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

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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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86
resolv/resolv_conf.c
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@ -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. */