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authorTorvald Riegel <triegel@redhat.com>2016-08-17 13:56:11 +0200
committerTorvald Riegel <triegel@redhat.com>2016-08-18 20:53:37 +0200
commit6f9d4f595e4073807ad0e844cbb3b3d7158b76d5 (patch)
treed0064a3e11f1326fcdfcad1b239076044da9a762 /resolv
parentb65f0b7b2ecd144800830633a4c2719f11775572 (diff)
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Fix incorrect double-checked locking related to _res_hconf.initialized.
_res_hconf.initialized was not suitable for use in a multi-threaded environment due to the lack of atomics and memory barriers. Use of it was also unnecessary because _res_hconf_init did the right thing by using __libc_once. This patch fixes the glibc-internal uses by just calling _res_hconf_init unconditionally, and switches to a release MO atomic store for _res_hconf.initialized to fix the glibc side of the synchronization problem (which will maintain backward compatibility, but cannot fix the lack of acquire MO on any glibc-external loads). [BZ #20477] * resolv/res_hconf.c (do_init): Use atomic access. * resolv/res_hconf.h: Add comments. * nscd/aicache.c (addhstaiX): Call _res_hconf_init unconditionally. * nss/getXXbyYY_r.c (REENTRANT_NAME): Likewise. * sysdeps/posix/getaddrinfo.c (gaih_inet): Likewise.
Diffstat (limited to 'resolv')
-rw-r--r--resolv/res_hconf.c3
-rw-r--r--resolv/res_hconf.h9
2 files changed, 11 insertions, 1 deletions
diff --git a/resolv/res_hconf.c b/resolv/res_hconf.c
index 5cd128916d..093c26837f 100644
--- a/resolv/res_hconf.c
+++ b/resolv/res_hconf.c
@@ -348,7 +348,8 @@ do_init (void)
arg_trimdomain_list (ENV_TRIM_OVERR, 1, envval);
}
- _res_hconf.initialized = 1;
+ /* See comments on the declaration of _res_hconf. */
+ atomic_store_release (&_res_hconf.initialized, 1);
}
diff --git a/resolv/res_hconf.h b/resolv/res_hconf.h
index b97734df9e..a3d23f3e58 100644
--- a/resolv/res_hconf.h
+++ b/resolv/res_hconf.h
@@ -25,6 +25,15 @@
struct hconf
{
+ /* We keep the INITIALIZED member only for backwards compatibility. New
+ code should just call _res_hconf_init unconditionally. For this field
+ to be used safely, users must ensure that either (1) a call to
+ _res_hconf_init happens-before any load from INITIALIZED, or (2) an
+ assignment of zero to INITIALIZED happens-before any load from it, and
+ these loads use acquire MO if the intent is to skip calling
+ _res_hconf_init if the load returns a nonzero value. Such acquire MO
+ loads will then synchronize with the release MO store to INITIALIZED
+ in do_init in res_hconf.c; see pthread_once for more detail. */
int initialized;
int unused1;
int unused2[4];