[PATCH v5] ifaddrs: Get rid of alloca

[Adhemerval Zanella Netto]

On 01/06/23 13:43, Joe Simmons-Talbott via Libc-alpha wrote:
> Use scratch_buffer and malloc rather than alloca to avoid potential stack
> overflows.
> ---
> Changes to v4:
>   * Don't use a cleanup handler.
> 
>  sysdeps/unix/sysv/linux/ifaddrs.c | 46 ++++++++++++++-----------------
>  1 file changed, 20 insertions(+), 26 deletions(-)
> 
> diff --git a/sysdeps/unix/sysv/linux/ifaddrs.c b/sysdeps/unix/sysv/linux/ifaddrs.c
> index 184ee224cb..a10e8d9292 100644
> --- a/sysdeps/unix/sysv/linux/ifaddrs.c
> +++ b/sysdeps/unix/sysv/linux/ifaddrs.c
> @@ -16,13 +16,13 @@
>     License along with the GNU C Library; if not, see
>     <https://www.gnu.org/licenses/>.  */
>  
> -#include <alloca.h>
>  #include <assert.h>
>  #include <errno.h>
>  #include <ifaddrs.h>
>  #include <net/if.h>
>  #include <netinet/in.h>
>  #include <netpacket/packet.h>
> +#include <scratch_buffer.h>
>  #include <stdbool.h>
>  #include <stdint.h>
>  #include <stdlib.h>
> @@ -131,26 +131,14 @@ __netlink_request (struct netlink_handle *h, int type)
>    ssize_t read_len;
>    bool done = false;
>  
> -#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
> -  bool use_malloc = false;
> -  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_fail;
> -    }
> +  /* Netlink requires that user buffer needs to be either 8kb or page size
> +     (whichever is bigger), however this has been changed over time and now
> +     8Kb is sufficient (check NLMSG_DEFAULT_SIZE on Linux
> +     linux/include/linux/netlink.h).  */
> +  const size_t buf_size = 8192;
> +  char *buf = malloc (buf_size);
> +  if (buf == NULL)
> +    goto out_fail;
>  
>    struct iovec iov = { buf, buf_size };
>  
> @@ -229,13 +217,11 @@ __netlink_request (struct netlink_handle *h, int type)
>        h->end_ptr = nlm_next;
>      }
>  
> -  if (use_malloc)
> -    free (buf);
> +  free(buf);
>    return 0;
>  
>  out_fail:
> -  if (use_malloc)
> -    free (buf);
> +  free(buf);
>    return -1;
>  }
>  
> @@ -324,6 +310,8 @@ getifaddrs_internal (struct ifaddrs **ifap)
>    char *ifa_data_ptr;	/* Pointer to the unused part of memory for
>  				ifa_data.  */
>    int result = 0;
> +  struct scratch_buffer buf;
> +  scratch_buffer_init (&buf);
>  
>    *ifap = NULL;
>  
> @@ -425,7 +413,12 @@ getifaddrs_internal (struct ifaddrs **ifap)
>      }
>  
>    /* Table for mapping kernel index to entry in our list.  */
> -  map_newlink_data = alloca (newlink * sizeof (int));
> +  if (!scratch_buffer_set_array_size (&buf, 1, newlink * sizeof (int)))

I just realized that this defeats the overflow checks done by 
__libc_scratch_buffer_set_array_size.  The correct usage is as:

  if (!scratch_buffer_set_array_size (&buf, newlink, sizeof (int)))
     [...]

Could you please send a new version so Siddhesh STATS script do not
scream at us?

> +    {
> +      result = -1;
> +      goto exit_free;
> +    }
> +  map_newlink_data = buf.data;
>    memset (map_newlink_data, '\xff', newlink * sizeof (int));
>  
>    ifa_data_ptr = (char *) &ifas[newlink + newaddr];
> @@ -820,6 +813,7 @@ getifaddrs_internal (struct ifaddrs **ifap)
>   exit_free:
>    __netlink_free_handle (&nh);
>    __netlink_close (&nh);
> +  scratch_buffer_free (&buf);
>  
>    return result;
>  }