Re: [PATCH] getaddrinfo: Get rid of alloca

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 20/06/23 18:40, Joe Simmons-Talbott via Libc-alpha wrote:
> Use a scratch_buffer rather than alloca to avoid potential stack
> overflow.
> ---
>  sysdeps/posix/getaddrinfo.c | 22 ++++++++--------------
>  1 file changed, 8 insertions(+), 14 deletions(-)
> 
> diff --git a/sysdeps/posix/getaddrinfo.c b/sysdeps/posix/getaddrinfo.c
> index 0356b622be..442475d621 100644
> --- a/sysdeps/posix/getaddrinfo.c
> +++ b/sysdeps/posix/getaddrinfo.c
> @@ -2404,22 +2404,17 @@ getaddrinfo (const char *name, const char *service,
>        struct addrinfo *q;
>        struct addrinfo *last = NULL;
>        char *canonname = NULL;
> -      bool malloc_results;
>        size_t alloc_size = nresults * (sizeof (*results) + sizeof (size_t));
> +      struct scratch_buffer buf;
> +      scratch_buffer_init (&buf);
>  
> -      malloc_results
> -	= !__libc_use_alloca (alloc_size);
> -      if (malloc_results)
> +      if (!scratch_buffer_set_array_size (&buf, 1, alloc_size))

I think it would be better to use:

  if (!scratch_buffer_set_array_size (&buf, nresults, 
				      sizeof (*results) + sizeof (size_t)))
    [...]

To use the overflow checks done by scratch_buffer_set_array_size (
sizeof (*results) + sizeof (size_t) is always safe so I think there is no
need to add extra checks).

>  	{
> -	  results = malloc (alloc_size);
> -	  if (results == NULL)
> -	    {
> -	      __free_in6ai (in6ai);
> -	      return EAI_MEMORY;
> -	    }
> +	  __free_in6ai (in6ai);
> +	  return EAI_MEMORY;
>  	}
> -      else
> -	results = alloca (alloc_size);
> +      results = buf.data;
> +
>        order = (size_t *) (results + nresults);
>  
>        /* Now we definitely need the interface information.  */
> @@ -2590,8 +2585,7 @@ getaddrinfo (const char *name, const char *service,
>        /* Fill in the canonical name into the new first entry.  */
>        p->ai_canonname = canonname;
>  
> -      if (malloc_results)
> -	free (results);
> +      scratch_buffer_free (&buf);
>      }
>  
>    __free_in6ai (in6ai);