[2.26 COMMITTED] getlogin_r: return early when linux sentinel value is set

[2.26 COMMITTED] getlogin_r: return early when linux sentinel value is set

[Florian Weimer]

From: Jesse Hathaway <jesse@mbuki-mvuki.org>

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>
(cherry picked from commit cc8a1620eb97ccddd337d157263c13c57b39ab71)

2018-03-27  Jesse Hathaway  <jesse@mbuki-mvuki.org>

	[BZ #23024]
	* sysdeps/unix/sysv/linux/getlogin_r.c (__getlogin_r_loginuid): Return
	early when linux sentinel value is set.

diff --git a/NEWS b/NEWS
index 8c432b3e6e..dcbb2b0ff6 100644
--- a/NEWS
+++ b/NEWS
@@ -118,6 +118,7 @@ The following bugs are resolved with this release:
   [22685] powerpc: Fix syscalls during early process initialization
   [22715] x86-64: Properly align La_x86_64_retval to VEC_SIZE
   [22774] malloc: Integer overflow in malloc (CVE-2018-6551)
+  [23024] getlogin_r: return early when linux sentinel value is set
   [23037] resolv: Fully initialize struct mmsghdr in send_dg
 \f
 Version 2.26
diff --git a/sysdeps/unix/sysv/linux/getlogin_r.c b/sysdeps/unix/sysv/linux/getlogin_r.c
index 05ac36b491..fe81fd196a 100644
--- 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_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;
+    }
+
   size_t buflen = 1024;
   char *buf = alloca (buflen);
   bool use_malloc = false;