libxl: Protect fds with CLOEXEC even with forking threads

We introduce a new "carefd" concept, which relates to fds that we care
about not being inherited by long-lived children.

As yet we do not use this anywhere in libxl.  Until all locations in
libxl which make such fds are converted, libxl__postfork may not work
entirely properly.  If these locations do not use O_CLOEXEC (or use
calls for which there is no O_CLOEXEC) then multithreaded programs may
not work properly.

This introduces a new API call libxl_postfork_child_noexec which must
be called by applications which make long-running non-execing
children.  Add the appropriate call to xl's postfork function.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <Ian.Jackson@eu.citrix.com>

tools/libxl/Makefile

@@ -53,7 +53,7 @@ LIBXL_LIBS += -lyajl
 LIBXL_OBJS = flexarray.o libxl.o libxl_create.o libxl_dm.o libxl_pci.o \
 			libxl_dom.o libxl_exec.o libxl_xshelp.o libxl_device.o \
 			libxl_internal.o libxl_utils.o libxl_uuid.o libxl_json.o \
-			libxl_qmp.o libxl_event.o $(LIBXL_OBJS-y)
+			libxl_qmp.o libxl_event.o libxl_fork.o $(LIBXL_OBJS-y)
 LIBXL_OBJS += _libxl_types.o libxl_flask.o _libxl_types_internal.o
 
 $(LIBXL_OBJS): CFLAGS += $(CFLAGS_libxenctrl) $(CFLAGS_libxenguest) $(CFLAGS_libxenstore) $(CFLAGS_libblktapctl) -include $(XEN_ROOT)/tools/config.h

tools/libxl/libxl.c

@@ -68,6 +68,9 @@ int libxl_ctx_alloc(libxl_ctx **pctx, int version,
 
     /* Now ctx is safe for ctx_free; failures simply set rc and "goto out" */
 
+    rc = libxl__atfork_init(ctx);
+    if (rc) goto out;
+
     rc = libxl__poller_init(ctx, &ctx->poller_app);
     if (rc) goto out;
 

tools/libxl/libxl_event.h

@@ -371,6 +371,19 @@ void libxl_osevent_occurred_fd(libxl_ctx *ctx, void *for_libxl,
  */
 void libxl_osevent_occurred_timeout(libxl_ctx *ctx, void *for_libxl);
 
+
+/*
+ * An application which initialises a libxl_ctx in a parent process
+ * and then forks a child which does not quickly exec, must
+ * instead libxl_postfork_child_noexec in the child.  One call
+ * on any existing (or specially made) ctx is sufficient; after
+ * this all previously existing libxl_ctx's are invalidated and
+ * must not be used - or even freed.  It is harmless to call this
+ * postfork function and then exec anyway.
+ */
+void libxl_postfork_child_noexec(libxl_ctx *ctx);
+
+
 #endif
 
 /*

tools/libxl/libxl_fork.c

@@ -0,0 +1,149 @@
+/*
+ * Copyright (C) 2012      Citrix Ltd.
+ *
+ * This program 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; version 2.1 only. with the special
+ * exception on linking described in file LICENSE.
+ *
+ * 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 Lesser General Public License for more details.
+ */
+/*
+ * Internal child process machinery for use by other parts of libxl
+ */
+
+#include "libxl_osdeps.h" /* must come before any other headers */
+
+#include "libxl_internal.h"
+
+/*
+ * carefd arrangements
+ *
+ * carefd_begin and _unlock take out the no_forking lock, which we
+ * also take and release in our pthread_atfork handlers.  So when this
+ * lock is held the whole process cannot fork.  We therefore protect
+ * our fds from leaking into children made by other threads.
+ *
+ * We maintain a list of all the carefds, so that if the application
+ * wants to fork a long-running but non-execing child, we can close
+ * them all.
+ *
+ * So the record function sets CLOEXEC for the benefit of execing
+ * children, and makes a note of the fd for the benefit of non-execing
+ * ones.
+ */
+
+struct libxl__carefd {
+    LIBXL_LIST_ENTRY(libxl__carefd) entry;
+    int fd;
+};
+
+static pthread_mutex_t no_forking = PTHREAD_MUTEX_INITIALIZER;
+static int atfork_registered;
+static LIBXL_LIST_HEAD(, libxl__carefd) carefds =
+    LIBXL_LIST_HEAD_INITIALIZER(carefds);
+
+static void atfork_lock(void)
+{
+    int r = pthread_mutex_lock(&no_forking);
+    assert(!r);
+}
+
+static void atfork_unlock(void)
+{
+    int r = pthread_mutex_unlock(&no_forking);
+    assert(!r);
+}
+
+int libxl__atfork_init(libxl_ctx *ctx)
+{
+    int r, rc;
+    
+    atfork_lock();
+    if (atfork_registered) { rc = 0; goto out; }
+
+    r = pthread_atfork(atfork_lock, atfork_unlock, atfork_unlock);
+    if (r) {
+        assert(r == ENOMEM);
+        libxl__alloc_failed(ctx, __func__, 0,0);
+    }
+
+    atfork_registered = 1;
+    rc = 0;
+ out:
+    atfork_unlock();
+    return rc;
+}
+
+void libxl__carefd_begin(void) { atfork_lock(); }
+void libxl__carefd_unlock(void) { atfork_unlock(); }
+
+libxl__carefd *libxl__carefd_record(libxl_ctx *ctx, int fd)
+{
+    libxl__carefd *cf = 0;
+
+    libxl_fd_set_cloexec(ctx, fd, 1);
+    cf = libxl__zalloc(NULL, sizeof(*cf));
+    cf->fd = fd;
+    LIBXL_LIST_INSERT_HEAD(&carefds, cf, entry);
+    return cf;
+}
+
+libxl__carefd *libxl__carefd_opened(libxl_ctx *ctx, int fd)
+{
+    libxl__carefd *cf = 0;
+
+    cf = libxl__carefd_record(ctx, fd);
+    libxl__carefd_unlock();
+    return cf;
+}
+
+void libxl_postfork_child_noexec(libxl_ctx *ctx)
+{
+    libxl__carefd *cf, *cf_tmp;
+    int r;
+
+    atfork_lock();
+    LIBXL_LIST_FOREACH_SAFE(cf, &carefds, entry, cf_tmp) {
+        if (cf->fd >= 0) {
+            r = close(cf->fd);
+            if (r)
+                LIBXL__LOG_ERRNO(ctx, LIBXL__LOG_WARNING,
+                                 "failed to close fd=%d"
+                                 " (perhaps of another libxl ctx)", cf->fd);
+        }
+        free(cf);
+    }
+    LIBXL_LIST_INIT(&carefds);
+    atfork_unlock();
+}
+
+int libxl__carefd_close(libxl__carefd *cf)
+{
+    if (!cf) return 0;
+    int r = cf->fd < 0 ? 0 : close(cf->fd);
+    int esave = errno;
+    atfork_lock();
+    LIBXL_LIST_REMOVE(cf, entry);
+    atfork_unlock();
+    free(cf);
+    errno = esave;
+    return r;
+}
+
+int libxl__carefd_fd(const libxl__carefd *cf)
+{
+    if (!cf) return -1;
+    return cf->fd;
+}
+
+/*
+ * Local variables:
+ * mode: C
+ * c-basic-offset: 4
+ * indent-tabs-mode: nil
+ * End:
+ */

