[PATCH] support_become_root: Don't fail when /proc/<pid/setgroups is missing

[Christian Brauner <christian dot brauner at ubuntu dot com>]

The requirement to write "deny" to /proc/<pid>/setgroups for a given user
namespace before being able to write a gid mapping was introduced in Linux 3.19.
Before that this requirement including the file did not exist. So don't fail
when errno == ENOENT.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 ChangeLog                     |  5 +++++
 support/support_become_root.c | 20 +++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/ChangeLog b/ChangeLog
index 604d571ca6..2f2d3b82ff 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2017-11-17  Christian Brauner <christian.brauner@ubuntu.com>
+
+	* support/support_become_root.c (setup_uid_gid_mapping): Don't fail when
+	/proc/<pid>/setgroups does not exist.
+
 2017-11-17  Tulio Magno Quites Machado Filho  <tuliom@linux.vnet.ibm.com>
 
 	* sysdeps/powerpc/bits/hwcap.h (PPC_FEATURE2_HTM_NO_SUSPEND): New
diff --git a/support/support_become_root.c b/support/support_become_root.c
index 5086570251..01a386c20c 100644
--- a/support/support_become_root.c
+++ b/support/support_become_root.c
@@ -18,6 +18,7 @@
 
 #include <support/namespace.h>
 
+#include <errno.h>
 #include <fcntl.h>
 #include <sched.h>
 #include <stdio.h>
@@ -50,11 +51,20 @@ setup_uid_gid_mapping (uid_t original_uid, gid_t original_gid)
   xwrite (fd, buf, ret);
   xclose (fd);
 
-  /* Disable setgroups before mapping groups, otherwise that would
-     fail with EPERM.  */
-  fd = xopen ("/proc/self/setgroups", O_WRONLY, 0);
-  xwrite (fd, "deny\n", strlen ("deny\n"));
-  xclose (fd);
+  /* Linux 3.19 introduced the setgroups file. We need write "deny" to this file
+     otherwise writing to gid_map will fail with EPERM. */
+  fd = open64 ("/proc/self/setgroups", O_WRONLY, 0);
+  if (fd < 0)
+    {
+      if (errno != ENOENT)
+        FAIL_EXIT1 ("open64 (\"/proc/self/setgroups\", 0x%x, 0%o): %m", O_WRONLY, 0);
+      /* This kernel doesn't expose the setgroups file so simply move on. */
+    }
+  else
+    {
+      xwrite (fd, "deny\n", strlen ("deny\n"));
+      xclose (fd);
+    }
 
   /* Now map our own GID, like we did for the user ID.  */
   fd = xopen ("/proc/self/gid_map", O_WRONLY, 0);
-- 
2.14.1