[PATCH] __libc_start_main: slightly simplify stage2 pointer setup

[PATCH] __libc_start_main: slightly simplify stage2 pointer setup

[Alexander Monakov]

Use "+r" in the asm instead of implementing a non-transparent copy by
applying "0" constraint to the source value. Introduce a typedef for
the function type to avoid spelling it out twice.
---

I didn't get credited in the asm bugfix, but I still want to leave my mark ;)

Thanks.
Alexander

 src/env/__libc_start_main.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c
index b4965d7f..7c95f822 100644
--- a/src/env/__libc_start_main.c
+++ b/src/env/__libc_start_main.c
@@ -66,7 +66,8 @@ static void libc_start_init(void)
 
 weak_alias(libc_start_init, __libc_start_init);
 
-static int libc_start_main_stage2(int (*)(int,char **,char **), int, char **);
+typedef int lsm2_fn(int (*)(int,char **,char **), int, char **);
+static lsm2_fn libc_start_main_stage2;
 
 int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
 {
@@ -79,8 +80,8 @@ int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
 
 	/* Barrier against hoisting application code or anything using ssp
 	 * or thread pointer prior to its initialization above. */
-	int (*stage2)(int (*)(int,char **,char **), int, char **);
-	__asm__ ( "" : "=r"(stage2) : "0"(libc_start_main_stage2) : "memory" );
+	lsm2_fn *stage2 = libc_start_main_stage2;
+	__asm__ ( "" : "+r"(stage2) : : "memory" );
 	return stage2(main, argc, argv);
 }
 
-- 
2.11.0

Re: [PATCH] __libc_start_main: slightly simplify stage2 pointer setup

[Rich Felker]

On Sun, Oct 21, 2018 at 12:27:44AM +0300, Alexander Monakov wrote:
> Use "+r" in the asm instead of implementing a non-transparent copy by
> applying "0" constraint to the source value. Introduce a typedef for
> the function type to avoid spelling it out twice.
> ---
> 
> I didn't get credited in the asm bugfix, but I still want to leave my mark ;)
> 
> Thanks.
> Alexander
> 
>  src/env/__libc_start_main.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/src/env/__libc_start_main.c b/src/env/__libc_start_main.c
> index b4965d7f..7c95f822 100644
> --- a/src/env/__libc_start_main.c
> +++ b/src/env/__libc_start_main.c
> @@ -66,7 +66,8 @@ static void libc_start_init(void)
>  
>  weak_alias(libc_start_init, __libc_start_init);
>  
> -static int libc_start_main_stage2(int (*)(int,char **,char **), int, char **);
> +typedef int lsm2_fn(int (*)(int,char **,char **), int, char **);
> +static lsm2_fn libc_start_main_stage2;
>  
>  int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
>  {
> @@ -79,8 +80,8 @@ int __libc_start_main(int (*main)(int,char **,char **), int argc, char **argv)
>  
>  	/* Barrier against hoisting application code or anything using ssp
>  	 * or thread pointer prior to its initialization above. */
> -	int (*stage2)(int (*)(int,char **,char **), int, char **);
> -	__asm__ ( "" : "=r"(stage2) : "0"(libc_start_main_stage2) : "memory" );
> +	lsm2_fn *stage2 = libc_start_main_stage2;
> +	__asm__ ( "" : "+r"(stage2) : : "memory" );
>  	return stage2(main, argc, argv);
>  }
>  
> -- 
> 2.11.0

This looks better, especially the aspect of using the typedef'd
function type for DRY (assignment to an explicitly declared function
pointer with the right argument type would also achieve that, but
would involve repetition). If nobody objects I'll apply this very
soon, once I get through with some unrelated queued changes.

Rich