diff options
Diffstat (limited to 'resolv')
| -rw-r--r-- | resolv/res_hconf.c | 70 |
1 files changed, 62 insertions, 8 deletions
diff --git a/resolv/res_hconf.c b/resolv/res_hconf.c index 692d948308..a4cca76727 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 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 +430,19 @@ _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. No + ordering is required for the load because we have the lock, + and num_ifs is only updated under the lock. Also see (3) in + the analysis 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 +491,14 @@ _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 +506,43 @@ _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_ifs yields a positive value. The write to ifaddrs is + sequenced-before leaving the critical section. Leaving the + critical section happens-before we entered the critical + section ourselves, which means that the write to ifaddrs + 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; |