tequilaOS/platform_bionic
2
0
Fork 0
Code Issues Pull requests Projects Releases Packages Wiki Activity

Don't open /dev/null until we need to.

Browse source 
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
This commit is contained in:
Elliott Hughes 2021-10-26 17:31:03 -07:00
parent d8854293d1
commit 7a2386bf89
1 changed files with 14 additions and 39 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

53
libc/bionic/libc_init_common.cpp
View file

@ -149,50 +149,25 @@ __noreturn static void __early_abort(int line) {
_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__);
}
}
}