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resolv: Automatically reload a changed /etc/resolv.conf file [BZ #984]

Browse Source 
This commit enhances the stub resolver to reload the configuration
in the per-thread _res object if the /etc/resolv.conf file has
changed.  The resolver checks whether the application has modified
_res and will not overwrite the _res object in that case.

The struct resolv_context mechanism is used to check the
configuration file only once per name lookup.
This commit is contained in:
Florian Weimer 2017-07-03 21:06:23 +02:00
parent a1c4eb8794
commit aef16cc8a4
11 changed files with 273 additions and 95 deletions
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33
ChangeLog
View File

@ -1,3 +1,36 @@
2017-06-30 Florian Weimer <fweimer@redhat.com>
[BZ #984]
Reload /etc/resolv.conf if has been changed on disk.
* resolv/resolv_conf.h (resolv_conf): Remove initstamp member.
(__resolv_conf_load, __resolv_conf_get_current): Declare.
* resolv/resolv_conf.c (struct resolv_conf_global): Add
conf_current, conf_mtime, conf_ctime, conf_size, conf_ino members.
(__resolv_conf_get_current): New function.
(__resolv_conf_allocate): Do not initialize initstamp.
(freeres): Deallocate global->conf_current.
* resolv/resolv.h (RES_NORELOAD): Define.
* resolv/res_debug.c (p_option): Add RES_NORELOAD.
* resolv/resolv-internal.h (__res_initstamp): Remove declaration.
* resolv/resolv_context.c (replicated_configuration_matches): New.
(maybe_init): Call it. Use __resolv_conf_get_current to obtain
the current configration.
* resolv/res_init.c (__res_initstamp): Remove variable definition.
(has_preinit_values): New function.
(__resolv_conf_load): Renamed from __res_vinit. Drop res_state
parameter and do not call __resolv_conf_attach.
(__res_vinit): Reimplement based __resolv_conf_load.
(res_options): Handle no-reload.
* resolv/res_libc.c (atomicinclock, atomicincunlock, atomicinc)
(lock): Remove.
(res_int): Do not update __res_initstamp.
* resolv/tst-resolv-res_init-skeleton.c (print_resp): Handle
RES_NORELOAD.
(test_cases): Test no-reload.
(special_test_call_res_init): Remove.
(special_test_callback): Rely on automated reloading. Add tests
for no-reload.
2017-06-30 Florian Weimer <fweimer@redhat.com>
Mirror the entire resolver configuration in struct resolv_conf.

4
NEWS
View File

@ -245,6 +245,10 @@ Version 2.26
array, which is limited to six entries) will only use the first six search
domains, as before.
* The glibc DNS stub resolver now automatically reloads /etc/resolv.conf,
to pick up changed configuration settings. The new “no-reload”
(RES_NORELOAD) resolver option disables this behavior.
Security related changes:
* The DNS stub resolver limits the advertised UDP buffer size to 1200 bytes,

1
resolv/res_debug.c
View File

@ -613,6 +613,7 @@ p_option(u_long option) {
case RES_SNGLKUPREOP: return "single-request-reopen";
case RES_USE_DNSSEC: return "dnssec";
case RES_NOTLDQUERY: return "no-tld-query";
case RES_NORELOAD: return "no-reload";
/* XXX nonreentrant */
default: sprintf(nbuf, "?0x%lx?", (u_long)option);
return (nbuf);

