From 447da99f0b8f0608981899127433c3d2c61a1dc8 Mon Sep 17 00:00:00 2001
From: Bananymous <oskari.alaranta@bananymous.com>
Date: Sat, 11 Nov 2023 23:16:52 +0200
Subject: [PATCH] Kernel/LibC: Implement readlink and readlinkat

---
 kernel/include/kernel/Process.h |  3 +++
 kernel/kernel/Process.cpp       | 41 +++++++++++++++++++++++++++++++++
 kernel/kernel/Syscall.cpp       |  6 +++++
 libc/include/sys/syscall.h      |  2 ++
 libc/unistd.cpp                 | 10 ++++++++
 5 files changed, 62 insertions(+)

diff --git a/kernel/include/kernel/Process.h b/kernel/include/kernel/Process.h
index 033fcd154..92cfc37f1 100644
--- a/kernel/include/kernel/Process.h
+++ b/kernel/include/kernel/Process.h
@@ -101,6 +101,9 @@ namespace Kernel
 		BAN::ErrorOr<long> sys_create(const char*, mode_t);
 		BAN::ErrorOr<long> sys_create_dir(const char*, mode_t);
 		BAN::ErrorOr<long> sys_unlink(const char*);
+		BAN::ErrorOr<long> readlink_impl(BAN::StringView absolute_path, char* buffer, size_t bufsize);
+		BAN::ErrorOr<long> sys_readlink(const char* path, char* buffer, size_t bufsize);
+		BAN::ErrorOr<long> sys_readlinkat(int fd, const char* path, char* buffer, size_t bufsize);
 
 		BAN::ErrorOr<long> sys_chmod(const char*, mode_t);
 
diff --git a/kernel/kernel/Process.cpp b/kernel/kernel/Process.cpp
index e8bee629f..c709741f7 100644
--- a/kernel/kernel/Process.cpp
+++ b/kernel/kernel/Process.cpp
@@ -782,6 +782,47 @@ namespace Kernel
 		return 0;
 	}
 
+	BAN::ErrorOr<long> Process::readlink_impl(BAN::StringView absolute_path, char* buffer, size_t bufsize)
+	{
+		auto inode = TRY(VirtualFileSystem::get().file_from_absolute_path(m_credentials, absolute_path, O_NOFOLLOW | O_RDONLY)).inode;
+		
+		// FIXME: no allocation needed
+		auto link_target = TRY(inode->link_target());
+
+		size_t byte_count = BAN::Math::min<size_t>(link_target.size(), bufsize);
+		memcpy(buffer, link_target.data(), byte_count);
+
+		return byte_count;
+	}
+
+	BAN::ErrorOr<long> Process::sys_readlink(const char* path, char* buffer, size_t bufsize)
+	{
+		LockGuard _(m_lock);
+		validate_string_access(path);
+		validate_pointer_access(buffer, bufsize);
+
+		auto absolute_path = TRY(absolute_path_of(path));
+
+		return readlink_impl(absolute_path.sv(), buffer, bufsize);
+	}
+
+	BAN::ErrorOr<long> Process::sys_readlinkat(int fd, const char* path, char* buffer, size_t bufsize)
+	{
+		LockGuard _(m_lock);
+		validate_string_access(path);
+		validate_pointer_access(buffer, bufsize);
+
+		// FIXME: handle O_SEARCH in fd
+		auto parent_path = TRY(m_open_file_descriptors.path_of(fd));
+
+		BAN::String absolute_path;
+		TRY(absolute_path.append(parent_path));
+		TRY(absolute_path.push_back('/'));
+		TRY(absolute_path.append(path));
+
+		return readlink_impl(absolute_path.sv(), buffer, bufsize);
+	}
+
 	BAN::ErrorOr<long> Process::sys_chmod(const char* path, mode_t mode)
 	{
 		if (mode & S_IFMASK)
diff --git a/kernel/kernel/Syscall.cpp b/kernel/kernel/Syscall.cpp
index ab7c3ce85..b3c574cdb 100644
--- a/kernel/kernel/Syscall.cpp
+++ b/kernel/kernel/Syscall.cpp
@@ -211,6 +211,12 @@ namespace Kernel
 		case SYS_UNLINK:
 			ret = Process::current().sys_unlink((const char*)arg1);
 			break;
+		case SYS_READLINK:
+			ret = Process::current().sys_readlink((const char*)arg1, (char*)arg2, (size_t)arg3);
+			break;
+		case SYS_READLINKAT:
+			ret = Process::current().sys_readlinkat((int)arg1, (const char*)arg2, (char*)arg3, (size_t)arg4);
+			break;
 		default:
 			dwarnln("Unknown syscall {}", syscall);
 			break;
diff --git a/libc/include/sys/syscall.h b/libc/include/sys/syscall.h
index b6eacdf8e..b07079a4a 100644
--- a/libc/include/sys/syscall.h
+++ b/libc/include/sys/syscall.h
@@ -59,6 +59,8 @@ __BEGIN_DECLS
 #define SYS_CREATE 56 // creat, mkfifo
 #define SYS_CREATE_DIR 57 // mkdir
 #define SYS_UNLINK 58
+#define SYS_READLINK 59
+#define SYS_READLINKAT 60
 
 __END_DECLS
 
diff --git a/libc/unistd.cpp b/libc/unistd.cpp
index b118da951..0e35d6259 100644
--- a/libc/unistd.cpp
+++ b/libc/unistd.cpp
@@ -78,6 +78,16 @@ ssize_t write(int fildes, const void* buf, size_t nbyte)
 	return syscall(SYS_WRITE, fildes, buf, nbyte);
 }
 
+ssize_t readlink(const char* __restrict path, char* __restrict buf, size_t bufsize)
+{
+	return syscall(SYS_READLINK, path, buf, bufsize);
+}
+
+ssize_t readlinkat(int fd, const char* __restrict path, char* __restrict buf, size_t bufsize)
+{
+	return syscall(SYS_READLINKAT, fd, path, buf, bufsize);
+}
+
 int dup(int fildes)
 {
 	return syscall(SYS_DUP, fildes);