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/* SPDX-License-Identifier: LGPL-2.1-or-later */
#include <errno.h>
#include <stddef.h>
#include <sys/prctl.h>
#include <sys/wait.h>
#include <unistd.h>
#include "async.h"
#include "errno-util.h"
#include "fd-util.h"
#include "log.h"
#include "macro.h"
#include "process-util.h"
#include "signal-util.h"
int asynchronous_sync(pid_t *ret_pid) {
int r;
/* This forks off an invocation of fork() as a child process, in order to initiate synchronization to
* disk. Note that we implement this as helper process rather than thread as we don't want the sync() to hang our
* original process ever, and a thread would do that as the process can't exit with threads hanging in blocking
* syscalls. */
r = safe_fork("(sd-sync)", FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS|(ret_pid ? 0 : FORK_DETACH), ret_pid);
if (r < 0)
return r;
if (r == 0) {
/* Child process */
sync();
_exit(EXIT_SUCCESS);
}
return 0;
}
/* We encode the fd to close in the userdata pointer as an unsigned value. The highest bit indicates whether
* we need to fork again */
#define NEED_DOUBLE_FORK (1U << (sizeof(unsigned) * 8 - 1))
static int close_func(void *p) {
unsigned v = PTR_TO_UINT(p);
(void) prctl(PR_SET_NAME, (unsigned long*) "(sd-close)");
/* Note: 💣 This function is invoked in a child process created via glibc's clone() wrapper. In such
* children memory allocation is not allowed, since glibc does not release malloc mutexes in
* clone() 💣 */
if (v & NEED_DOUBLE_FORK) {
pid_t pid;
v &= ~NEED_DOUBLE_FORK;
/* This inner child will be reparented to the subreaper/PID 1. Here we turn on SIGCHLD, so
* that the reaper knows when it's time to reap. */
pid = clone_with_nested_stack(close_func, SIGCHLD|CLONE_FILES, UINT_TO_PTR(v));
if (pid >= 0)
return 0;
}
close((int) v); /* no assert() here, we are in the child and the result would be eaten up anyway */
return 0;
}
int asynchronous_close(int fd) {
unsigned v;
pid_t pid;
int r;
/* This is supposed to behave similar to safe_close(), but actually invoke close() asynchronously, so
* that it will never block. Ideally the kernel would have an API for this, but it doesn't, so we
* work around it, and hide this as a far away as we can.
*
* It is important to us that we don't use threads (via glibc pthread) in PID 1, hence we'll do a
* minimal subprocess instead which shares our fd table via CLONE_FILES. */
if (fd < 0)
return -EBADF; /* already invalid */
PROTECT_ERRNO;
v = (unsigned) fd;
/* We want to fork off a process that is automatically reaped. For that we'd usually double-fork. But
* we can optimize this a bit: if we are PID 1 or a subreaper anyway (the systemd service manager
* process qualifies as this), we can avoid the double forking, since the double forked process would
* be reparented back to us anyway. */
r = is_reaper_process();
if (r < 0)
log_debug_errno(r, "Cannot determine if we are a reaper process, assuming we are not: %m");
if (r <= 0)
v |= NEED_DOUBLE_FORK;
pid = clone_with_nested_stack(close_func, CLONE_FILES | ((v & NEED_DOUBLE_FORK) ? 0 : SIGCHLD), UINT_TO_PTR(v));
if (pid < 0)
safe_close(fd); /* local fallback */
else if (v & NEED_DOUBLE_FORK) {
/* Reap the intermediate child. Key here is that we specify __WCLONE, since we didn't ask for
* any signal to be sent to us on process exit, and otherwise waitid() would refuse waiting
* then.
*
* We usually prefer calling waitid(), but before kernel 4.7 it didn't support __WCLONE while
* waitpid() did. Hence let's use waitpid() here, it's good enough for our purposes here. */
for (;;)
if (waitpid(pid, NULL, __WCLONE) >= 0 || errno != EINTR)
break;
}
return -EBADF; /* return an invalidated fd */
}
int asynchronous_rm_rf(const char *p, RemoveFlags flags) {
int r;
assert(p);
/* Forks off a child that destroys the specified path. This will be best effort only, i.e. the child
* will attempt to do its thing, but we won't wait for it or check its success. */
r = safe_fork("(sd-rmrf)", FORK_RESET_SIGNALS|FORK_CLOSE_ALL_FDS|FORK_DETACH, NULL);
if (r != 0)
return r;
/* Child */
r = rm_rf(p, flags);
if (r < 0) {
log_debug_errno(r, "Failed to rm -rf '%s', ignoring: %m", p);
_exit(EXIT_FAILURE); /* This is a detached process, hence no one really cares, but who knows
* maybe it's good for debugging/tracing to return an exit code
* indicative of our failure here. */
}
_exit(EXIT_SUCCESS);
}
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