80
resolv/res_init.c
View File

@ -106,8 +106,6 @@
static uint32_t net_mask (struct in_addr);
unsigned long long int __res_initstamp;
int
res_ninit (res_state statp)
{
@ -164,6 +162,16 @@ struct resolv_conf_parser
struct resolv_conf template;
};
/* Return true if *PREINIT contains actual preinitialization. */
static bool
has_preinit_values (const struct __res_state *preinit)
{
return (preinit->retrans != 0 && preinit->retrans != RES_TIMEOUT)
|| (preinit->retry != 0 && preinit->retry != RES_DFLRETRY)
|| (preinit->options != 0
&& (preinit->options & ~RES_INIT) != RES_DEFAULT);
}
static void
resolv_conf_parser_init (struct resolv_conf_parser *parser,
const struct __res_state *preinit)
@ -531,14 +539,8 @@ res_vinit_1 (FILE *fp, struct resolv_conf_parser *parser)
return true;
}
/* Set up default settings. If the /etc/resolv.conf configuration
file exist, the values there will have precedence. Otherwise, the
server address is set to INADDR_LOOPBACK and the default domain
name comes from gethostname. The RES_OPTIONS and LOCALDOMAIN
environment variables can be used to override some settings.
Return 0 if completes successfully, -1 on error. */
int
__res_vinit (res_state statp, int preinit)
struct resolv_conf *
__resolv_conf_load (struct __res_state *preinit)
{
/* Ensure that /etc/hosts.conf has been loaded (once). */
_res_hconf_init ();
@ -559,20 +561,14 @@ __res_vinit (res_state statp, int preinit)
default:
/* Other errors refer to resource allocation problems and
need to be handled by the application. */
return -1;
return NULL;
}
struct resolv_conf_parser parser;
if (preinit)
{
resolv_conf_parser_init (&parser, statp);
statp->id = res_randomid ();
}
else
resolv_conf_parser_init (&parser, NULL);
resolv_conf_parser_init (&parser, preinit);
bool ok = res_vinit_1 (fp, &parser);
if (ok)
struct resolv_conf *conf = NULL;
if (res_vinit_1 (fp, &parser))
{
parser.template.nameserver_list
= nameserver_list_begin (&parser.nameserver_list);
@ -583,21 +579,42 @@ __res_vinit (res_state statp, int preinit)
= search_list_size (&parser.search_list);
parser.template.sort_list = sort_list_begin (&parser.sort_list);
parser.template.sort_list_size = sort_list_size (&parser.sort_list);
struct resolv_conf *conf = __resolv_conf_allocate (&parser.template);
if (conf == NULL)
ok = false;
else
{
ok = __resolv_conf_attach (statp, conf);
__resolv_conf_put (conf);
}
conf = __resolv_conf_allocate (&parser.template);
}
resolv_conf_parser_free (&parser);
if (!ok)
return -1;
return conf;
}
/* Set up default settings. If the /etc/resolv.conf configuration
file exist, the values there will have precedence. Otherwise, the
server address is set to INADDR_LOOPBACK and the default domain
name comes from gethostname. The RES_OPTIONS and LOCALDOMAIN
environment variables can be used to override some settings.
Return 0 if completes successfully, -1 on error. */
int
__res_vinit (res_state statp, int preinit)
{
struct resolv_conf *conf;
if (preinit && has_preinit_values (statp))
/* For the preinit case, we cannot use the cached configuration
because some settings could be different. */
conf = __resolv_conf_load (statp);
else
return 0;
conf = __resolv_conf_get_current ();
if (conf == NULL)
return -1;
bool ok = __resolv_conf_attach (statp, conf);
__resolv_conf_put (conf);
if (ok)
{
if (preinit)
statp->id = res_randomid ();
return 0;
}
else
return -1;
}
static void
@ -652,6 +669,7 @@ res_setoptions (struct resolv_conf_parser *parser, const char *options)
{ STRnLEN ("single-request"), 0, RES_SNGLKUP },
{ STRnLEN ("no_tld_query"), 0, RES_NOTLDQUERY },
{ STRnLEN ("no-tld-query"), 0, RES_NOTLDQUERY },
{ STRnLEN ("no-reload"), 0, RES_NORELOAD },
{ STRnLEN ("use-vc"), 0, RES_USEVC }
};
#define noptions (sizeof (options) / sizeof (options[0]))

18
resolv/res_libc.c
View File

@ -42,18 +42,6 @@
#include <libc-lock.h>
#include <resolv-internal.h>
/* We have atomic increment operations on 64-bit platforms. */
#if __WORDSIZE == 64
# define atomicinclock(lock) (void) 0
# define atomicincunlock(lock) (void) 0
# define atomicinc(var) catomic_increment (&(var))
#else
__libc_lock_define_initialized (static, lock);
# define atomicinclock(lock) __libc_lock_lock (lock)
# define atomicincunlock(lock) __libc_lock_unlock (lock)
# define atomicinc(var) ++var
#endif
int
res_init (void)
{
@ -90,12 +78,6 @@ res_init (void)
if (!_res.id)
_res.id = res_randomid ();
atomicinclock (lock);
/* Request all threads to re-initialize their resolver states,
resolv.conf might have changed. */
atomicinc (__res_initstamp);
atomicincunlock (lock);
return __res_vinit (&_res, 1);
}

