commit cc8a1620eb97ccddd337d157263c13c57b39ab71
Author: Jesse Hathaway <jesse@mbuki-mvuki.org>
Date:   Tue Mar 27 21:17:59 2018 +0000

    getlogin_r: return early when linux sentinel value is set
    
    When there is no login uid Linux sets /proc/self/loginid to the sentinel
    value of, (uid_t) -1. If this is set we can return early and avoid
    needlessly looking up the sentinel value in any configured nss
    databases.
    
    Checked on aarch64-linux-gnu.
    
            * sysdeps/unix/sysv/linux/getlogin_r.c (__getlogin_r_loginuid): Return
            early when linux sentinel value is set.
    
    Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>

Index: b/sysdeps/unix/sysv/linux/getlogin_r.c
===================================================================
--- a/sysdeps/unix/sysv/linux/getlogin_r.c
+++ b/sysdeps/unix/sysv/linux/getlogin_r.c
@@ -54,6 +54,15 @@ __getlogin_r_loginuid (char *name, size_
 	  endp == uidbuf || *endp != '\0'))
     return -1;
 
+  /* If there is no login uid, linux sets /proc/self/loginid to the sentinel
+     value of, (uid_t) -1, so check if that value is set and return early to
+     avoid making unneeded nss lookups. */
+  if (uid == (uid_t) -1)
+    {
+      __set_errno (ENXIO);
+      return ENXIO;
+    }
+
   size_t buflen = 1024;
   char *buf = alloca (buflen);
   bool use_malloc = false;