Don't open /dev/null until we need to.
This saves a couple of syscalls in the common case, and also lets static
binaries run in a chroot without /dev/null as long as
stdin/stdout/stderr are actually connected to something (which the
toybox maintainer tried to do).
Test: manual with strace
Change-Id: Ic9a28896a07304a3bd428acfd9ddca9d22015f6e
diff --git a/libc/bionic/libc_init_common.cpp b/libc/bionic/libc_init_common.cpp
index dd623a5..8084e73 100644
--- a/libc/bionic/libc_init_common.cpp
+++ b/libc/bionic/libc_init_common.cpp
@@ -149,50 +149,25 @@
_exit(EXIT_FAILURE);
}
-// Force any of the closed stdin, stdout and stderr to be associated with /dev/null.
+// Force any of the stdin/stdout/stderr file descriptors that aren't
+// open to be associated with /dev/null.
static void __nullify_closed_stdio() {
- int dev_null = TEMP_FAILURE_RETRY(open("/dev/null", O_RDWR));
- if (dev_null == -1) {
- // init won't have /dev/null available, but SELinux provides an equivalent.
- dev_null = TEMP_FAILURE_RETRY(open("/sys/fs/selinux/null", O_RDWR));
- }
- if (dev_null == -1) {
- __early_abort(__LINE__);
- }
-
- // If any of the stdio file descriptors is valid and not associated
- // with /dev/null, dup /dev/null to it.
for (int i = 0; i < 3; i++) {
- // If it is /dev/null already, we are done.
- if (i == dev_null) {
- continue;
- }
+ if (TEMP_FAILURE_RETRY(fcntl(i, F_GETFL)) == -1) {
+ // The only error we allow is that the file descriptor does not exist.
+ if (errno != EBADF) __early_abort(__LINE__);
- // Is this fd already open?
- int status = TEMP_FAILURE_RETRY(fcntl(i, F_GETFL));
- if (status != -1) {
- continue;
- }
-
- // The only error we allow is that the file descriptor does not
- // exist, in which case we dup /dev/null to it.
- if (errno == EBADF) {
- // Try dupping /dev/null to this stdio file descriptor and
- // repeat if there is a signal. Note that any errors in closing
- // the stdio descriptor are lost.
- status = TEMP_FAILURE_RETRY(dup2(dev_null, i));
- if (status == -1) {
+ // This file descriptor wasn't open, so open /dev/null.
+ // init won't have /dev/null available, but SELinux provides an equivalent.
+ // This takes advantage of the fact that open() will take the lowest free
+ // file descriptor, and we're iterating in order from 0, but we'll
+ // double-check we got the right fd anyway...
+ int fd;
+ if (((fd = TEMP_FAILURE_RETRY(open("/dev/null", O_RDWR))) == -1 &&
+ (fd = TEMP_FAILURE_RETRY(open("/sys/fs/selinux/null", O_RDWR))) == -1) ||
+ fd != i) {
__early_abort(__LINE__);
}
- } else {
- __early_abort(__LINE__);
- }
- }
-
- // If /dev/null is not one of the stdio file descriptors, close it.
- if (dev_null > 2) {
- if (close(dev_null) == -1) {
- __early_abort(__LINE__);
}
}
}