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

[Peter Zijlstra <peterz@infradead.org>]

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@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>
> Signed-off-by: Rong Xu <xur@google.com>
> Suggested-by: Sriraman Tallam <tmsriram@google.com>
> Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.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
>