Open file handlers manually using open() and fdopen() instead of using fopen()

A previous commit (d51124e) added the `e` flag to the `fopen()` calls. However
this flag is not supported on all platforms (MacOS) and will be silently
dropped (see #145). This patch works around those issues by manually opening
the file descriptor using `open()` with the `O_CLOEXEC` flag, and invoking
`fd_open()` on the resulting file descriptor to open an appropriate `FILE`
stream.

This makes sure that all files used by pam_yubico will be opened with the
`O_CLOEXEC` flag on all supported platforms to mitigate issues with missing
`fclose()` invocation (see #136).

pam_yubico.c

@@ -814,12 +814,16 @@ parse_cfg (int flags, int argc, const char **argv, struct cfg *cfg)
           else
             {
               struct stat st;
+              int fd;
               FILE *file;
               if(lstat(filename, &st) == 0)
                 {
                   if(S_ISREG(st.st_mode))
                     {
-                      file = fopen(filename, "ae");
+                      fd = open(filename, O_WRONLY | O_CREAT | O_APPEND | O_CLOEXEC, S_IRUSR | S_IWUSR | S_IRGRP);
+                      if (fd >= 0)
+                        {
+                          file = fdopen(fd, "a");
                           if (file)
                             {
                               cfg->debug_file = file;
@@ -829,6 +833,7 @@ parse_cfg (int flags, int argc, const char **argv, struct cfg *cfg)
                 }
             }
         }
+    }
 
   DBG ("called.");
   DBG ("flags %d argc %d", flags, argc);

util.c

@@ -187,8 +187,14 @@ int generate_random(void *buf, int len)
 {
 	FILE *u;
 	int res;
+	int fd;
 
-	u = fopen("/dev/urandom", "re");
+	fd = open("/dev/urandom", O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		return -1;
+	}
+
+	u = fdopen(fd, "r");
 	if (!u) {
 		return -1;
 	}

ykpamcfg.c

@@ -38,6 +38,7 @@
 #include <errno.h>
 #include <sys/stat.h>
 #include <sys/types.h>
+#include <fcntl.h>
 
 #include <ykpers.h>
 
@@ -143,6 +144,7 @@ do_add_hmac_chalresp(YK_KEY *yk, uint8_t slot, bool verbose, char *output_dir, u
   unsigned int response_len;
   char *fn;
   struct passwd *p;
+  int fd;
   FILE *f = NULL;
   struct stat st;
 
@@ -237,11 +239,16 @@ do_add_hmac_chalresp(YK_KEY *yk, uint8_t slot, bool verbose, char *output_dir, u
 
   umask(077);
 
-  f = fopen (fn, "we");
+  fd = open (fn, O_WRONLY | O_CREAT | O_TRUNC | O_CLOEXEC, S_IRUSR | S_IWUSR);
-  if (! f) {
+  if (fd < 0) {
     fprintf (stderr, "Failed opening '%s' for writing : %s\n", fn, strerror (errno));
     goto out;
   }
+  f = fdopen (fd, "w");
+  if (! f) {
+    fprintf (stderr, "fdopen: %s\n", strerror (errno));
+    goto out;
+  }
 
   if (! write_chalresp_state (f, &state))
     goto out;