init: use SELinux /dev/null if available
SELinux provides it's own /dev/null character device at
/sys/fs/selinux/null. This character device is exactly the same
as /dev/null, including the same major/minor numbers, and can
be used wherever /dev/null is used.
Use /sys/fs/selinux/null instead of trying to create our own
/dev/__null__ device. This moves us one step closer to eliminating
all uses of mknod() by init.
/sys/fs/selinux/null is only available once the /sys/fs/selinux filesystem
is mounted. It's not available to the first stage init, so we
still have to fall back to mknod then.
Change-Id: Ic733767ea6220a130537de33cc478ae79578ce20
diff --git a/init/util.cpp b/init/util.cpp
index 332aa2a..b7fb867 100644
--- a/init/util.cpp
+++ b/init/util.cpp
@@ -379,23 +379,28 @@
void open_devnull_stdio(void)
{
- int fd;
- static const char *name = "/dev/__null__";
- if (mknod(name, S_IFCHR | 0600, (1 << 8) | 3) == 0) {
- fd = open(name, O_RDWR);
- unlink(name);
- if (fd >= 0) {
- dup2(fd, 0);
- dup2(fd, 1);
- dup2(fd, 2);
- if (fd > 2) {
- close(fd);
- }
- return;
+ // Try to avoid the mknod() call if we can. Since SELinux makes
+ // a /dev/null replacement available for free, let's use it.
+ int fd = open("/sys/fs/selinux/null", O_RDWR);
+ if (fd == -1) {
+ // OOPS, /sys/fs/selinux/null isn't available, likely because
+ // /sys/fs/selinux isn't mounted. Fall back to mknod.
+ static const char *name = "/dev/__null__";
+ if (mknod(name, S_IFCHR | 0600, (1 << 8) | 3) == 0) {
+ fd = open(name, O_RDWR);
+ unlink(name);
+ }
+ if (fd == -1) {
+ exit(1);
}
}
- exit(1);
+ dup2(fd, 0);
+ dup2(fd, 1);
+ dup2(fd, 2);
+ if (fd > 2) {
+ close(fd);
+ }
}
void import_kernel_cmdline(int in_qemu,