kernel: Add initial support for symbolic links :D

kernel/src/fs/VFS.cpp

@@ -15,28 +15,60 @@ namespace VFS
         return *root_fs->root_inode();
     }
 
-    Result<SharedPtr<Inode>> resolve_path(const char* path, Credentials auth, SharedPtr<Inode> working_directory)
+    static constexpr int MAX_SYMLINKS = 8;
+
+    Result<SharedPtr<Inode>> resolve_path_impl(const char* path, Credentials auth, SharedPtr<Inode> current_inode,
+                                               bool follow_last_symlink, int& symlinks_followed)
     {
+        if (symlinks_followed >= MAX_SYMLINKS) return err(ELOOP);
+
         if (*path == '\0') return err(ENOENT);
 
         auto parser = TRY(PathParser::create(path));
 
-        SharedPtr<Inode> current_inode;
-
-        if (parser.is_absolute() || !working_directory) current_inode = root_fs->root_inode();
-        else
-            current_inode = working_directory;
+        SharedPtr<Inode> parent_inode = current_inode;
 
         const char* section;
         while (parser.next().try_set_value(section))
         {
             if (!can_execute(current_inode, auth)) return err(EACCES);
             current_inode = TRY(current_inode->find(section));
+
+            if (current_inode->type() == VFS::InodeType::Symlink && (follow_last_symlink || parser.has_next()))
+            {
+                auto link = TRY(current_inode->readlink());
+
+                SharedPtr<VFS::Inode> symlink_root;
+
+                if (PathParser::is_absolute(link.chars())) symlink_root = root_fs->root_inode();
+                else
+                    symlink_root = parent_inode;
+
+                symlinks_followed++;
+                current_inode = TRY(resolve_path_impl(link.chars(), auth, symlink_root, true, symlinks_followed));
+                symlinks_followed--;
+            }
+
+            parent_inode = current_inode;
         }
 
         return current_inode;
     }
 
+    Result<SharedPtr<Inode>> resolve_path(const char* path, Credentials auth, SharedPtr<VFS::Inode> working_directory,
+                                          bool follow_last_symlink)
+    {
+        SharedPtr<Inode> current_inode;
+
+        if (PathParser::is_absolute(path) || !working_directory) current_inode = root_fs->root_inode();
+        else
+            current_inode = working_directory;
+
+        int symlinks_followed = 0;
+
+        return resolve_path_impl(path, auth, current_inode, follow_last_symlink, symlinks_followed);
+    }
+
     Result<SharedPtr<Inode>> create_directory(const char* path, Credentials auth, SharedPtr<Inode> working_directory)
     {
         auto parser = TRY(PathParser::create(path));

kernel/src/fs/VFS.h

@@ -12,7 +12,8 @@ namespace VFS
     {
         RegularFile,
         Directory,
-        Device
+        Device,
+        Symlink
     };
 
     class Inode;
@@ -28,6 +29,8 @@ namespace VFS
 
         virtual Result<SharedPtr<Inode>> create_device_inode(u32 major, u32 minor) = 0;
 
+        virtual Result<SharedPtr<Inode>> create_symlink_inode(StringView link) = 0;
+
         virtual Result<void> set_mount_dir(SharedPtr<Inode> parent) = 0;
 
         virtual u64 handles() const
@@ -92,6 +95,12 @@ namespace VFS
 
         virtual bool blocking() const = 0;
 
+        // Symlink-specific methods
+        virtual Result<StringView> readlink()
+        {
+            return StringView {};
+        }
+
         // Metadata accessors
         virtual usize size() const = 0;
 
@@ -250,7 +259,8 @@ namespace VFS
     };
 
     Result<SharedPtr<Inode>> resolve_path(const char* path, Credentials auth,
-                                          SharedPtr<VFS::Inode> working_directory = {});
+                                          SharedPtr<VFS::Inode> working_directory = {},
+                                          bool follow_last_symlink = true);
 
     Result<SharedPtr<Inode>> create_directory(const char* path, Credentials auth,
                                               SharedPtr<VFS::Inode> working_directory = {});

kernel/src/fs/tmpfs/FileSystem.cpp

@@ -24,6 +24,15 @@ namespace TmpFS
         return (SharedPtr<VFS::Inode>)inode;
     }
 
