Re: [PATCH -mm] elf: init pt_regs pointer later

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 19 Apr 2019 23:03:43 +0300 Alexey Dobriyan <adobriyan@gmail.com> wrote:

> Get "current_pt_regs" pointer right before usage.
> 
> Space savings on x86_64:
> 
> 	add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-180 (-180)
> 	Function                           old     new   delta
> 	load_elf_binary                   5806    5626    -180 !!!

-256 bytes with my setup.

> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -704,12 +704,12 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  	unsigned long start_code, end_code, start_data, end_data;
>  	unsigned long reloc_func_desc __maybe_unused = 0;
>  	int executable_stack = EXSTACK_DEFAULT;
> -	struct pt_regs *regs = current_pt_regs();
>  	struct {
>  		struct elfhdr elf_ex;
>  		struct elfhdr interp_elf_ex;
>  	} *loc;
>  	struct arch_elf_state arch_state = INIT_ARCH_ELF_STATE;
> +	struct pt_regs *regs;
>  
>  	loc = kmalloc(sizeof(*loc), GFP_KERNEL);
>  	if (!loc) {
> @@ -1159,6 +1159,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  				MAP_FIXED | MAP_PRIVATE, 0);
>  	}
>  
> +	regs = current_pt_regs();
>  #ifdef ELF_PLAT_INIT
>  	/*
>  	 * The ABI may specify that certain registers be set up in special

Why the heck does this make such a difference?