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>

1 files changed, 9 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/getlogin_r.c b/sysdeps/unix/sysv/linux/getlogin_r.c
index 73ea14c8f9..14587712a7 100644
--- a/sysdeps/unix/sysv/linux/getlogin_r.c
+++ b/sysdeps/unix/sysv/linux/getlogin_r.c
@@ -55,6 +55,15 @@ __getlogin_r_loginuid (char *name, size_t namesize)
 	  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;
+    }
+
   struct passwd pwd;
   struct passwd *tpwd;
   int result = 0;