+    Result<SharedPtr<VFS::Inode>> FileSystem::create_symlink_inode(StringView link)
+    {
+        SharedPtr<SymlinkInode> inode = TRY(make_shared<SymlinkInode>());
+        inode->set_fs(*this, {});
+        TRY(inode->set_link(link, {}));
+        inode->set_inode_number(m_next_inode_number++, {});
+        return (SharedPtr<VFS::Inode>)inode;
+    }
+
     Result<SharedPtr<VFS::Inode>> FileSystem::create_dir_inode(SharedPtr<VFS::Inode> parent)
     {
         SharedPtr<DirInode> inode = TRY(make_shared<DirInode>());

kernel/src/fs/tmpfs/FileSystem.h

@@ -5,6 +5,7 @@
 #include <luna/Badge.h>
 #include <luna/Buffer.h>
 #include <luna/StaticString.h>
+#include <luna/String.h>
 #include <luna/Vector.h>
 
 namespace TmpFS
@@ -20,6 +21,7 @@ namespace TmpFS
         Result<SharedPtr<VFS::Inode>> create_file_inode() override;
         Result<SharedPtr<VFS::Inode>> create_dir_inode(SharedPtr<VFS::Inode> parent) override;
         Result<SharedPtr<VFS::Inode>> create_device_inode(u32 major, u32 minor) override;
+        Result<SharedPtr<VFS::Inode>> create_symlink_inode(StringView link) override;
 
         Result<void> set_mount_dir(SharedPtr<VFS::Inode> parent) override;
 
@@ -120,6 +122,115 @@ namespace TmpFS
         u32 m_nlinks { 0 };
     };
 
+    class SymlinkInode : public VFS::FileInode
+    {
+      public:
+        SymlinkInode() = default;
+
+        VFS::InodeType type() const override
+        {
+            return VFS::InodeType::Symlink;
+        }
+
+        void set_fs(FileSystem& fs, Badge<FileSystem>)
+        {
+            m_fs = &fs;
+        }
+
+        void set_inode_number(usize inum, Badge<FileSystem>)
+        {
+            m_inode_number = inum;
+        }
+
+        Result<void> set_link(StringView link, Badge<FileSystem>)
+        {
+            m_link = TRY(String::from_string_view(link));
+            return {};
+        }
+
+        VFS::FileSystem* fs() const override
+        {
+            return m_fs;
+        }
+
+        usize inode_number() const override
+        {
+            return m_inode_number;
+        }
+
+        Result<usize> read(u8*, usize, usize) const override
+        {
+            return err(ENOTSUP);
+        }
+
+        Result<usize> write(const u8*, usize, usize) override
+        {
+            return err(ENOTSUP);
+        }
+
+        Result<void> truncate(usize) override
+        {
+            return err(ENOTSUP);
+        }
+
+        usize size() const override
+        {
+            return m_link.length();
+        }
+
+        mode_t mode() const override
+        {
+            return 0777;
+        }
+
+        u32 uid() const override
+        {
+            return m_uid;
+        }
+
+        u32 gid() const override
+        {
+            return m_gid;
+        }
+
+        Result<void> chmod(mode_t) override
+        {
+            return {};
+        }
+
+        Result<void> chown(u32 uid, u32 gid) override
+        {
+            m_uid = uid;
+            m_gid = gid;
+            return {};
+        }
+
+        void did_link() override
+        {
+            m_nlinks++;
+        }
+
+        void did_unlink() override
+        {
+            m_nlinks--;
+        }
+
+        Result<StringView> readlink() override
+        {
+            return m_link.view();
+        }
+
+        virtual ~SymlinkInode() = default;
+
+      private:
+        VFS::FileSystem* m_fs;
+        String m_link;
+        usize m_inode_number;
+        u32 m_uid { 0 };
+        u32 m_gid { 0 };
+        u32 m_nlinks { 0 };
+    };
+
     class DeviceInode : public VFS::DeviceInode
     {
       public:

kernel/src/sys/id.cpp

@@ -104,7 +104,7 @@ Result<u64> sys_fchmodat(Registers*, SyscallArgs args)
 
     auto* current = Scheduler::current();
 
-    auto inode = TRY(current->resolve_atfile(dirfd, path, flags & AT_EMPTY_PATH));
+    auto inode = TRY(current->resolve_atfile(dirfd, path, flags & AT_EMPTY_PATH, !(flags & AT_SYMLINK_NOFOLLOW)));
 
     if (current->auth.euid != 0 && current->auth.euid != inode->uid()) return err(EPERM);
 