3
resolv/resolv-internal.h
View File

@ -97,7 +97,4 @@ int __res_nopt (struct resolv_context *, int n0,
int __inet_pton_length (int af, const char *src, size_t srclen, void *);
libc_hidden_proto (__inet_pton_length)
/* Used to propagate the effect of res_init calls across threads. */
extern unsigned long long int __res_initstamp attribute_hidden;
#endif /* _RESOLV_INTERNAL_H */

1
resolv/resolv.h
View File

@ -134,6 +134,7 @@ struct res_sym {
#define RES_USE_DNSSEC 0x00800000 /* use DNSSEC using OK bit in OPT */
#define RES_NOTLDQUERY 0x01000000 /* Do not look up unqualified name
as a TLD. */
#define RES_NORELOAD 0x02000000 /* No automatic configuration reload. */
#define RES_DEFAULT (RES_RECURSE|RES_DEFNAMES|RES_DNSRCH)

86
resolv/resolv_conf.c
View File

@ -22,6 +22,7 @@
#include <assert.h>
#include <libc-lock.h>
#include <resolv-internal.h>
#include <sys/stat.h>
/* _res._u._ext.__glibc_extension_index is used as an index into a
struct resolv_conf_array object. The intent of this construction
@ -54,6 +55,15 @@ struct resolv_conf_global
the array element is overwritten with NULL. */
struct resolv_conf_array array;
/* Cached current configuration object for /etc/resolv.conf. */
struct resolv_conf *conf_current;
/* These properties of /etc/resolv.conf are used to check if the
configuration needs reloading. */
struct timespec conf_mtime;
struct timespec conf_ctime;
off64_t conf_size;
ino64_t conf_ino;
};
/* Lazily allocated storage for struct resolv_conf_global. */
@ -100,6 +110,75 @@ conf_decrement (struct resolv_conf *conf)
free (conf);
}
struct resolv_conf *
__resolv_conf_get_current (void)
{
struct stat64 st;
if (stat64 (_PATH_RESCONF, &st) != 0)
{
switch (errno)
{
case EACCES:
case EISDIR:
case ELOOP:
case ENOENT:
case ENOTDIR:
case EPERM:
/* Ignore errors due to file system contents. */
memset (&st, 0, sizeof (st));
break;
default:
/* Other errors are fatal. */
return NULL;
}
}
struct resolv_conf_global *global_copy = get_locked_global ();
if (global_copy == NULL)
return NULL;
struct resolv_conf *conf;
if (global_copy->conf_current != NULL
&& (global_copy->conf_mtime.tv_sec == st.st_mtim.tv_sec
&& global_copy->conf_mtime.tv_nsec == st.st_mtim.tv_nsec
&& global_copy->conf_ctime.tv_sec == st.st_ctim.tv_sec
&& global_copy->conf_ctime.tv_nsec == st.st_ctim.tv_nsec
&& global_copy->conf_ino == st.st_ino
&& global_copy->conf_size == st.st_size))
/* We can reuse the cached configuration object. */
conf = global_copy->conf_current;
else
{
/* Parse configuration while holding the lock. This avoids
duplicate work. */
conf = __resolv_conf_load (NULL);
if (conf != NULL)
{
if (global_copy->conf_current != NULL)
conf_decrement (global_copy->conf_current);
global_copy->conf_current = conf; /* Takes ownership. */
/* Update file modification stamps. The configuration we
read could be a newer version of the file, but this does
not matter because this will lead to an extraneous reload
later. */
global_copy->conf_mtime = st.st_mtim;
global_copy->conf_ctime = st.st_ctim;
global_copy->conf_ino = st.st_ino;
global_copy->conf_size = st.st_size;
}
}
if (conf != NULL)
{
/* Return an additional reference. */
assert (conf->__refcount > 0);
++conf->__refcount;
assert (conf->__refcount > 0);
}
put_locked_global (global_copy);
return conf;
}
/* Internal implementation of __resolv_conf_get, without validation
against *RESP. */
static struct resolv_conf *
@ -320,7 +399,6 @@ __resolv_conf_allocate (const struct resolv_conf *init)
conf->retry = init->retry;
conf->options = init->options;
conf->ndots = init->ndots;
conf->initstamp = __res_initstamp;
/* Allocate the arrays with pointers. These must come first because
they have the highets alignment. */
@ -580,6 +658,12 @@ freeres (void)
if (global == NULL)
return;
if (global->conf_current != NULL)
{
conf_decrement (global->conf_current);
global->conf_current = NULL;
}
/* Note that this frees only the array itself. The pointed-to
configuration objects should have been deallocated by res_nclose
and per-thread cleanup functions. */

