Add DropPrivileges()

util/posix/drop_privileges.h

Index: util/posix/drop_privileges.h
diff --git a/util/posix/drop_privileges.h b/util/posix/drop_privileges.h
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_UTIL_POSIX_DROP_PRIVILEGES_H_
+#define CRASHPAD_UTIL_POSIX_DROP_PRIVILEGES_H_
+
+namespace crashpad {
+
+//! \brief Permanently drops privileges conferred by being a setuid or setgid
+//!     executable.
+//!
+//! The effective user ID and saved set-user ID are set to the real user ID,
+//! negating any effects of being a setuid executable. The effective group ID
+//! and saved set-group ID are set to the real group ID, negating any effects of
+//! being a setgid executable. Because the saved set-user ID and saved set-group
+//! ID are reset, there is no way to restore the prior privileges, and the drop
+//! is permanent.
+//!
+//! This function drops privileges correctly when running setuid root and in
+//! other circumstances, including when running setuid non-root. If the program
+//! is not a setuid or setgid executable, this function has no effect.
+//!
+//! No changes are made to the supplementary group list, which is normally not
+//! altered for setuid or setgid executables.
+void DropPrivileges();
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_UTIL_POSIX_DROP_PRIVILEGES_H_