@@ -123,7 +123,7 @@ Result<u64> sys_fchownat(Registers*, SyscallArgs args)
 
     auto* current = Scheduler::current();
 
-    auto inode = TRY(current->resolve_atfile(dirfd, path, flags & AT_EMPTY_PATH));
+    auto inode = TRY(current->resolve_atfile(dirfd, path, flags & AT_EMPTY_PATH, !(flags & AT_SYMLINK_NOFOLLOW)));
 
     if (current->auth.euid != 0) return err(EPERM);
 

kernel/src/sys/link.cpp

@@ -21,7 +21,7 @@ Result<u64> sys_unlinkat(Registers*, SyscallArgs args)
 
     kinfoln("unlinkat: remove %s from directory %s, dirfd is %d", basename.chars(), dirname.chars(), dirfd);
 
-    auto inode = TRY(current->resolve_atfile(dirfd, dirname, false));
+    auto inode = TRY(current->resolve_atfile(dirfd, dirname, false, false));
     if (!VFS::can_write(inode, current->auth)) return err(EACCES);
 
     if (flags > 0)
@@ -34,3 +34,31 @@ Result<u64> sys_unlinkat(Registers*, SyscallArgs args)
 
     return 0;
 }
+
+Result<u64> sys_symlinkat(Registers*, SyscallArgs args)
+{
+    auto target = TRY(MemoryManager::strdup_from_user(args[0]));
+    int dirfd = (int)args[1];
+    auto linkpath = TRY(MemoryManager::strdup_from_user(args[2]));
+
+    if (target.is_empty()) return err(ENOENT);
+
+    auto* current = Scheduler::current();
+
+    auto parser = TRY(PathParser::create(linkpath.chars()));
+    auto parent = TRY(parser.dirname());
+
+    auto parent_inode = TRY(current->resolve_atfile(dirfd, parent, false, true));
+
+    if (!VFS::can_write(parent_inode, current->auth)) return err(EACCES);
+
+    auto child_name = TRY(parser.basename());
+
+    TRY(VFS::validate_filename(child_name.view()));
+
+    auto inode = TRY(parent_inode->fs()->create_symlink_inode(target.view()));
+    TRY(inode->chown(current->auth.euid, current->auth.egid));
+    TRY(parent_inode->add_entry(inode, child_name.chars()));
+
+    return 0;
+}

kernel/src/sys/open.cpp

@@ -31,7 +31,8 @@ Result<u64> sys_openat(Registers*, SyscallArgs args)
 
     int error;
     SharedPtr<VFS::Inode> parent_inode;
-    bool ok = current->resolve_atfile(dirfd, path, false, &parent_inode).try_set_value_or_error(inode, error);
+    bool ok = current->resolve_atfile(dirfd, path, false, !(flags & O_NOFOLLOW), &parent_inode)
+                  .try_set_value_or_error(inode, error);
     if (!ok)
     {
         if (error == ENOENT && (flags & O_CREAT) && !path.is_empty())
@@ -51,6 +52,9 @@ Result<u64> sys_openat(Registers*, SyscallArgs args)
         if ((flags & O_WRONLY) && !VFS::can_write(inode, current->auth)) return err(EACCES);
     }
 
+    // This should only be possible if O_NOFOLLOW was in flags.
+    if (inode->type() == VFS::InodeType::Symlink) return err(ELOOP);
+
     if (flags & O_TMPFILE)
     {
         if (inode->type() != VFS::InodeType::Directory) return err(EINVAL);

kernel/src/sys/stat.cpp

@@ -14,6 +14,7 @@ static mode_t make_mode(mode_t mode, VFS::InodeType type)
     case VFS::InodeType::RegularFile: result |= S_IFREG; break;
     case VFS::InodeType::Directory: result |= S_IFDIR; break;
     case VFS::InodeType::Device: result |= S_IFCHR; break;
+    case VFS::InodeType::Symlink: result |= S_IFLNK; break;
     default: break;
     }
 
@@ -29,7 +30,7 @@ Result<u64> sys_fstatat(Registers*, SyscallArgs args)
 
     Thread* current = Scheduler::current();
 
