notify-recv: add generic implementation of sd_notify() server side dgram recv code

The code is not trivial, and we implemented this at various places
already, introduce a common implementation for this we can reuse.

src/shared/meson.build

@@ -126,6 +126,7 @@ shared_sources = files(
         'netif-naming-scheme.c',
         'netif-sriov.c',
         'netif-util.c',
+        'notify-recv.c',
         'nsflags.c',
         'nsresource.c',
         'numa-util.c',

src/shared/notify-recv.c

@@ -0,0 +1,106 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "fd-util.h"
+#include "notify-recv.h"
+#include "socket-util.h"
+
+int notify_recv(int fd, char **ret_text, struct ucred *ret_ucred, PidRef *ret_pidref) {
+        CMSG_BUFFER_TYPE(CMSG_SPACE(sizeof(struct ucred)) +
+                         CMSG_SPACE(sizeof(int)) + /* SCM_PIDFD */
+                         CMSG_SPACE(sizeof(int) * NOTIFY_FD_MAX)) control;
+        struct iovec iovec;
+        struct msghdr msghdr = {
+                .msg_iov = &iovec,
+                .msg_iovlen = 1,
+                .msg_control = &control,
+                .msg_controllen = sizeof(control),
+        };
+        ssize_t n;
+
+        assert(fd >= 0);
+
+        /* Receives a $NOTIFY_SOCKET message (aka sd_notify()). Does various validations. Returns -EAGAIN in
+         * case an invalid message is received (following the logic that an invalid message shall be ignored,
+         * and treated like no message at all). */
+
+        _cleanup_free_ char *buf = new(char, NOTIFY_BUFFER_MAX+1);
+        if (!buf)
+                return log_oom_debug();
+
+        iovec = (struct iovec) {
+                .iov_base = buf,
+                .iov_len = NOTIFY_BUFFER_MAX,
+        };
+
+        n = recvmsg_safe(fd, &msghdr, MSG_DONTWAIT|MSG_CMSG_CLOEXEC);
+        if (ERRNO_IS_NEG_TRANSIENT(n))
+                return -EAGAIN;
+        if (n == -ECHRNG) {
+                log_debug_errno(n, "Got message with truncated control data (unexpected fds sent?), ignoring.");
+                return -EAGAIN;
+        }
+        if (n == -EXFULL) {
+                log_debug_errno(n, "Got message with truncated payload data, ignoring.");
+                return -EAGAIN;
+        }
+        if (n < 0)
+                return (int) n;
+
+        const struct ucred *ucred = NULL;
+        _cleanup_close_ int pidfd = -EBADF;
+        struct cmsghdr *cmsg;
+        CMSG_FOREACH(cmsg, &msghdr) {
+                if (cmsg->cmsg_level != SOL_SOCKET)
+                        continue;
+
+                switch (cmsg->cmsg_type) {
+
+                case SCM_RIGHTS:
+                        /* For now, just close every fd */
+                        close_many(CMSG_TYPED_DATA(cmsg, int),
+                                   (cmsg->cmsg_len - CMSG_LEN(0)) / sizeof(int));
+                        break;
+
+                case SCM_PIDFD:
+                        assert(cmsg->cmsg_len == CMSG_LEN(sizeof(int)));
+                        pidfd = *CMSG_TYPED_DATA(cmsg, int);
+                        break;
+
+                case SCM_CREDENTIALS:
+                        assert(cmsg->cmsg_len == CMSG_LEN(sizeof(struct ucred)));
+                        ucred = CMSG_TYPED_DATA(cmsg, struct ucred);
+                        break;
+                }
+        }
+
+        if ((ret_ucred || ret_pidref) && (!ucred || ucred->pid <= 0))
+                return log_debug_errno(SYNTHETIC_ERRNO(EAGAIN), "Got notification datagram lacking valid credential information, ignoring.");
+
+        if (n == 0)
+                return log_debug_errno(SYNTHETIC_ERRNO(EAGAIN), "Got empty notification message, ignoring.");
+        if (memchr(buf, 0, n - 1))
+                return log_debug_errno(SYNTHETIC_ERRNO(EAGAIN), "Got notification message with embedded NUL, ignoring.");
+
+        if (ret_pidref) {
+                assert(ucred);
+                assert(ucred->pid > 0);
+
+                if (pidfd >= 0)
+                        *ret_pidref = (PidRef) {
+                                .pid = ucred->pid,
+                                .fd = TAKE_FD(pidfd),
+                        };
+                else
+                        *ret_pidref = PIDREF_MAKE_FROM_PID(ucred->pid);
+        }
+
+        if (ret_text) {
+                buf[n] = 0;
+                *ret_text = TAKE_PTR(buf);
+        }
+
+        if (ret_ucred)
+                *ret_ucred = *ucred;
+
+        return 0;
+}

src/shared/notify-recv.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <sys/socket.h>
+
+#include "pidref.h"
+
+int notify_recv(int fd, char **ret_text, struct ucred *ret_ucred, PidRef *ret_pidref);