17
resolv/resolv_conf.h
View File

@ -35,11 +35,6 @@ struct resolv_sortlist_entry
object. */
struct resolv_conf
{
/* Used to propagate the effect of res_init across threads. This
member is mutable and prevents sharing of the same struct
resolv_conf object among multiple struct __res_state objects. */
unsigned long long int initstamp;
/* Reference counter. The object is deallocated once it reaches
zero. For internal use within resolv_conf only. */
size_t __refcount;
@ -69,6 +64,18 @@ struct resolv_conf
struct __res_state;
/* Read /etc/resolv.conf and return a configuration object, or NULL if
/etc/resolv.conf cannot be read due to memory allocation errors.
If PREINIT is not NULL, some configuration values are taken from the
struct __res_state object. */
struct resolv_conf *__resolv_conf_load (struct __res_state *preinit)
attribute_hidden __attribute__ ((warn_unused_result));
/* Return a configuration object for the current /etc/resolv.conf
settings, or NULL on failure. The object is cached. */
struct resolv_conf *__resolv_conf_get_current (void)
attribute_hidden __attribute__ ((warn_unused_result));
/* Return the extended resolver state for *RESP, or NULL if it cannot
be determined. A call to this function must be paired with a call
to __resolv_conf_put. */

53
resolv/resolv_context.c
View File

@ -51,6 +51,20 @@
resolver state. */
static __thread struct resolv_context *current attribute_tls_model_ie;
/* The resolv_conf handling will gives us a ctx->conf pointer even if
these fields do not match because a mis-match does not cause a loss
of state (_res objects can store the full information). This
function checks to ensure that there is a full patch, to prevent
overwriting a patched configuration. */
static bool
replicated_configuration_matches (const struct resolv_context *ctx)
{
return ctx->resp->options == ctx->conf->options
&& ctx->resp->retrans == ctx->conf->retrans
&& ctx->resp->retry == ctx->conf->retry
&& ctx->resp->ndots == ctx->conf->ndots;
}
/* Initialize *RESP if RES_INIT is not yet set in RESP->options, or if
res_init in some other thread requested re-initializing. */
static __attribute__ ((warn_unused_result)) bool
@ -59,27 +73,36 @@ maybe_init (struct resolv_context *ctx, bool preinit)
struct __res_state *resp = ctx->resp;
if (resp->options & RES_INIT)
{
if (resp->options & RES_NORELOAD)
/* Configuration reloading was explicitly disabled. */
return true;
/* If there is no associated resolv_conf object despite the
initialization, something modified *ctx->resp. Do not
override those changes. */
if (ctx->conf != NULL && ctx->conf->initstamp != __res_initstamp)
if (ctx->conf != NULL && replicated_configuration_matches (ctx))
{
if (resp->nscount > 0)
/* This call will detach the extended resolver state. */
__res_iclose (resp, true);
/* And this call will attach it again. */
if (__res_vinit (resp, 1) < 0)
struct resolv_conf *current = __resolv_conf_get_current ();
if (current == NULL)
return false;
/* Check if the configuration changed. */
if (current != ctx->conf)
{
/* The configuration no longer matches after failed
initialization. */
__resolv_conf_put (ctx->conf);
ctx->conf = NULL;
return false;
/* This call will detach the extended resolver state. */
if (resp->nscount > 0)
__res_iclose (resp, true);
/* Reattach the current configuration. */
if (__resolv_conf_attach (ctx->resp, current))
{
__resolv_conf_put (ctx->conf);
/* ctx takes ownership, so we do not release current. */
ctx->conf = current;
}
}
/* Delay the release of the old configuration until this
point, so that __res_vinit can reuse it if possible. */
__resolv_conf_put (ctx->conf);
ctx->conf = __resolv_conf_get (ctx->resp);
else
/* No change. Drop the reference count for current. */
__resolv_conf_put (current);
}
return true;
}

