Kernel+SystemServer: Add /dev/tty

This file refers to the controlling terminal associated with the current
process. It's specified by POSIX, and is used by ports like openssh to
interface with the terminal even if the standard input/output is
redirected to somewhere else.

Our implementation leverages ProcFS's existing facilities to create
process-specific symbolic links. In our setup, `/dev/tty` is a symbolic
link to `/proc/self/tty`, which itself is a symlink to the appropriate
`/dev/pts` entry. If no TTY is attached, `/dev/tty` is left dangling.

Kernel/FileSystem/ProcFS.cpp

@@ -318,6 +318,8 @@ ErrorOr<NonnullRefPtr<Inode>> ProcFSProcessDirectoryInode::lookup(StringView nam
         return TRY(ProcFSProcessPropertyInode::try_create_for_pid_property(procfs(), SegmentedProcFSIndex::MainProcessProperty::PerformanceEvents, associated_pid()));
     if (name == "vm"sv)
         return TRY(ProcFSProcessPropertyInode::try_create_for_pid_property(procfs(), SegmentedProcFSIndex::MainProcessProperty::VirtualMemoryStats, associated_pid()));
+    if (name == "tty"sv)
+        return TRY(ProcFSProcessPropertyInode::try_create_for_pid_property(procfs(), SegmentedProcFSIndex::MainProcessProperty::TTYLink, associated_pid()));
     return ENOENT;
 }
 
@@ -454,6 +456,8 @@ static mode_t determine_procfs_process_inode_mode(SegmentedProcFSIndex::ProcessS
         return S_IFLNK | 0777;
     if (main_property == SegmentedProcFSIndex::MainProcessProperty::CurrentWorkDirectoryLink)
         return S_IFLNK | 0777;
+    if (main_property == SegmentedProcFSIndex::MainProcessProperty::TTYLink)
+        return S_IFLNK | 0777;
     return S_IFREG | 0400;
 }
 
@@ -557,6 +561,8 @@ ErrorOr<void> ProcFSProcessPropertyInode::try_to_acquire_data(Process& process,
         return process.procfs_get_perf_events(builder);
     case SegmentedProcFSIndex::MainProcessProperty::VirtualMemoryStats:
         return process.procfs_get_virtual_memory_stats(builder);
+    case SegmentedProcFSIndex::MainProcessProperty::TTYLink:
+        return process.procfs_get_tty_link(builder);
     default:
         VERIFY_NOT_REACHED();
     }

Kernel/Process.h

@@ -576,6 +576,7 @@ public:
     ErrorOr<size_t> procfs_get_file_description_link(unsigned fd, KBufferBuilder& builder) const;
     ErrorOr<void> traverse_file_descriptions_directory(FileSystemID, Function<ErrorOr<void>(FileSystem::DirectoryEntryView const&)> callback) const;
     ErrorOr<NonnullRefPtr<Inode>> lookup_file_descriptions_directory(const ProcFS&, StringView name) const;
+    ErrorOr<void> procfs_get_tty_link(KBufferBuilder& builder) const;
 
 private:
     inline PerformanceEventBuffer* current_perf_events_buffer()

Kernel/ProcessExposed.h

@@ -29,6 +29,7 @@ enum class MainProcessProperty {
     CurrentWorkDirectoryLink = 5,
     PerformanceEvents = 6,
     VirtualMemoryStats = 7,
+    TTYLink = 8,
 };
 
 enum class ProcessSubDirectory {

Kernel/ProcessProcFSTraits.cpp

@@ -62,6 +62,7 @@ ErrorOr<void> Process::ProcessProcFSTraits::traverse_as_directory(FileSystemID f
     TRY(callback({ "cwd", { fsid, SegmentedProcFSIndex::build_segmented_index_for_main_property_in_pid_directory(process->pid(), SegmentedProcFSIndex::MainProcessProperty::CurrentWorkDirectoryLink) }, DT_LNK }));
     TRY(callback({ "perf_events", { fsid, SegmentedProcFSIndex::build_segmented_index_for_main_property_in_pid_directory(process->pid(), SegmentedProcFSIndex::MainProcessProperty::PerformanceEvents) }, DT_REG }));
     TRY(callback({ "vm", { fsid, SegmentedProcFSIndex::build_segmented_index_for_main_property_in_pid_directory(process->pid(), SegmentedProcFSIndex::MainProcessProperty::VirtualMemoryStats) }, DT_REG }));
+    TRY(callback({ "tty", { fsid, SegmentedProcFSIndex::build_segmented_index_for_main_property_in_pid_directory(process->pid(), SegmentedProcFSIndex::MainProcessProperty::TTYLink) }, DT_LNK }));
     return {};
 }
 

Kernel/ProcessSpecificExposed.cpp

@@ -15,6 +15,7 @@
 #include <Kernel/Memory/MemoryManager.h>
 #include <Kernel/Process.h>
 #include <Kernel/ProcessExposed.h>
+#include <Kernel/TTY/TTY.h>
 
 namespace Kernel {
 
@@ -281,4 +282,11 @@ ErrorOr<void> Process::procfs_get_binary_link(KBufferBuilder& builder) const
     return builder.append(custody->absolute_path().bytes());
 }
 
+ErrorOr<void> Process::procfs_get_tty_link(KBufferBuilder& builder) const
+{
+    if (m_tty.is_null())
+        return Error::from_errno(ENOENT);
+    return builder.append(m_tty->tty_name().characters());
+}
+
 }

Userland/Services/SystemServer/main.cpp

@@ -363,6 +363,11 @@ static void prepare_synthetic_filesystems()
         VERIFY_NOT_REACHED();
     }
 
+    rc = symlink("/proc/self/tty", "/dev/tty");
+    if (rc < 0) {
+        VERIFY_NOT_REACHED();
+    }
+
     populate_devfs();
 
     rc = mkdir("/dev/pts", 0755);