posix: Add posix_spawn_file_actions_addclosefrom_np

This patch adds a way to close a range of file descriptors on
posix_spawn as a new file action.  The API is similar to the one
provided by Solaris 11 [1], where the file action causes the all open
file descriptors greater than or equal to input on to be closed when
the new process is spawned.

The function posix_spawn_file_actions_addclosefrom_np is safe to be
implemented by iterating over /proc/self/fd, since the Linux spawni.c
helper process does not use CLONE_FILES, so its has own file descriptor
table and any failure (in /proc operation) aborts the process creation
and returns an error to the caller.

I am aware that this file action might be redundant to the current
approach of POSIX in promoting O_CLOEXEC in more interfaces. However
O_CLOEXEC is still not the default and for some specific usages, the
caller needs to close all possible file descriptors to avoid them
leaking.  Some examples are CPython (discussed in BZ#10353) and OpenJDK
jspawnhelper [2] (where OpenJDK spawns a helper process to exactly
closes all file descriptors).  Most likely any environment which calls
functions that might open file descriptor under the hood and aim to use
posix_spawn might face the same requirement.

Checked on x86_64-linux-gnu and i686-linux-gnu on kernel 5.11 and 4.15.

[1] https://docs.oracle.com/cd/E36784_01/html/E36874/posix-spawn-file-actions-addclosefrom-np-3c.html
[2] https://github.com/openjdk/jdk/blob/master/src/java.base/unix/native/libjava/childproc.c#L82

1 files changed, 57 insertions, 0 deletions

diff --git a/posix/spawn_faction_addclosefrom.c b/posix/spawn_faction_addclosefrom.c
new file mode 100644
index 0000000000..7630db8925
--- /dev/null
+++ b/posix/spawn_faction_addclosefrom.c
@@ -0,0 +1,57 @@
+/* Add a closefrom to a file action list for posix_spawn.
+   Copyright (C) 2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library 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
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <errno.h>
+#include <spawn.h>
+#include <unistd.h>
+#include <spawn_int.h>
+
+int
+__posix_spawn_file_actions_addclosefrom (posix_spawn_file_actions_t
+					 *file_actions, int from)
+{
+#if __SPAWN_SUPPORT_CLOSEFROM
+  struct __spawn_action *rec;
+
+  if (!__spawn_valid_fd (from))
+    return EBADF;
+
+  /* Allocate more memory if needed.  */
+  if (file_actions->__used == file_actions->__allocated
+      && __posix_spawn_file_actions_realloc (file_actions) != 0)
+    /* This can only mean we ran out of memory.  */
+    return ENOMEM;
+
+  /* Add the new value.  */
+  rec = &file_actions->__actions[file_actions->__used];
+  rec->tag = spawn_do_closefrom;
+  rec->action.closefrom_action.from = from;
+
+  /* Account for the new entry.  */
+  ++file_actions->__used;
+
+  return 0;
+#else
+  return EINVAL;
+#endif
+}
+weak_alias (__posix_spawn_file_actions_addclosefrom,
+	    posix_spawn_file_actions_addclosefrom_np)
+#if !__SPAWN_SUPPORT_CLOSEFROM
+stub_warning (posix_spawn_file_actions_addclosefrom_np)
+#endif