Use anonymous shared memory for tempfiles

SHM_ANON on FreeBSD, memfd on recent Linux.

- avoids usage of posix_fallocate on Copy-on-Write filesystems like ZFS
  when XDG_RUNTIME_DIR is something like ~/.tmp instead of a tmpfs
  (FreeBSD 12 does not even allow it on ZFS anymore)
- avoids touching the filesystem, which increases sandboxing potential

Author: valpackett

configure.ac

@@ -102,7 +102,7 @@ AC_CHECK_DECL(TFD_CLOEXEC,[],
 AC_CHECK_DECL(CLOCK_MONOTONIC,[],
 	      [AC_MSG_ERROR("CLOCK_MONOTONIC is needed to compile weston")],
 	      [[#include <time.h>]])
-AC_CHECK_HEADERS([execinfo.h])
+AC_CHECK_HEADERS([execinfo.h linux/memfd.h])
 
 AC_CHECK_FUNCS([mkostemp strchrnul initgroups posix_fallocate])
 

shared/os-compatibility.c

@@ -25,6 +25,13 @@
 
 #include "config.h"
 
+#ifdef __FreeBSD__
+#include <sys/mman.h>
+#elif HAVE_LINUX_MEMFD_H
+#define _GNU_SOURCE
+#include <sys/syscall.h>
+#include <linux/memfd.h>
+#endif
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <unistd.h>
@@ -107,12 +114,15 @@ os_epoll_create_cloexec(void)
 	return set_cloexec_or_close(fd);
 }
 
+#ifndef __FreeBSD__
 static int
 create_tmpfile_cloexec(char *tmpname)
 {
 	int fd;
 
-#ifdef HAVE_MKOSTEMP
+#ifdef HAVE_LINUX_MEMFD_H
+	fd = syscall(SYS_memfd_create, tmpname, MFD_CLOEXEC);
+#elif HAVE_MKOSTEMP
 	fd = mkostemp(tmpname, O_CLOEXEC);
 	if (fd >= 0)
 		unlink(tmpname);
@@ -126,6 +136,7 @@ create_tmpfile_cloexec(char *tmpname)
 
 	return fd;
 }
+#endif
 
 /*
  * Create a new, unique, anonymous file of the given size, and
@@ -151,11 +162,13 @@ create_tmpfile_cloexec(char *tmpname)
 int
 os_create_anonymous_file(off_t size)
 {
+	int fd, ret;
+#ifdef __FreeBSD__
+	fd = shm_open(SHM_ANON, O_CREAT | O_RDWR, 0600); // shm_open is always CLOEXEC
+#else
 	static const char template[] = "/weston-shared-XXXXXX";
 	const char *path;
 	char *name;
-	int fd;
-	int ret;
 
 	path = getenv("XDG_RUNTIME_DIR");
 	if (!path) {
@@ -173,11 +186,12 @@ os_create_anonymous_file(off_t size)
 	fd = create_tmpfile_cloexec(name);
 
 	free(name);
+#endif
 
 	if (fd < 0)
 		return -1;
 
-#ifdef HAVE_POSIX_FALLOCATE
+#if defined(HAVE_POSIX_FALLOCATE) && !defined(__FreeBSD__)
 	do {
 		ret = posix_fallocate(fd, 0, size);
 	} while (ret == EINTR);