[PATCH] check_native: Get rid of alloca

[Joe Simmons-Talbott]

On Wed, May 31, 2023 at 09:01:22AM -0400, Joe Simmons-Talbott wrote:
> Use malloc rather than alloca to avoid potential stack overflow.

This is failing on 32bit builds[1] and I'm not sure how to fix it or why
it's failing here but didn't with the same construct in my ifaddrs patch
[2]

Thanks,
Joe

[1] https://www.delorie.com/trybots/32bit/20795/make.tail.txt
[2] https://sourceware.org/pipermail/libc-alpha/2023-May/148681.html

> ---
>  sysdeps/unix/sysv/linux/check_native.c | 27 +++++++++-----------------
>  1 file changed, 9 insertions(+), 18 deletions(-)
> 
> diff --git a/sysdeps/unix/sysv/linux/check_native.c b/sysdeps/unix/sysv/linux/check_native.c
> index 34876ca624..45b328f240 100644
> --- a/sysdeps/unix/sysv/linux/check_native.c
> +++ b/sysdeps/unix/sysv/linux/check_native.c
> @@ -37,6 +37,12 @@
>  
>  #include "netlinkaccess.h"
>  
> +static void
> +ifree (char **ptr)
> +{
> +  free (*ptr);
> +}
> +
>  void
>  __check_native (uint32_t a1_index, int *a1_native,
>  		uint32_t a2_index, int *a2_native)
> @@ -48,7 +54,6 @@ __check_native (uint32_t a1_index, int *a1_native,
>    nladdr.nl_family = AF_NETLINK;
>  
>    socklen_t addr_len = sizeof (nladdr);
> -  bool use_malloc = false;
>  
>    if (fd < 0)
>      return;
> @@ -82,24 +87,13 @@ __check_native (uint32_t a1_index, int *a1_native,
>    nladdr.nl_family = AF_NETLINK;
>  
>  #ifdef PAGE_SIZE
> -  /* Help the compiler optimize out the malloc call if PAGE_SIZE
> -     is constant and smaller or equal to PTHREAD_STACK_MIN/4.  */
>    const size_t buf_size = PAGE_SIZE;
>  #else
>    const size_t buf_size = __getpagesize ();
>  #endif
> -  char *buf;
> -
> -  if (__libc_use_alloca (buf_size))
> -    buf = alloca (buf_size);
> -  else
> -    {
> -      buf = malloc (buf_size);
> -      if (buf != NULL)
> -	use_malloc = true;
> -      else
> -	goto out;
> -    }
> +  char *buf __attribute__ ((__cleanup__ (ifree))) = malloc (buf_size);
> +  if (buf == NULL)
> +    goto out;
>  
>    struct iovec iov = { buf, buf_size };
>  
> @@ -170,7 +164,4 @@ __check_native (uint32_t a1_index, int *a1_native,
>  
>  out:
>    __close_nocancel_nostatus (fd);
> -
> -  if (use_malloc)
> -    free (buf);
>  }
> -- 
> 2.39.2
>