gethostid (Linux variant): Switch to struct scratch_buffer [BZ #18023]

  Previously, extend_alloca was used without alloca accounting,
which could have been problematic with large NSS results.

2018-06-26  Florian Weimer  <fweimer@redhat.com>

	[BZ #18023]
	* sysdeps/unix/sysv/linux/gethostid.c (gethostid): Use struct
	scratch_buffer instead of extend_alloca.  Update comments.
  

On 26/06/2018 12:46, Florian Weimer wrote:
> Previously, extend_alloca was used without alloca accounting,
> which could have been problematic with large NSS results.
> 
> 2018-06-26  Florian Weimer  <fweimer@redhat.com>
> 
> 	[BZ #18023]
> 	* sysdeps/unix/sysv/linux/gethostid.c (gethostid): Use struct
> 	scratch_buffer instead of extend_alloca.  Update comments.
> 
> diff --git a/sysdeps/unix/sysv/linux/gethostid.c b/sysdeps/unix/sysv/linux/gethostid.c
> index 73c56e57e5..c8a33fe14d 100644
> --- a/sysdeps/unix/sysv/linux/gethostid.c
> +++ b/sysdeps/unix/sysv/linux/gethostid.c
> @@ -21,6 +21,7 @@
>  #include <unistd.h>
>  #include <netdb.h>
>  #include <not-cancel.h>
> +#include <stdbool.h>
>  
>  #define HOSTIDFILE "/etc/hostid"
>  
> @@ -63,13 +64,12 @@ sethostid (long int id)
>  # include <sys/param.h>
>  # include <resolv/netdb.h>
>  # include <netinet/in.h>
> +# include <scratch_buffer.h>
>  
>  long int
>  gethostid (void)
>  {
>    char hostname[MAXHOSTNAMELEN + 1];
> -  size_t buflen;
> -  char *buffer;
>    struct hostent hostbuf, *hp;
>    int32_t id;
>    struct in_addr in;
> @@ -88,29 +88,40 @@ gethostid (void)
>  	return id;
>      }
>  
> -  /* Getting from the file was not successful.  An intelligent guess for
> -     a unique number of a host is its IP address.  Return this.  */
> +  /* Getting from the file was not successful.  An intelligent guess
> +     for a unique number of a host is its IP address.  To get the IP
> +     address we need to know the host name.  */
>    if (__gethostname (hostname, MAXHOSTNAMELEN) < 0 || hostname[0] == '\0')
>      /* This also fails.  Return and arbitrary value.  */
>      return 0;
>  
> -  buflen = 1024;
> -  buffer = __alloca (buflen);
> -
> -  /* To get the IP address we need to know the host name.  */
> -  while (__gethostbyname_r (hostname, &hostbuf, buffer, buflen, &hp, &herr)
> -	 != 0
> -	 || hp == NULL)
> -    if (herr != NETDB_INTERNAL || errno != ERANGE)
> -      return 0;
> -    else
> -      /* Enlarge buffer.  */
> -      buffer = extend_alloca (buffer, buflen, 2 * buflen);
> +  /* Determine the IP address of the host name.  */
> +  struct scratch_buffer tmpbuf;
> +  scratch_buffer_init (&tmpbuf);
> +  while (true)
> +    {
> +      int ret = __gethostbyname_r (hostname, &hostbuf,
> +				   tmpbuf.data, tmpbuf.length, &hp, &herr);
> +      if (ret == 0)
> +	break;
> +      else
> +	{
> +	  /* Enlarge the buffer on ERANGE.  */
> +	  if (herr == NETDB_INTERNAL && errno == ERANGE)
> +	    {
> +	      if (!scratch_buffer_grow (&tmpbuf))
> +		return 0;
> +	    }
> +	  else
> +	    /* Other errors are a failure.  Return an arbitrary value.  */

Shouldn' it call 'scratch_buffer_free' here for the case the buffer is
grown and a subsequent __gethostbyname_r results something different
than ERANGE (assuming it is possible)?

> +	    return 0;
> +	}
> +    }
>  
>    in.s_addr = 0;
>    memcpy (&in, hp->h_addr,
>  	  (int) sizeof (in) < hp->h_length ? (int) sizeof (in) : hp->h_length);
> -
> +  scratch_buffer_free (&tmpbuf);
>    /* For the return value to be not exactly the IP address we do some
>       bit fiddling.  */
>    return (int32_t) (in.s_addr << 16 | in.s_addr >> 16);
>