[glibc/maskray/grte] getlogin_r: return early when linux sentinel value is set

[glibc/maskray/grte] getlogin_r: return early when linux sentinel value is set

[Fangrui Song]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=dadc309087c7acab517ed3e42546aa859ae9fb45

commit dadc309087c7acab517ed3e42546aa859ae9fb45
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>
    (cherry picked from commit cc8a1620eb97ccddd337d157263c13c57b39ab71)

Diff:
---
 ChangeLog                            | 5 +++++
 sysdeps/unix/sysv/linux/getlogin_r.c | 9 +++++++++
 2 files changed, 14 insertions(+)

diff --git a/ChangeLog b/ChangeLog
index 89ecd49d19..670c6d94b8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2018-03-27  Jesse Hathaway  <jesse@mbuki-mvuki.org>
+
+	* sysdeps/unix/sysv/linux/getlogin_r.c (__getlogin_r_loginuid): Return
+	early when linux sentinel value is set.
+
 2018-03-27  Andreas Schwab  <schwab@suse.de>
 
 	[BZ #23005]
diff --git a/sysdeps/unix/sysv/linux/getlogin_r.c b/sysdeps/unix/sysv/linux/getlogin_r.c
index 84c51d0ecd..7a814ea92f 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;