72
resolv/tst-resolv-res_init-skeleton.c
View File

@ -151,6 +151,7 @@ print_resp (FILE *fp, res_state resp)
print_option_flag (fp, &options, RES_SNGLKUPREOP,
"single-request-reopen");
print_option_flag (fp, &options, RES_NOTLDQUERY, "no-tld-query");
print_option_flag (fp, &options, RES_NORELOAD, "no-reload");
fputc ('\n', fp);
if (options != 0)
fprintf (fp, "; error: unresolved option bits: 0x%x\n", options);
@ -470,6 +471,28 @@ struct test_case test_cases[] =
"nameserver 192.0.2.1\n"
"; nameserver[0]: [192.0.2.1]:53\n"
},
{.name = "basic no-reload",
.conf = "options no-reload\n"
"search corp.example.com example.com\n"
"nameserver 192.0.2.1\n",
.expected = "options no-reload\n"
"search corp.example.com example.com\n"
"; search[0]: corp.example.com\n"
"; search[1]: example.com\n"
"nameserver 192.0.2.1\n"
"; nameserver[0]: [192.0.2.1]:53\n"
},
{.name = "basic no-reload via RES_OPTIONS",
.conf = "search corp.example.com example.com\n"
"nameserver 192.0.2.1\n",
.expected = "options no-reload\n"
"search corp.example.com example.com\n"
"; search[0]: corp.example.com\n"
"; search[1]: example.com\n"
"nameserver 192.0.2.1\n"
"; nameserver[0]: [192.0.2.1]:53\n",
.res_options = "no-reload"
},
{.name = "whitespace",
.conf = "# This test covers comment and whitespace processing "
" (trailing whitespace,\n"
@ -722,18 +745,7 @@ test_file_contents (const struct test_case *t)
}
/* Special tests which do not follow the general pattern. */
enum { special_tests_count = 7 };
#if TEST_THREAD
/* Called from test number 3-6 to trigger reloading of the
configuration. */
static void *
special_test_call_res_init (void *closure)
{
TEST_VERIFY (res_init () == 0);
return NULL;
}
#endif
enum { special_tests_count = 11 };
/* Implementation of special tests. */
static void
@ -800,20 +812,29 @@ special_test_callback (void *closure)
case 4:
case 5:
case 6:
/* Test res_init change broadcast. This requires a second
thread to trigger the reload. */
#if TEST_THREAD
support_write_file_string (_PATH_RESCONF,
"options edns0\n"
"nameserver 192.0.2.1\n");
goto reload_tests;
case 7: /* 7 and the following tests are with no-reload. */
case 8:
case 9:
case 10:
support_write_file_string (_PATH_RESCONF,
"options edns0 no-reload\n"
"nameserver 192.0.2.1\n");
/* Fall through. */
reload_tests:
for (int iteration = 0; iteration < 2; ++iteration)
{
switch (test_index)
{
case 3:
case 7:
TEST_VERIFY (res_init () == 0);
break;
case 4:
case 8:
{
unsigned char buf[512];
TEST_VERIFY
@ -822,37 +843,44 @@ special_test_callback (void *closure)
}
break;
case 5:
case 9:
gethostbyname (test_hostname);
break;
case 6:
case 10:
{
struct addrinfo *ai;
(void) getaddrinfo (test_hostname, NULL, NULL, &ai);
}
break;
}
if (iteration == 0)
/* test_index == 7 is res_init and performs a reload even
with no-reload. */
if (iteration == 0 || test_index > 7)
{
TEST_VERIFY (_res.options & RES_USE_EDNS0);
TEST_VERIFY (!(_res.options & RES_ROTATE));
if (test_index < 7)
TEST_VERIFY (!(_res.options & RES_NORELOAD));
else
TEST_VERIFY (_res.options & RES_NORELOAD);
TEST_VERIFY (_res.nscount == 1);
/* File change triggers automatic reloading. */
support_write_file_string (_PATH_RESCONF,
"options rotate\n"
"nameserver 192.0.2.1\n"
"nameserver 192.0.2.2\n");
xpthread_join (xpthread_create
(NULL, special_test_call_res_init, NULL));
}
else
{
/* edns0 was dropped, but the flag is not cleared. See
bug 21701. */
/* TEST_VERIFY (!(_res.options & RES_USE_EDNS0)); */
if (test_index != 3 && test_index != 7)
/* test_index 3, 7 are res_init; this function does
not reset flags. See bug 21701. */
TEST_VERIFY (!(_res.options & RES_USE_EDNS0));
TEST_VERIFY (_res.options & RES_ROTATE);
TEST_VERIFY (_res.nscount == 2);
}
}
#endif
break;
}
}