Re: [PATCH v3] Fix double-checked locking in _res_hconf_reorder_addrs [BZ #19074]

[Torvald Riegel <triegel at redhat dot com>]

On Mon, 2015-10-12 at 17:44 +0200, Florian Weimer wrote:
> diff --git a/resolv/res_hconf.c b/resolv/res_hconf.c
> index 692d948..00b620d 100644
> --- a/resolv/res_hconf.c
> +++ b/resolv/res_hconf.c
> @@ -45,6 +45,7 @@
>  #include "ifreq.h"
>  #include "res_hconf.h"
>  #include <wchar.h>
> +#include <atomic.h>
>  
>  #if IS_IN (libc)
>  # define fgets_unlocked __fgets_unlocked
> @@ -391,9 +392,14 @@ _res_hconf_reorder_addrs (struct hostent *hp)
>  {
>  #if defined SIOCGIFCONF && defined SIOCGIFNETMASK
>    int i, j;
> -  /* Number of interfaces.  */
> +  /* Number of interfaces.  Also serves as a flag for the
> +     double-checked locking idiom.  */
>    static int num_ifs = -1;
> -  /* We need to protect the dynamic buffer handling.  */
> +  /* Local copy of num_ifs, for non-atomic access.  */
> +  int num_ifs_local;
> +  /* We need to protect the dynamic buffer handling.  The lock is only
> +     acquired during initialization.  Afterwards, a positive num_ifs
> +     value indicates initialization.  */

I'd say "completed initialization".

>    __libc_lock_define_initialized (static, lock);
>  
>    /* Only reorder if we're supposed to.  */
> @@ -404,7 +410,10 @@ _res_hconf_reorder_addrs (struct hostent *hp)
>    if (hp->h_addrtype != AF_INET)
>      return;
>  
> -  if (num_ifs <= 0)
> +  /* This load synchronizes with the release MO store in the
> +     initialization block below.  */
> +  num_ifs_local = atomic_load_acquire (&num_ifs);
> +  if (num_ifs_local <= 0)
>      {
>        struct ifreq *ifr, *cur_ifr;
>        int sd, num, i;
> @@ -421,9 +430,18 @@ _res_hconf_reorder_addrs (struct hostent *hp)
>        /* Get lock.  */
>        __libc_lock_lock (lock);
>  
> -      /* Recheck, somebody else might have done the work by now.  */
> -      if (num_ifs <= 0)
> +      /* Recheck, somebody else might have done the work by now.  A
> +	 relaxed load is sufficient because we have the lock, and
> +	 num_ifs is only updated under the lock.  */

That's not really the reason why the relaxed MO load is okay.  You cite
just concurrent updates to num_ifs, but the relation to these is not
affected by the MO here.  The MOs are about ordering, so you'd need a
reason related to ordering here, informally.
I would instead just refer to point (3) below.

> +      num_ifs_local = atomic_load_relaxed (num_ifs);
> +      if (num_ifs_local <= 0)
>  	{
> +	  /* This is the only block which writes to num_ifs.  It can
> +	     be executed several times (sequentially) if
> +	     initialization does not yield any interfaces, and num_ifs
> +	     remains zero.  However, once we stored a positive value
> +	     in num_ifs below, this block cannot be entered again due
> +	     to the condition above.  */
>  	  int new_num_ifs = 0;
>  
>  	  /* Get a list of interfaces.  */
> @@ -472,7 +490,14 @@ _res_hconf_reorder_addrs (struct hostent *hp)
>  	  /* Release lock, preserve error value, and close socket.  */
>  	  errno = save;
>  
> -	  num_ifs = new_num_ifs;
> +	  /* Advertise successful initialization if new_num_ifs is
> +	     positive (and no updates to ifaddrs are permitted after
> +	     that).  Otherwise, num_ifs remains unchanged, at zero.
> +	     This store synchronizes with the initial acquire MO
> +	     load.  */
> +	  atomic_store_release (&num_ifs, new_num_ifs);
> +	  /* Keep the local copy current, to save another load.  */
> +	  num_ifs_local = new_num_ifs;
>  	}
>  
>        __libc_lock_unlock (lock);
> @@ -480,15 +505,43 @@ _res_hconf_reorder_addrs (struct hostent *hp)
>        __close (sd);
>      }
>  
> -  if (num_ifs == 0)
> +  /* num_ifs_local cannot be negative because the if statement above
> +     covered this case.  It can still be zero if we just performed
> +     initialization, but could not find any interfaces.  */
> +  if (num_ifs_local == 0)
>      return;
>  
> +  /* The code below accesses ifaddrs, so we need to ensure that the
> +     initialization happens-before this point.
> +
> +     The actual initialization is sequenced-before the release store
> +     to num_ifs, and sequenced-before the end of the critical section.
> +
> +     This means there are three possible executions:
> +
> +     (1) The thread that initialized the data also uses it, so
> +         sequenced-before is sufficient to ensure happens-before.
> +
> +     (2) The release MO store of num_ifs synchronizes-with the acquire
> +         MO load, and the acquire MO load is sequenced before the use
> +         of the initialized data below.
> +
> +     (3) We enter the critical section, and the relaxed MO load of
> +         num_fis yields a positive value.  The write to ifaddrs is

s/num_fis/num_ifs/ and ...

> +         sequenced-before leaving the critical section.  Leaving the
> +         critical section happens-before we entered the critical
> +         section ourselves, which means that the the write to ifaddrs

s/the the/the/.  Sorry for not spotting these earlier :)

> +         happens-before this point.
> +
> +     Consequently, all potential writes to ifaddrs (and the data it
> +     points to) happens-before this point.  */
> +
>    /* Find an address for which we have a direct connection.  */
>    for (i = 0; hp->h_addr_list[i]; ++i)
>      {
>        struct in_addr *haddr = (struct in_addr *) hp->h_addr_list[i];
>  
> -      for (j = 0; j < num_ifs; ++j)
> +      for (j = 0; j < num_ifs_local; ++j)
>  	{
>  	  u_int32_t if_addr    = ifaddrs[j].u.ipv4.addr;
>  	  u_int32_t if_netmask = ifaddrs[j].u.ipv4.mask;

Okay with the changes above.  Thanks!