Kernel/Userspace: Add basic init process

This process parses /etc/passwd and promps login screen.
When an username is entered, it will launch that users shell

userspace/init/CMakeLists.txt

@@ -0,0 +1,16 @@
+cmake_minimum_required(VERSION 3.26)
+
+project(init CXX)
+
+set(SOURCES
+	main.cpp
+)
+
+add_executable(init ${SOURCES})
+target_compile_options(init PUBLIC -O2 -g)
+target_link_libraries(init PUBLIC libc ban)
+
+add_custom_target(init-install
+	COMMAND cp ${CMAKE_CURRENT_BINARY_DIR}/init ${BANAN_BIN}/
+	DEPENDS init
+)

userspace/init/main.cpp

@@ -0,0 +1,153 @@
+#include <BAN/String.h>
+#include <BAN/Optional.h>
+#include <BAN/Vector.h>
+
+#include <ctype.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+struct User
+{
+	BAN::String name;
+	uid_t uid;
+	gid_t gid;
+	BAN::String home;
+	BAN::String shell;
+};
+
+BAN::Optional<id_t> parse_id(BAN::StringView value)
+{
+	// NOTE: we only allow 10^9 uids
+	if (value.size() < 1 || value.size() > 9)
+		return {};
+
+	id_t id { 0 };
+	for (char c : value)
+	{
+		if (!isdigit(c))
+			return {};
+		id = (id * 10) + (c - '0');
+	}
+
+	return id;
+}
+
+BAN::Optional<User> parse_user(BAN::StringView line)
+{
+	auto parts = MUST(line.split(':', true));
+	if (parts.size() != 7)
+		return {};
+	User user;
+	user.name = parts[0];
+	user.uid = ({ auto id = parse_id(parts[2]); if (!id.has_value()) return {}; id.value(); });
+	user.gid = ({ auto id = parse_id(parts[3]); if (!id.has_value()) return {}; id.value(); });
+	user.home = parts[5];
+	user.shell = parts[6];
+	return user;
+}
+
+BAN::Vector<User> parse_users()
+{
+	FILE* fp = fopen("/etc/passwd", "r");
+	if (fp == nullptr)
+	{
+		fprintf(stderr, "could not open /etc/passwd\n");
+		perror("fopen");
+	}
+
+	BAN::Vector<User> users;
+
+	char buffer[1024];
+	while (fgets(buffer, sizeof(buffer), fp))
+	{
+		*strchrnul(buffer, '\n') = '\0';
+		auto user = parse_user(buffer);
+		if (user.has_value())
+			MUST(users.push_back(user.release_value()));
+	}
+
+	if (ferror(fp))
+	{
+		perror("fread");
+		fclose(fp);
+		return {};
+	}
+
+	fclose(fp);
+
+	return users;
+}
+
+void initialize_stdio()
+{
+	char tty[L_ctermid];
+	ctermid(tty);
+	if (open(tty, O_RDONLY) != 0) _exit(1);
+	if (open(tty, O_WRONLY) != 1) _exit(1);
+	if (open(tty, O_WRONLY) != 2) _exit(1);
+}
+
+int main()
+{
+	initialize_stdio();
+
+	while (true)
+	{
+		auto users = parse_users();
+
+		char name_buffer[128];
+		BAN::StringView name;
+
+		while (true)
+		{
+			printf("username: ");
+			fflush(stdout);
+
+			size_t nread = fread(name_buffer, 1, sizeof(name_buffer) - 1, stdin);
+			if (nread == 0)
+			{
+				if (ferror(stdin))
+				{
+					fprintf(stderr, "Could not read from stdin\n");
+					return 1;
+				}
+				continue;
+			}
+			if (nread == 1)
+				continue;		
+
+			name = BAN::StringView(name_buffer, nread - 1);
+			break;
+		}
+
+		for (const User& user : users)
+		{
+			if (user.name == name)
+			{
+				pid_t pid = fork();
+				if (pid == 0)
+				{
+					printf("Welcome back %s!\n", user.name.data());
+
+					execl(user.shell.data(), user.shell.data(), nullptr);
+					perror("execl");
+					
+					exit(1);
+				}
+				
+				if (pid == -1)
+				{
+					perror("fork");
+					break;
+				}
+
+				int status;
+				waitpid(pid, &status, 0);
+			}
+		}
+	}
+
+
+}