Re: [PATCH v3 44/64] x86/jump_label: Define ELF section entry size for jump labels

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 26, 2025 at 04:55:31PM -0700, Josh Poimboeuf wrote:
> In preparation for the objtool klp diff subcommand, define the entry
> size for the __jump_table section in its ELF header.  This will allow
> tooling to extract individual entries.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
> ---
>  arch/x86/include/asm/jump_label.h | 17 +++++++++++------
>  kernel/bounds.c                   |  4 ++++
>  2 files changed, 15 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index cd21554b3675..7a6b0e5d85c1 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -12,12 +12,17 @@
>  #include <linux/stringify.h>
>  #include <linux/types.h>
>  
> -#define JUMP_TABLE_ENTRY(key, label)			\
> -	".pushsection __jump_table,  \"a\"\n\t"		\
> -	_ASM_ALIGN "\n\t"				\
> -	".long 1b - . \n\t"				\
> -	".long " label " - . \n\t"			\
> -	_ASM_PTR " " key " - . \n\t"			\
> +#ifndef COMPILE_OFFSETS
> +#include <generated/bounds.h>
> +#endif
> +
> +#define JUMP_TABLE_ENTRY(key, label)				\
> +	".pushsection __jump_table,  \"aM\", @progbits, "	\
> +	__stringify(JUMP_ENTRY_SIZE) "\n\t"			\

Argh, can you please not do this line-break. Yes it'll be long, but this
is most confusing.

> +	_ASM_ALIGN "\n\t"					\
> +	".long 1b - . \n\t"					\
> +	".long " label " - . \n\t"				\
> +	_ASM_PTR " " key " - . \n\t"				\
>  	".popsection \n\t"
>  
>  /* This macro is also expanded on the Rust side. */
> diff --git a/kernel/bounds.c b/kernel/bounds.c
> index 02b619eb6106..e4c7ded3dc48 100644
> --- a/kernel/bounds.c
> +++ b/kernel/bounds.c
> @@ -13,6 +13,7 @@
>  #include <linux/kbuild.h>
>  #include <linux/log2.h>
>  #include <linux/spinlock_types.h>
> +#include <linux/jump_label.h>
>  
>  int main(void)
>  {
> @@ -29,6 +30,9 @@ int main(void)
>  #else
>  	DEFINE(LRU_GEN_WIDTH, 0);
>  	DEFINE(__LRU_REFS_WIDTH, 0);
> +#endif
> +#if defined(CONFIG_HAVE_ARCH_JUMP_LABEL_RELATIVE) && defined(CONFIG_JUMP_LABEL)

How is HAVE_ARCH_JUMP_LABEL_RELATIVE relevant here?

> +	DEFINE(JUMP_ENTRY_SIZE, sizeof(struct jump_entry));
>  #endif
>  	/* End of constants */
>  
> -- 
> 2.49.0
>