libc: simplify devname() fallback

[Philip Guenther]

Inspired by espie@'s ttyname.c diff, here's a simplification of libc's
fallback-to-scanning-/dev code for devname().  Since devname() returns
the "name under /dev", this eliminates the string manipulation == win!

Works when tested with "stat -Lf %Sr" on a symlink to a device with
/var/run/dev.db chmoded to 600.

ok?

Philip Guenther


Index: lib/libc/gen/devname.c
===================================================================
RCS file: /data/src/openbsd/src/lib/libc/gen/devname.c,v
retrieving revision 1.12
diff -u -p -r1.12 devname.c
--- lib/libc/gen/devname.c      13 Sep 2015 08:31:47 -0000      1.12
+++ lib/libc/gen/devname.c      28 Jun 2016 07:44:03 -0000
@@ -28,23 +28,20 @@
  * SUCH DAMAGE.
  */
 
-#include <sys/stat.h>
 #include <sys/types.h>
+#include <sys/stat.h>
 
 #include <db.h>
 #include <dirent.h>
 #include <fcntl.h>
-#include <limits.h>
 #include <paths.h>
 #include <stdbool.h>
-#include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
 
 static char *
 devname_nodb(dev_t dev, mode_t type)
 {
-       static char buf[sizeof(_PATH_DEV) + NAME_MAX];
        char *name = NULL;
        struct dirent *dp;
        struct stat sb;
@@ -52,19 +49,13 @@ devname_nodb(dev_t dev, mode_t type)
 
        if ((dirp = opendir(_PATH_DEV)) == NULL)
                return (NULL);
-       if (strlcpy(buf, _PATH_DEV, sizeof(buf)) >= sizeof(buf))
-               return (NULL);
        while ((dp = readdir(dirp)) != NULL) {
                if (dp->d_type != DT_UNKNOWN && DTTOIF(dp->d_type) != type)
                        continue;
-               buf[sizeof(_PATH_DEV) - 1] = '\0';
-               if (strlcat(buf, dp->d_name, sizeof(buf)) >= sizeof(buf))
-                       continue;
-               if (lstat(buf, &sb) == -1)
-                       continue;
-               if (sb.st_rdev != dev || (sb.st_mode & S_IFMT) != type)
+               if (fstatat(dirfd(dirp), dp->d_name, &sb, AT_SYMLINK_NOFOLLOW)
+                   || sb.st_rdev != dev || (sb.st_mode & S_IFMT) != type)
                        continue;
-               name = buf + sizeof(_PATH_DEV) - 1;
+               name = dp->d_name;
                break;
        }
        closedir(dirp);