s3:lib: add per_thread_cwd_{check,supported,disable,activate}() helper functions

Only Linux it's possible to have a per thread current working directory
using unshare(CLONE_FS).

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>

source3/include/proto.h

@@ -1008,4 +1008,11 @@ void contend_level2_oplocks_begin(files_struct *fsp,
 void contend_level2_oplocks_end(files_struct *fsp,
 				enum level2_contention_type type);
 
+/* The following definitions come from lib/per_thread_cwd.c */
+
+void per_thread_cwd_check(void);
+bool per_thread_cwd_supported(void);
+void per_thread_cwd_disable(void);
+void per_thread_cwd_activate(void);
+
 #endif /*  _PROTO_H_  */

source3/lib/per_thread_cwd.c

@@ -0,0 +1,156 @@
+/*
+   Unix SMB/Netbios implementation.
+
+   Copyright (C) Ralph Boehme 2019
+   Copyright (C) Stefan Metzmacher 2019
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include "includes.h"
+#include "system/filesys.h"
+#include "system/threads.h"
+#ifdef HAVE_UNSHARE_CLONE_FS
+#include <sched.h>
+#endif /* HAVE_UNSHARE_CLONE_FS */
+
+static bool _per_thread_cwd_checked;
+static bool _per_thread_cwd_supported;
+#ifdef HAVE_UNSHARE_CLONE_FS
+static __thread bool _per_thread_cwd_disabled;
+static __thread bool _per_thread_cwd_activated;
+#endif /* HAVE_UNSHARE_CLONE_FS */
+
+/*
+ * This is the first function to be called!
+ * Typically in the main() function before
+ * any threads are created.
+ *
+ * This can be called multiple times
+ * as the result is cached the first time.
+ */
+void per_thread_cwd_check(void)
+{
+	if (_per_thread_cwd_checked) {
+		return;
+	}
+
+#ifdef HAVE_UNSHARE_CLONE_FS
+	/*
+	 * While unshare(CLONE_FS) is available on
+	 * Linux for ages, unshare() is also
+	 * used to implement containers with various
+	 * per container namespaces.
+	 *
+	 * It's possible that the whole unshare()
+	 * is blocked in order to disallow neested
+	 * containers.
+	 *
+	 * That's why we sadly need a runtime check
+	 * for this.
+	 */
+	{
+		int res;
+
+		res = unshare(CLONE_FS);
+		if (res == 0) {
+			_per_thread_cwd_supported = true;
+		}
+	}
+
+	/*
+	 * We're in the main thread, so we should disallow
+	 * per_thread_cwd_activate() here.
+	 */
+	_per_thread_cwd_disabled = true;
+#endif /* HAVE_UNSHARE_CLONE_FS */
+
+	_per_thread_cwd_checked = true;
+}
+
+/*
+ * In order to use per_thread_cwd_supported()
+ * per_thread_cwd_check() needs to be called first!
+ * Otherwise an assert will be triggered!
+ */
+bool per_thread_cwd_supported(void)
+{
+	SMB_ASSERT(_per_thread_cwd_checked);
+	return _per_thread_cwd_supported;
+}
+
+/*
+ * In order to use per_thread_cwd_disable()
+ * should be called after any fork() in order
+ * to mark the main thread of the process,
+ * which should disallow per_thread_cwd_activate().
+ *
+ * This can be called without calling
+ * per_thread_cwd_check() first.
+ *
+ * And it can't be called after calling
+ * per_thread_cwd_activate()!
+ * Otherwise an assert will be triggered!
+ *
+ * This can be called multiple times
+ * as the result is cached the first time.
+ */
+void per_thread_cwd_disable(void)
+{
+#ifdef HAVE_UNSHARE_CLONE_FS
+	SMB_ASSERT(!_per_thread_cwd_activated);
+	if (_per_thread_cwd_disabled) {
+		return;
+	}
+	_per_thread_cwd_disabled = true;
+#endif /* HAVE_UNSHARE_CLONE_FS */
+}
+
+/*
+ * In order to use per_thread_cwd_activate()
+ * per_thread_cwd_supported() needs to be checked first!
+ * Otherwise an assert will be triggered!
+ *
+ * This MUST only be called within helper threads!
+ *
+ * That means it can't be called after calling
+ * per_thread_cwd_disable()!
+ * Otherwise an assert will be triggered!
+ *
+ * This can be called multiple times
+ * as the result is cached the first time.
+ */
+void per_thread_cwd_activate(void)
+{
+	SMB_ASSERT(_per_thread_cwd_checked);
+	SMB_ASSERT(_per_thread_cwd_supported);
+
+#ifdef HAVE_UNSHARE_CLONE_FS
+	if (_per_thread_cwd_activated) {
+		return;
+	}
+
+	SMB_ASSERT(!_per_thread_cwd_disabled);
+
+	{
+		int ret;
+		ret = unshare(CLONE_FS);
+		SMB_ASSERT(ret == 0);
+	}
+
+	_per_thread_cwd_activated = true;
+#else /* not HAVE_UNSHARE_CLONE_FS */
+	smb_panic(__location__);
+#endif /* not HAVE_UNSHARE_CLONE_FS */
+}

source3/wscript_build

@@ -968,6 +968,7 @@ bld.SAMBA3_LIBRARY('samba3-util',
                           lib/namearray.c
                           lib/file_id.c
                           lib/cbuf.c
+                          lib/per_thread_cwd.c
                           ''',
                    deps='''
                         samba-util