Re: [musl] fix various warnings/theoretical UB

[Rich Felker <dalias@libc.org>]

On Mon, Jul 03, 2023 at 01:55:57PM -0400, Alex Xu (Hello71) wrote:
> See attached patches.

> From 978f2cded65ce73450277d3fde48f038b339d5f9 Mon Sep 17 00:00:00 2001
> From: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
> Date: Sun, 2 Jul 2023 20:28:23 -0400
> Subject: [PATCH 1/4] volatile static -> static volatile
> 
> C11 6.11.5p1:
> 
> > The placement of a storage-class specifier other than at the
> > beginning of the declaration specifiers in a declaration is an
> > obsolescent feature.
> 
> gcc also warns about this.
> ---
>  src/time/timer_create.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/src/time/timer_create.c b/src/time/timer_create.c
> index cd32c945..9216b3ab 100644
> --- a/src/time/timer_create.c
> +++ b/src/time/timer_create.c
> @@ -61,7 +61,7 @@ static void *start(void *arg)
>  
>  int timer_create(clockid_t clk, struct sigevent *restrict evp, timer_t *restrict res)
>  {
> -	volatile static int init = 0;
> +	static volatile int init = 0;
>  	pthread_t td;
>  	pthread_attr_t attr;
>  	int r;
> -- 
> 2.41.0

No objection to this change. It's contrary to usual style. I would say
let's convert to pthread_once, but this code is slated for removal
anyway once signals are no longer used for SIGEV_THREAD timers.

> From b98f243e7921ddff6978ee9b0ce9f08efaa17951 Mon Sep 17 00:00:00 2001
> From: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
> Date: Sun, 2 Jul 2023 20:29:41 -0400
> Subject: [PATCH 2/4] __year_to_secs: fix dangling pointer
> 
> C11 6.5.2.5p5:
> 
> > If the compound literal occurs outside the body of a function, the
> > object has static storage duration; otherwise, it has automatic
> > storage duration associated with the enclosing block.
> 
> gcc also warns about this.
> ---
>  src/time/__year_to_secs.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/src/time/__year_to_secs.c b/src/time/__year_to_secs.c
> index 2824ec6d..d215880a 100644
> --- a/src/time/__year_to_secs.c
> +++ b/src/time/__year_to_secs.c
> @@ -10,9 +10,9 @@ long long __year_to_secs(long long year, int *is_leap)
>  		return 31536000*(y-70) + 86400*leaps;
>  	}
>  
> -	int cycles, centuries, leaps, rem;
> +	int cycles, centuries, leaps, rem, tmp;
>  
> -	if (!is_leap) is_leap = &(int){0};
> +	if (!is_leap) is_leap = &tmp;
>  	cycles = (year-100) / 400;
>  	rem = (year-100) % 400;
>  	if (rem < 0) {
> -- 
> 2.41.0

Seems like a bogus warning. The enclosing block is the whole function,
the same as the lifetime of the pointer. This might merit
investigation on whether GCC is doing something wrong though..

> From a30c4ab397af040d10d978d97dd4a6835d4b99a8 Mon Sep 17 00:00:00 2001
> From: "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
> Date: Sun, 2 Jul 2023 20:54:45 -0400
> Subject: [PATCH 3/4] fix mismatched VLA parameter types
> 
> gcc warns about this, and it's probably technically UB
> ---
>  src/internal/procfdname.c | 2 +-
>  src/prng/seed48.c         | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/src/internal/procfdname.c b/src/internal/procfdname.c
> index fd7306ab..bfa3e7e5 100644
> --- a/src/internal/procfdname.c
> +++ b/src/internal/procfdname.c
> @@ -1,6 +1,6 @@
>  #include "syscall.h"
>  
> -void __procfdname(char *buf, unsigned fd)
> +void __procfdname(char buf[static 15+3*sizeof(int)], unsigned fd)
>  {
>  	unsigned i, j;
>  	for (i=0; (buf[i] = "/proc/self/fd/"[i]); i++);

This was raised/proposed before and is probably an okay change, but
I'd like to understand what the reason "it's probably technically UB"
is.

> diff --git a/src/prng/seed48.c b/src/prng/seed48.c
> index bce7b339..7b789086 100644
> --- a/src/prng/seed48.c
> +++ b/src/prng/seed48.c
> @@ -2,7 +2,7 @@
>  #include <string.h>
>  #include "rand48.h"
>  
> -unsigned short *seed48(unsigned short *s)
> +unsigned short *seed48(unsigned short s[3])
>  {
>  	static unsigned short p[3];
>  	memcpy(p, __seed48, sizeof p);
> -- 

This one is almost surely not UB because there's no static and the 3
is ignored. The question is just whether the static produces a
difference in the declaration type that makes them clash.

Rich