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Fix double-checked locking in _res_hconf_reorder_addrs [BZ #19074] 

[BZ #19074]
	* resolv/res_hconf.c (_res_hconf_reorder_addrs): Use atomics to
	load and store num_ifs.
This commit is contained in:
Florian Weimer 2015-10-14 16:19:24 +02:00
parent d95453ef5d
commit f463c7b183
3 changed files with 71 additions and 10 deletions
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6
ChangeLog
View File

@ -1,3 +1,9 @@
2015-10-14 Florian Weimer <fweimer@redhat.com>
[BZ #19074]
* resolv/res_hconf.c (_res_hconf_reorder_addrs): Use atomics to
load and store num_ifs.
2015-10-14 H.J. Lu <hongjiu.lu@intel.com>
[BZ #18822]

5
NEWS
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@ -18,8 +18,9 @@ Version 2.23
18790, 18795, 18796, 18803, 18820, 18823, 18824, 18825, 18857, 18863,
18870, 18872, 18873, 18875, 18887, 18921, 18951, 18952, 18956, 18961,
18966, 18967, 18969, 18970, 18977, 18980, 18981, 18985, 19003, 19007,
19012, 19016, 19018, 19032, 19046, 19049, 19050, 19059, 19071, 19076,
19077, 19078, 19079, 19085, 19086, 19088, 19094, 19095, 19124, 19125
19012, 19016, 19018, 19032, 19046, 19049, 19050, 19059, 19071, 19074,
19076, 19077, 19078, 19079, 19085, 19086, 19088, 19094, 19095, 19124,
19125
* The obsolete header <regexp.h> has been removed. Programs that require
this header must be updated to use <regex.h> instead.

70
resolv/res_hconf.c
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@ -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;