tools/libxl/libxl_internal.h

@@ -611,6 +611,9 @@ void libxl__poller_put(libxl_ctx *ctx, libxl__poller *p);
 void libxl__poller_wakeup(libxl__egc *egc, libxl__poller *p);
 
 
+int libxl__atfork_init(libxl_ctx *ctx);
+
+
 /* from xl_dom */
 _hidden libxl_domain_type libxl__domain_type(libxl__gc *gc, uint32_t domid);
 _hidden int libxl__domain_shutdown_reason(libxl__gc *gc, uint32_t domid);
@@ -1307,6 +1310,66 @@ _hidden void libxl__ao_complete(libxl__egc *egc, libxl__ao *ao, int rc);
 /* For use by ao machinery ONLY */
 _hidden void libxl__ao__destroy(libxl_ctx*, libxl__ao *ao);
 
+
+/*
+ * File descriptors and CLOEXEC
+ */
+
+/*
+ * For libxl functions which create file descriptors, at least one
+ * of the following must be true:
+ *  (a) libxl does not care if copies of this open-file are inherited
+ *      by random children and might remain open indefinitely
+ *  (b) libxl must take extra care for the fd (the actual descriptor,
+ *      not the open-file) as below.  We call this a "carefd".
+ *
+ * The rules for opening a carefd are:
+ *  (i)   Before bringing any carefds into existence,
+ *        libxl code must call libxl__carefd_begin.
+ *  (ii)  Then for each carefd brought into existence,
+ *        libxl code must call libxl__carefd_record
+ *        and remember the libxl__carefd_record*.
+ *  (iii) Then it must call libxl__carefd_unlock.
+ *  (iv)  When in a child process the fd is to be passed across
+ *        exec by libxl, the libxl code must unset FD_CLOEXEC
+ *        on the fd eg by using libxl_fd_set_cloexec.
+ *  (v)   Later, when the fd is to be closed in the same process,
+ *        libxl code must not call close.  Instead, it must call
+ *        libxl__carefd_close.
+ * Steps (ii) and (iii) can be combined by calling the convenience
+ * function libxl__carefd_opened.
+ */
+/* libxl__carefd_begin and _unlock (or _opened) must be called always
+ * in pairs.  They may be called with the CTX lock held.  In between
+ * _begin and _unlock, the following are prohibited:
+ *   - anything which might block
+ *   - any callbacks to the application
+ *   - nested calls to libxl__carefd_begin
+ *   - fork (libxl__fork)
+ * In general nothing should be done before _unlock that could be done
+ * afterwards.
+ */
+typedef struct libxl__carefd libxl__carefd;
+
+void libxl__carefd_begin(void);
+void libxl__carefd_unlock(void);
+
+/* fd may be -1, in which case this returns a dummy libxl__fd_record
+ * on which it _carefd_close is a no-op.  Cannot fail. */
+libxl__carefd *libxl__carefd_record(libxl_ctx *ctx, int fd);
+
+/* Combines _record and _unlock in a single call.  If fd==-1,
+ * still does the unlock, but returns 0.  Cannot fail. */
+libxl__carefd *libxl__carefd_opened(libxl_ctx *ctx, int fd);
+
+/* Works just like close(2).  You may pass NULL, in which case it's
+ * a successful no-op. */
+int libxl__carefd_close(libxl__carefd*);
+
+/* You may pass NULL in which case the answer is -1. */
+int libxl__carefd_fd(const libxl__carefd*);
+
+
 /*
  * Convenience macros.
  */

tools/libxl/xl.c

@@ -96,6 +96,9 @@ static void parse_global_config(const char *configfile,
 
 void postfork(void)
 {
+    libxl_postfork_child_noexec(ctx); /* in case we don't exit/exec */
+    ctx = 0;
+
     if (libxl_ctx_alloc(&ctx, LIBXL_VERSION, 0, (xentoollog_logger*)logger)) {
         fprintf(stderr, "cannot reinit xl context after fork\n");
         exit(-1);