Re: [PATCH v4 3/6] Change the symbols order when --ffuntion-sections is enabled

[Masahiro Yamada <masahiroy@kernel.org>]

On Tue, Oct 15, 2024 at 6:33 AM Rong Xu <xur@google.com> 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 "..".



Why not for ".text.fixup"?

$ git grep --name-only '\.text\.fixup' | xargs sed -i
's/\.text\.fixup/.text..fixup/g'



I do not know how to rename other sections that are generated by compilers.




> 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 | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index eeadbaeccf88..5df589c60401 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -554,9 +554,21 @@
>   * 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                                                      \


Why did you do this conditionally?

You are making this even more unmaintainable.





> +               ALIGN_FUNCTION();                                       \
> +               *(.text.asan.* .text.tsan.*)                            \
> +               *(.text.unknown .text.unknown.*)                        \
> +               *(.text.unlikely .text.unlikely.*)                      \
> +               . = ALIGN(PAGE_SIZE);                                   \
> +               *(.text.hot .text.hot.*)                                \
> +               *(TEXT_MAIN .text.fixup)                                \
> +               NOINSTR_TEXT                                            \
> +               *(.ref.text)
> +#else
>  #define TEXT_TEXT                                                      \
>                 ALIGN_FUNCTION();                                       \
>                 *(.text.hot .text.hot.*)                                \
> @@ -566,6 +578,7 @@
>                 NOINSTR_TEXT                                            \
>                 *(.ref.text)                                            \
>                 *(.text.asan.* .text.tsan.*)
> +#endif
>
>
>  /* sched.text is aling to function alignment to secure we have same
> --
> 2.47.0.rc1.288.g06298d1525-goog
>
>


--
Best Regards
Masahiro Yamada