linux-kernel - Re: [PATCH v2 3/6] Change the symbols order when --ffuntion-sections is enabled

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Message-ID: <20241003154320.GX5594@noisy.programming.kicks-ass.net>
Date: Thu, 3 Oct 2024 17:43:20 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Rong Xu <xur@...gle.com>
Cc: Han Shen <shenhan@...gle.com>, Sriraman Tallam <tmsriram@...gle.com>,
	David Li <davidxl@...gle.com>,
	Krzysztof Pszeniczny <kpszeniczny@...gle.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>, Bill Wendling <morbo@...gle.com>,
	Borislav Petkov <bp@...en8.de>, Breno Leitao <leitao@...ian.org>,
	Brian Gerst <brgerst@...il.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Heiko Carstens <hca@...ux.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Juergen Gross <jgross@...e.com>,
	Justin Stitt <justinstitt@...gle.com>, Kees Cook <kees@...nel.org>,
	linux-arch@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	llvm@...ts.linux.dev, Masahiro Yamada <masahiroy@...nel.org>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Nicolas Schier <nicolas@...sle.eu>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Samuel Holland <samuel.holland@...ive.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wei Yang <richard.weiyang@...il.com>, workflows@...r.kernel.org,
	x86@...nel.org, "Xin Li (Intel)" <xin@...or.com>
Subject: Re: [PATCH v2 3/6] Change the symbols order when --ffuntion-sections
 is enabled

On Wed, Oct 02, 2024 at 04:34:02PM -0700, Rong Xu wrote:
> When the -ffunction-sections compiler option is enabled, each function
> is placed in a separate section named .text.function_name rather than
> putting all functions in a single .text section.
> 
> However, using -function-sections can cause problems with the
> linker script. The comments included in include/asm-generic/vmlinux.lds.h
> note these issues.:
>   “TEXT_MAIN here will match .text.fixup and .text.unlikely if dead
>    code elimination is enabled, so these sections should be converted
>    to use ".." first.”
> 
> It is unclear whether there is a straightforward method for converting
> a suffix to "..". This patch modifies the order of subsections within the
> text output section when the -ffunction-sections flag is enabled.
> Specifically, it repositions sections with certain fixed patterns (for
> example .text.unlikely) before TEXT_MAIN, ensuring that they are grouped
> and matched together.
> 
> Note that the limitation arises because the linker script employs glob
> patterns instead of regular expressions for string matching. While there
> is a method to maintain the current order using complex patterns, this
> significantly complicates the pattern and increases the likelihood of
> errors.

Is there a down-side to using the new order unconditionally?