ndb/cs: prevent deadlock with ndb/cs by mounting /srv/dns *AFTER* /net

the dnsquery() library function should not start mouting /srv/dns on
its own. this problem arrises only for ndb/cs as it is started before
ndb/dns.

the issue with mounting /srv/dns before /net is when ndb/cs attempts
to read the list of interfaces, accessing /net/ipifc, which triggers
a rpc to ndb/dns as it is ontop of the mount. this can yield a deadlock
when ndb/dns blocks its 9p loop waiting for requests to complete on
a refresh and the requests are stuck waiting for ndb/cs to translate
a dial string for announce().
This commit is contained in:
cinap_lenrek 2018-05-16 21:41:42 +02:00
parent 5c2b4dfc81
commit 6e284eaad5
2 changed files with 51 additions and 33 deletions

View file

@ -1081,12 +1081,8 @@ netinit(int background)
Network *np;
if(background){
switch(rfork(RFPROC|RFNOTEG|RFMEM|RFNOWAIT)){
case 0:
break;
default:
if(rfork(RFPROC|RFNOTEG|RFMEM|RFNOWAIT) != 0)
return;
}
qlock(&netlock);
}
@ -1627,6 +1623,43 @@ slave(char *host)
}
static int
mountdns(void)
{
static QLock mountlock;
static int mounted;
char buf[128], *p;
int fd;
if(mounted)
return 0;
qlock(&mountlock);
snprint(buf, sizeof(buf), "%s/dns", mntpt);
if(access(buf, AEXIST) == 0)
goto done;
if(strcmp(mntpt, "/net") == 0)
snprint(buf, sizeof(buf), "/srv/dns");
else {
snprint(buf, sizeof(buf), "/srv/dns%s", mntpt);
while((p = strchr(buf+8, '/')) != nil)
*p = '_';
}
if((fd = open(buf, ORDWR)) < 0){
err:
qunlock(&mountlock);
return -1;
}
if(mount(fd, -1, mntpt, MAFTER, "") < 0){
close(fd);
goto err;
}
done:
mounted = 1;
qunlock(&mountlock);
return 0;
}
static Ndbtuple*
dnsip6lookup(char *mntpt, char *buf, Ndbtuple *t)
{
@ -1662,6 +1695,11 @@ dnsiplookup(char *host, Ndbs *s, int v6)
return nil;
}
if(mountdns() < 0){
qlock(&dblock);
return nil;
}
if(strcmp(ipattr(host), "ip") == 0)
t = dnsquery(mntpt, host, "ptr");
else {

View file

@ -12,13 +12,12 @@ static Ndbtuple *doquery(int, char *dn, char *type);
* search for a tuple that has the given 'attr=val' and also 'rattr=x'.
* copy 'x' into 'buf' and return the whole tuple.
*
* return 0 if not found.
* return nil if not found.
*/
Ndbtuple*
dnsquery(char *net, char *val, char *type)
{
char rip[128];
char *p;
char buf[128];
Ndbtuple *t;
int fd;
@ -28,37 +27,18 @@ dnsquery(char *net, char *val, char *type)
if(net == nil)
net = "/net";
snprint(rip, sizeof(rip), "%s/dns", net);
fd = open(rip, ORDWR);
if(fd < 0){
if(strcmp(net, "/net") == 0)
snprint(rip, sizeof(rip), "/srv/dns");
else {
snprint(rip, sizeof(rip), "/srv/dns%s", net);
p = strrchr(rip, '/');
*p = '_';
}
fd = open(rip, ORDWR);
if(fd < 0)
return nil;
if(mount(fd, -1, net, MBEFORE, "") < 0){
close(fd);
return nil;
}
/* fd is now closed */
snprint(rip, sizeof(rip), "%s/dns", net);
fd = open(rip, ORDWR);
if(fd < 0)
return nil;
}
snprint(buf, sizeof(buf), "%s/dns", net);
if((fd = open(buf, ORDWR)) < 0)
return nil;
/* zero out the error string */
werrstr("");
/* if this is a reverse lookup, first lookup the domain name */
if(strcmp(type, "ptr") == 0){
mkptrname(val, rip, sizeof rip);
t = doquery(fd, rip, "ptr");
mkptrname(val, buf, sizeof buf);
t = doquery(fd, buf, "ptr");
} else
t = doquery(fd, val, type);