:15 PM, Peter Zijlstra wrote:
> > > > On Tue, Nov 05, 2024 at 08:39:06PM +0800, Tiezhu Yang wrote:
> > > > > For now, it is time to remove the compiler option -fno-jump-tables
> > > > > to enable jump table for objtool if the compiler is GCC and it has
> > > > > the compiler option -mannotate-tablejump, otherwise still keep the
> > > > > compiler option -fno-jump-tables to maintain compatibility with the
> > > > > older compilers.
> 
> ...
> 
> > > ifdef CONFIG_CC_HAS_ANNOTATE_TABLEJUMP
> > > KBUILD_CFLAGS                   += $(call cc-option,-mannotate-tablejump)
> > > else
> > > KBUILD_CFLAGS                   += -fno-jump-tables
> > > endif
> > 
> > Has -mannotate-tablejump been added to Clang?
> 
> Yes.
> 
> > IMO it's better to add it
> > to Clang first, and add Clang & GCC support at once into objtool.
> 
> Looks reasonable, the fact is that there are some corner issues
> compiled with Clang due to different compiler behaviors, most of
> the issues have been addressed and I need to do more test, I will
> send v3 with about 10 patches after the coming merge window.

Hm, I didn't know -mannotate-tablejump existed.  We really need
something which supports all arches, not just loongarch.

Others were looking at adding something similar (adding them to Cc).

-- 
Josh