Kernel: Support sending filedescriptors with sendmsg(2) and SCM_RIGHTS

This is necessary to support the wayland protocol.
I also moved the CMSG_* macros to the kernel API since they are used in
both kernel and userspace.
this does not break ntpquery/SCM_TIMESTAMP.

Kernel/API/POSIX/sys/socket.h

@@ -81,6 +81,32 @@ struct msghdr {
     int msg_flags;
 };
 
+// These three are non-POSIX, but common:
+#define CMSG_ALIGN(x) (((x) + sizeof(void*) - 1) & ~(sizeof(void*) - 1))
+#define CMSG_SPACE(x) (CMSG_ALIGN(sizeof(struct cmsghdr)) + CMSG_ALIGN(x))
+#define CMSG_LEN(x) (CMSG_ALIGN(sizeof(struct cmsghdr)) + (x))
+
+static inline struct cmsghdr* CMSG_FIRSTHDR(struct msghdr* msg)
+{
+    if (msg->msg_controllen < sizeof(struct cmsghdr))
+        return (struct cmsghdr*)0;
+    return (struct cmsghdr*)msg->msg_control;
+}
+
+static inline struct cmsghdr* CMSG_NXTHDR(struct msghdr* msg, struct cmsghdr* cmsg)
+{
+    struct cmsghdr* next = (struct cmsghdr*)((char*)cmsg + CMSG_ALIGN(cmsg->cmsg_len));
+    unsigned offset = (char*)next - (char*)msg->msg_control;
+    if (msg->msg_controllen < offset + sizeof(struct cmsghdr))
+        return (struct cmsghdr*)0;
+    return next;
+}
+
+static inline void* CMSG_DATA(struct cmsghdr* cmsg)
+{
+    return (void*)(cmsg + 1);
+}
+
 struct sockaddr {
     sa_family_t sa_family;
     char sa_data[14];

Kernel/Net/LocalSocket.cpp

@@ -520,6 +520,26 @@ ErrorOr<NonnullLockRefPtr<OpenFileDescription>> LocalSocket::recvfd(OpenFileDesc
     return queue.take_first();
 }
 
+ErrorOr<NonnullLockRefPtrVector<OpenFileDescription>> LocalSocket::recvfds(OpenFileDescription const& socket_description, int n)
+{
+    MutexLocker locker(mutex());
+    NonnullLockRefPtrVector<OpenFileDescription> fds;
+
+    auto role = this->role(socket_description);
+    if (role != Role::Connected && role != Role::Accepted)
+        return set_so_error(EINVAL);
+    auto& queue = recvfd_queue_for(socket_description);
+
+    for (int i = 0; i < n; ++i) {
+        if (queue.is_empty())
+            break;
+
+        fds.append(queue.take_first());
+    }
+
+    return fds;
+}
+
 ErrorOr<void> LocalSocket::try_set_path(StringView path)
 {
     m_path = TRY(KString::try_create(path));

Kernel/Net/LocalSocket.h

@@ -28,6 +28,7 @@ public:
 
     ErrorOr<void> sendfd(OpenFileDescription const& socket_description, NonnullLockRefPtr<OpenFileDescription> passing_description);
     ErrorOr<NonnullLockRefPtr<OpenFileDescription>> recvfd(OpenFileDescription const& socket_description);
+    ErrorOr<NonnullLockRefPtrVector<OpenFileDescription>> recvfds(OpenFileDescription const& socket_description, int n);
 
     static void for_each(Function<void(LocalSocket const&)>);
     static ErrorOr<void> try_for_each(Function<ErrorOr<void>(LocalSocket const&)>);

Kernel/Syscalls/socket.cpp

@@ -4,6 +4,7 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/ByteBuffer.h>
 #include <Kernel/FileSystem/OpenFileDescription.h>
 #include <Kernel/Net/LocalSocket.h>
 #include <Kernel/Process.h>
@@ -199,6 +200,24 @@ ErrorOr<FlatPtr> Process::sys$sendmsg(int sockfd, Userspace<const struct msghdr*
             Thread::current()->send_signal(SIGPIPE, &Process::current());
         return EPIPE;
     }
+
+    if (msg.msg_controllen > 0) {
+        // Handle command messages.
+        auto cmsg_buffer = TRY(ByteBuffer::create_uninitialized(msg.msg_controllen));
+        TRY(copy_from_user(cmsg_buffer.data(), msg.msg_control, msg.msg_controllen));
+        msg.msg_control = cmsg_buffer.data();
+        for (struct cmsghdr* cmsg = CMSG_FIRSTHDR(&msg); cmsg != nullptr; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
+            if (socket.is_local() && cmsg->cmsg_level == SOL_SOCKET && cmsg->cmsg_type == SCM_RIGHTS) {
+                auto& local_socket = static_cast<LocalSocket&>(socket);
+                int* fds = (int*)CMSG_DATA(cmsg);
+                size_t nfds = (cmsg->cmsg_len - CMSG_ALIGN(sizeof(struct cmsghdr))) / sizeof(int);
+                for (size_t i = 0; i < nfds; ++i) {
+                    TRY(local_socket.sendfd(*description, TRY(open_file_description(fds[i]))));
+                }
+            }
+        }
+    }
+
     auto data_buffer = TRY(UserOrKernelBuffer::for_user_buffer((u8*)iovs[0].iov_base, iovs[0].iov_len));
 
     while (true) {
@@ -267,21 +286,41 @@ ErrorOr<FlatPtr> Process::sys$recvmsg(int sockfd, Userspace<struct msghdr*> user
         msg_flags |= MSG_TRUNC;
     }
 