-    auto inode = TRY(current->resolve_atfile(dirfd, path, flags & AT_EMPTY_PATH));
+    auto inode = TRY(current->resolve_atfile(dirfd, path, flags & AT_EMPTY_PATH, !(flags & AT_SYMLINK_NOFOLLOW)));
 
     stat kstat;
 

kernel/src/thread/Thread.cpp

@@ -49,13 +49,15 @@ Result<FileDescriptor*> Thread::resolve_fd(int fd)
 }
 
 Result<SharedPtr<VFS::Inode>> Thread::resolve_atfile(int dirfd, const String& path, bool allow_empty_path,
-                                                     SharedPtr<VFS::Inode>* parent_inode)
+                                                     bool follow_last_symlink, SharedPtr<VFS::Inode>* parent_inode)
 {
     if (parent_inode) *parent_inode = this->current_directory;
 
-    if (PathParser::is_absolute(path.view())) return VFS::resolve_path(path.chars(), this->auth);
+    if (PathParser::is_absolute(path.view()))
+        return VFS::resolve_path(path.chars(), this->auth, {}, follow_last_symlink);
 
-    if (dirfd == AT_FDCWD) return VFS::resolve_path(path.chars(), this->auth, this->current_directory);
+    if (dirfd == AT_FDCWD)
+        return VFS::resolve_path(path.chars(), this->auth, this->current_directory, follow_last_symlink);
 
     auto descriptor = TRY(resolve_fd(dirfd));
 
@@ -63,7 +65,7 @@ Result<SharedPtr<VFS::Inode>> Thread::resolve_atfile(int dirfd, const String& pa
 
     if (path.is_empty() && allow_empty_path) return descriptor->inode;
 
-    return VFS::resolve_path(path.chars(), this->auth, descriptor->inode);
+    return VFS::resolve_path(path.chars(), this->auth, descriptor->inode, follow_last_symlink);
 }
 
 bool FileDescriptor::should_append()

kernel/src/thread/Thread.h

@@ -76,6 +76,7 @@ struct Thread : public LinkedListNode<Thread>
     Result<int> allocate_fd(int min);
     Result<FileDescriptor*> resolve_fd(int fd);
     Result<SharedPtr<VFS::Inode>> resolve_atfile(int dirfd, const String& path, bool allow_empty_path,
+                                                 bool follow_last_symlink,
                                                  SharedPtr<VFS::Inode>* parent_inode = nullptr);
 
     FPData fp_data;

libc/include/bits/atfile.h

@@ -6,6 +6,7 @@
 #define AT_FDCWD -100
 
 #define AT_EMPTY_PATH 1
+#define AT_SYMLINK_NOFOLLOW 2
 
 #define AT_REMOVEDIR 1
 

libc/include/bits/modes.h

@@ -5,6 +5,7 @@
 
 #define S_IFMT 070000
 #define S_IFREG 000000
+#define S_IFLNK 010000
 #define S_IFDIR 040000
 #define S_IFCHR 050000
 
@@ -13,6 +14,7 @@
 #define S_ISREG(mode) __CHECK_TYPE(mode, S_IFREG)
 #define S_ISDIR(mode) __CHECK_TYPE(mode, S_IFDIR)
 #define S_ISCHR(mode) __CHECK_TYPE(mode, S_IFCHR)
+#define S_ISLNK(mode) __CHECK_TYPE(mode, S_IFLNK)
 
 #define S_IRWXU 0700
 #define S_IRUSR 0400

libc/include/bits/open-flags.h

@@ -14,6 +14,7 @@
 #define O_CLOEXEC 128
 #define O_DIRECTORY 256
 #define O_TMPFILE 512
+#define O_NOFOLLOW 1024
 
 #define O_ACCMODE O_RDWR
 

libluna/include/luna/Syscall.h

@@ -5,7 +5,7 @@
         _e(lseek) _e(mkdir) _e(execve) _e(fork) _e(waitpid) _e(getppid) _e(fcntl) _e(getdents) _e(getuid) _e(geteuid)  \
             _e(getgid) _e(getegid) _e(setuid) _e(setgid) _e(seteuid) _e(setegid) _e(fchmodat) _e(fchownat) _e(ioctl)   \
                 _e(fstatat) _e(chdir) _e(getcwd) _e(unlinkat) _e(uname) _e(sethostname) _e(dup2) _e(pipe) _e(mount)    \
-                    _e(umount) _e(pstat) _e(getrusage)
+                    _e(umount) _e(pstat) _e(getrusage) _e(symlinkat)
 
 enum Syscalls
 {