safely handle failure to open hosts, services, resolv.conf files

previously, transient failures like fd exhaustion or other
resource-related errors were treated the same as non-existence of
these files, leading to fallbacks or false-negative results. in
particular:

- failure to open hosts resulted in fallback to dns, possibly yielding
  EAI_NONAME for a hostname that should be defined locally, or an
  unwanted result from dns that the hosts file was intended to
  replace.

- failure to open services resulted in EAI_SERVICE.

- failure to open resolv.conf resulted in querying localhost rather
  than the configured nameservers.

now, only permanent errors trigger the fallback behaviors above; all
other errors are reportable to the caller as EAI_SYSTEM.

1 files changed, 9 insertions, 1 deletions

diff --git a/src/network/lookup_serv.c b/src/network/lookup_serv.c
index 4faa5bc7..66ebaea2 100644
--- a/src/network/lookup_serv.c
+++ b/src/network/lookup_serv.c
@@ -4,6 +4,7 @@
 #include <ctype.h>
 #include <string.h>
 #include <fcntl.h>
+#include <errno.h>
 #include "lookup.h"
 #include "stdio_impl.h"
 
@@ -69,7 +70,14 @@ int __lookup_serv(struct service buf[static MAXSERVS], const char *name, int pro
 
 	unsigned char _buf[1032];
 	FILE _f, *f = __fopen_rb_ca("/etc/services", &_f, _buf, sizeof _buf);
-	if (!f) return EAI_SERVICE;
+	if (!f) switch (errno) {
+	case ENOENT:
+	case ENOTDIR:
+	case EACCES:
+		return EAI_SERVICE;
+	default:
+		return EAI_SYSTEM;
+	}
 
 	while (fgets(line, sizeof line, f) && cnt < MAXSERVS) {
 		if ((p=strchr(line, '#'))) *p++='\n', *p=0;