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

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Message-ID: <20240729093405.GC37996@noisy.programming.kicks-ass.net>
Date: Mon, 29 Jul 2024 11:34:05 +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>, Jonathan Corbet <corbet@....net>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
	Arnd Bergmann <arnd@...db.de>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,
	Bill Wendling <morbo@...gle.com>,
	Justin Stitt <justinstitt@...gle.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	John Moon <john@...on.dev>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Samuel Holland <samuel.holland@...ive.com>,
	Mike Rapoport <rppt@...nel.org>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Rafael Aquini <aquini@...hat.com>, Petr Pavlu <petr.pavlu@...e.com>,
	Eric DeVolder <eric.devolder@...cle.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Randy Dunlap <rdunlap@...radead.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Breno Leitao <leitao@...ian.org>,
	Wei Yang <richard.weiyang@...il.com>,
	Brian Gerst <brgerst@...il.com>, Juergen Gross <jgross@...e.com>,
	Palmer Dabbelt <palmer@...osinc.com>,
	Alexandre Ghiti <alexghiti@...osinc.com>,
	Kees Cook <kees@...nel.org>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Xiao Wang <xiao.w.wang@...el.com>,
	Jan Kiszka <jan.kiszka@...mens.com>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
	linux-efi@...r.kernel.org, linux-arch@...r.kernel.org,
	llvm@...ts.linux.dev, Krzysztof Pszeniczny <kpszeniczny@...gle.com>
Subject: Re: [PATCH 3/6] Change the symbols order when --ffuntion-sections is
 enabled

On Sun, Jul 28, 2024 at 01:29:56PM -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.
> 
> Co-developed-by: Han Shen <shenhan@...gle.com>
> Signed-off-by: Han Shen <shenhan@...gle.com>
> Signed-off-by: Rong Xu <xur@...gle.com>
> Suggested-by: Sriraman Tallam <tmsriram@...gle.com>
> Suggested-by: Krzysztof Pszeniczny <kpszeniczny@...gle.com>
> ---
>  include/asm-generic/vmlinux.lds.h | 19 ++++++++++++++++---
>  1 file changed, 16 insertions(+), 3 deletions(-)
> 
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 5703526d6ebf..f3de66bda293 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -582,9 +582,21 @@
>   * during second ld run in second ld pass when generating System.map
>   *
>   * 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.
> + * code elimination or function-section is enabled. Match these symbols
> + * first when in these builds.
>   */
> +#if defined(CONFIG_LD_DEAD_CODE_DATA_ELIMINATION) || defined(CONFIG_LTO_CLANG)
> +#define TEXT_TEXT							\
> +		*(.text.asan.* .text.tsan.*)				\
> +		*(.text.unknown .text.unknown.*)			\
> +		*(.text.unlikely .text.unlikely.*)			\
> +		ALIGN_FUNCTION();					\

Why leave the above text sections unaligned?

> +		*(.text.hot .text.hot.*)				\
> +		*(TEXT_MAIN .text.fixup)				\
> +		NOINSTR_TEXT						\
> +		*(.ref.text)						\
> +	MEM_KEEP(init.text*)
> +#else
>  #define TEXT_TEXT							\
>  		ALIGN_FUNCTION();					\
>  		*(.text.hot .text.hot.*)				\
> @@ -594,7 +606,8 @@
>  		NOINSTR_TEXT						\
>  		*(.ref.text)						\
>  		*(.text.asan.* .text.tsan.*)				\
> -	MEM_KEEP(init.text*)						\
> +	MEM_KEEP(init.text*)
> +#endif
>  
>  
>  /* sched.text is aling to function alignment to secure we have same
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
>