-    if (socket.wants_timestamp()) {
-        struct {
-            cmsghdr cmsg;
-            timeval timestamp;
-        } cmsg_timestamp;
-        socklen_t control_length = sizeof(cmsg_timestamp);
-        if (msg.msg_controllen < control_length) {
+    socklen_t current_cmsg_len = 0;
+    auto try_add_cmsg = [&](int level, int type, void const* data, socklen_t len) -> ErrorOr<bool> {
+        if (current_cmsg_len + len > msg.msg_controllen) {
             msg_flags |= MSG_CTRUNC;
-        } else {
-            cmsg_timestamp = { { control_length, SOL_SOCKET, SCM_TIMESTAMP }, timestamp.to_timeval() };
-            TRY(copy_to_user(msg.msg_control, &cmsg_timestamp, control_length));
+            return false;
         }
-        TRY(copy_to_user(&user_msg.unsafe_userspace_ptr()->msg_controllen, &control_length));
+
+        cmsghdr cmsg = { (socklen_t)CMSG_LEN(len), level, type };
+        cmsghdr* target = (cmsghdr*)(((char*)msg.msg_control) + current_cmsg_len);
+        TRY(copy_to_user(target, &cmsg));
+        TRY(copy_to_user(CMSG_DATA(target), data, len));
+        current_cmsg_len += CMSG_ALIGN(cmsg.cmsg_len);
+        return true;
+    };
+
+    if (socket.wants_timestamp()) {
+        timeval time = timestamp.to_timeval();
+        TRY(try_add_cmsg(SOL_SOCKET, SCM_TIMESTAMP, &time, sizeof(time)));
     }
 
+    int space_for_fds = (msg.msg_controllen - current_cmsg_len - sizeof(struct cmsghdr)) / sizeof(int);
+    if (space_for_fds > 0 && socket.is_local()) {
+        auto& local_socket = static_cast<LocalSocket&>(socket);
+        auto descriptions = TRY(local_socket.recvfds(description, space_for_fds));
+        Vector<int> fdnums;
+        for (auto& description : descriptions) {
+            auto fd_allocation = TRY(m_fds.with_exclusive([](auto& fds) { return fds.allocate(); }));
+            m_fds.with_exclusive([&](auto& fds) { fds[fd_allocation.fd].set(description, 0); });
+            fdnums.append(fd_allocation.fd);
+        }
+        TRY(try_add_cmsg(SOL_SOCKET, SCM_RIGHTS, fdnums.data(), fdnums.size() * sizeof(int)));
+    }
+
+    TRY(copy_to_user(&user_msg.unsafe_userspace_ptr()->msg_controllen, &current_cmsg_len));
+
     TRY(copy_to_user(&user_msg.unsafe_userspace_ptr()->msg_flags, &msg_flags));
     return result.value();
 }

Userland/Libraries/LibC/sys/socket.h

@@ -33,30 +33,4 @@ int socketpair(int domain, int type, int protocol, int sv[2]);
 int sendfd(int sockfd, int fd);
 int recvfd(int sockfd, int options);
 
-// These three are non-POSIX, but common:
-#define CMSG_ALIGN(x) (((x) + sizeof(void*) - 1) & ~(sizeof(void*) - 1))
-#define CMSG_SPACE(x) (CMSG_ALIGN(sizeof(struct cmsghdr)) + CMSG_ALIGN(x))
-#define CMSG_LEN(x) (CMSG_ALIGN(sizeof(struct cmsghdr)) + (x))
-
-static inline struct cmsghdr* CMSG_FIRSTHDR(struct msghdr* msg)
-{
-    if (msg->msg_controllen < sizeof(struct cmsghdr))
-        return 0;
-    return (struct cmsghdr*)msg->msg_control;
-}
-
-static inline struct cmsghdr* CMSG_NXTHDR(struct msghdr* msg, struct cmsghdr* cmsg)
-{
-    struct cmsghdr* next = (struct cmsghdr*)((char*)cmsg + CMSG_ALIGN(cmsg->cmsg_len));
-    unsigned offset = (char*)next - (char*)msg->msg_control;
-    if (msg->msg_controllen < offset + sizeof(struct cmsghdr))
-        return NULL;
-    return next;
-}
-
-static inline void* CMSG_DATA(struct cmsghdr* cmsg)
-{
-    return (void*)(cmsg + 1);
-}
-
 __END_DECLS