[PATCH v5] ifaddrs: Get rid of alloca

[Joe Simmons-Talbott]

On Wed, Jun 21, 2023 at 04:14:11PM -0300, Adhemerval Zanella Netto wrote:
> 
> 
> 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?

I've sent a v6 with this fix.  Thanks for catching it.

Thanks,
Joe