[Bug 1432378] Re: libresolv res_init() does not correctly inititalize internals
Joshua Rogers
1432378 at bugs.launchpad.net
Sat Aug 1 14:43:30 UTC 2015
tl;dr:
res_init() does not correctly initialize the _res struct.
The code:
res_init();
if(_res.options & RES_INIT) {
printf("RES_INIT set.\n");
} else {
printf("RES_INIT not set.\n");
}
outputs "RES_INIT set." correctly, and that bit is set. res_init() does all of its setting through res_init.c, https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_init.c;h=66561ffac2ffc039707676ed8b4bf36ee50ee889;hb=HEAD#l151
That's all fine and dandy. -- (The actual resetting happens through the function __res_vinit())
However, res_init() neglects to set things that the ubuntu-specific eglibc requires for it to be a valid struct, such as the last modified time of /etc/resolv.conf -- "last_mtime".
Thus, the first call to res_query() resets the _res struct, effectively making any changes before it and after the original res_init(), useless.
As we can see in res_data.c, https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_data.c;h=81c9ae5bfd7ef71ebb986b5c9572c1859684ba39;hb=HEAD#l185 , res_query() calls __res_maybe_init (), which decides whether we need to re-call res_init() or not.
In the comments for that __res_maybe_init() function:
/* Initialize resp if RES_INIT is not yet set or if res_init in some other
thread requested re-initializing. */
On the first run of __res_maybe_init()[thus, effectively, res_query), it
will always call __res_vinit(), because res_init() does not set the
'last_mtime'. -- That's because the last_mtime is an Ubuntu-specific
feature, likely added for security reasons.
>>>>>> if ((ret == 0) && (last_mtime != statbuf.st_mtime)) { <<<<<<<
last_mtime = statbuf.st_mtime;
atomicinc (__res_initstamp);
}
__libc_lock_unlock (lock);
if (__res_initstamp != resp->_u._ext.initstamp) {
if (resp->nscount > 0)
__res_iclose (resp, true);
return __res_vinit (resp, 1);
}
even shorter tl;dr:
'last_mtime' is an ubuntu-specific feature added to eglibc's resolv library, which is only set inside the __res_maybe_init() function.
When calling res_init(), it does not set 'last_mtime', as that uses __res_vinit(), not __res_maybe_init().
When calling res_query() for the first time, all the changes made to the _res struct are wiped, with the exception of int retrans, int retry, u_long options. This is because res_query uses __res_maybe_init(), which will reset _res if 'last_mtime' has not been set(or is old)
A quick fix is to replace __res_vinit()'s usage within the res_query()
function with __res_maybe_init(), which takes the exact same parameters.
--
You received this bug notification because you are a member of Ubuntu
Foundations Bugs, which is subscribed to glibc in Ubuntu.
https://bugs.launchpad.net/bugs/1432378
Title:
libresolv res_init() does not correctly inititalize internals
Status in glibc package in Ubuntu:
Confirmed
Bug description:
As reported here:
https://sourceware.org/bugzilla/show_bug.cgi?id=18126
The bug, however, is not in the sourceware sourcecode, but in the
Ubuntu one.
https://sourceware.org/git/?p=glibc.git;a=blob;f=resolv/res_libc.c;h=ee3fa2114b7051b86f6f9676f1151d1435dedb9d;hb=HEAD#l97
Contrary to what one would think, res_init() does not correctly inititialize the internals for further use by the libresolv family, and others.
When you call res_init(), it correctly "keeps" these:
if (!_res.retrans)
_res.retrans = RES_TIMEOUT;
if (!_res.retry)
_res.retry = 4;
if (!(_res.options & RES_INIT))
_res.options = RES_DEFAULT;
else if (_res.nscount > 0)
__res_iclose (&_res, true); /* Close any VC sockets. */
then calls __res_vinit():
return (__res_vinit(&_res, 1));
However, programs that use the libresolv family and others, use the
hidden function, "__res_maybe_init".
__res_maybe_init determines if res_init(__res_vinit()) needs to be
called or not.
It does this:
static time_t last_mtime;
struct stat statbuf;
int ret;
if (resp->options & RES_INIT) {
ret = stat (_PATH_RESCONF, &statbuf);
__libc_lock_lock (lock);
if ((ret == 0) && (last_mtime != statbuf.st_mtime)) {
last_mtime = statbuf.st_mtime;
atomicinc (__res_initstamp);
}
__libc_lock_unlock (lock);
if (__res_initstamp != resp->_u._ext.initstamp) {
if (resp->nscount > 0)
__res_iclose (resp, true);
return __res_vinit (resp, 1);
}
return 0;
Since the internals have been initialized by res_init(), we don't need to reinitalize, normally. The program checks if we do need to reinitalize, such as due to the change in modifcation date of /etc/resolv.conf.
However, "last_mtime" is never set when using res_init(), so upon the
first run of __res_maybe_init(), it will always run __res_vinit().
This will wipe all changes except for the ones that are kept,
mentioned above.
"last_mtime" should be taken into consideration and handled, when calling res_init().
(for reference)
Only these are kept on res_init(), and thus are only kept with the first call to __res_maybe_init:
int retrans; /* retransmition time interval */
int retry; /* number of times to retransmit */
u_long options; /* option flags - see below. */
These are wiped, due to this bug:
int nscount; /* number of name servers */
struct sockaddr_in
nsaddr_list[MAXNS]; /* address of name server */
# define nsaddr nsaddr_list[0] /* for backward compatibility */
u_short id; /* current message id */
/* 2 byte hole here. */
char *dnsrch[MAXDNSRCH+1]; /* components of domain to search */
char defdname[256]; /* default domain (deprecated) */
u_long pfcode; /* RES_PRF_ flags - see below. */
unsigned ndots:4; /* threshold for initial abs. query */
unsigned nsort:4; /* number of elements in sort_list[] */
unsigned ipv6_unavail:1; /* connecting to IPv6 server failed */
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1432378/+subscriptions
More information about the foundations-bugs
mailing list