kernel+libc+ln: Add support for userspace hard link creation

apps/ln.cpp

@@ -5,16 +5,21 @@
 Result<int> luna_main(int argc, char** argv)
 {
     StringView target;
-    StringView link;
+    StringView linkpath;
+
+    bool create_symlink { false };
 
     os::ArgumentParser parser;
-    parser.add_description("Create symbolic links.");
+    parser.add_description("Create links between files.");
     parser.add_system_program_info("ln"_sv);
     parser.add_positional_argument(target, "target", true);
-    parser.add_positional_argument(link, "linkpath", true);
+    parser.add_positional_argument(linkpath, "linkpath", true);
+    parser.add_switch_argument(create_symlink, 's', "symbolic"_sv, "create symlinks instead of hard links"_sv);
     parser.parse(argc, argv);
 
-    if (symlink(target.chars(), link.chars()) < 0)
+    auto link_function = create_symlink ? symlink : link;
+
+    if (link_function(target.chars(), linkpath.chars()) < 0)
     {
         perror("ln");
         return 1;

kernel/src/fs/Mount.h

@@ -62,6 +62,11 @@ class MountInode : public VFS::Inode, public LinkedListNode<MountInode>
         return m_mount_root_inode->gid();
     }
 
+    nlink_t nlinks() const override
+    {
+        return m_mount_root_inode->nlinks();
+    }
+
     Result<void> chmod(mode_t mode) override
     {
         return m_mount_root_inode->chmod(mode);

kernel/src/fs/VFS.h

@@ -116,6 +116,11 @@ namespace VFS
 
         virtual mode_t mode() const = 0;
 
+        virtual nlink_t nlinks() const
+        {
+            return 1;
+        }
+
         virtual u32 uid() const
         {
             return 0;

kernel/src/fs/tmpfs/FileSystem.h

@@ -94,6 +94,11 @@ namespace TmpFS
             return m_gid;
         }
 
+        nlink_t nlinks() const override
+        {
+            return (nlink_t)m_nlinks;
+        }
+
         Result<void> chmod(mode_t mode) override
         {
             m_mode = mode;
@@ -200,6 +205,11 @@ namespace TmpFS
             return m_gid;
         }
 
+        nlink_t nlinks() const override
+        {
+            return (nlink_t)m_nlinks;
+        }
+
         Result<void> chmod(mode_t) override
         {
             return {};
@@ -329,6 +339,11 @@ namespace TmpFS
             return m_gid;
         }
 
+        nlink_t nlinks() const override
+        {
+            return (nlink_t)m_nlinks;
+        }
+
         Result<void> chmod(mode_t mode) override
         {
             m_mode = mode;

kernel/src/sys/link.cpp

@@ -2,6 +2,7 @@
 #include "memory/MemoryManager.h"
 #include "sys/Syscall.h"
 #include "thread/Scheduler.h"
+#include <bits/atfile.h>
 #include <luna/PathParser.h>
 
 Result<u64> sys_unlinkat(Registers*, SyscallArgs args)
@@ -88,3 +89,36 @@ Result<u64> sys_readlinkat(Registers*, SyscallArgs args)
 
     return nread;
 }
+
+Result<u64> sys_linkat(Registers*, SyscallArgs args)
+{
+    int olddirfd = (int)args[0];
+    auto oldpath = TRY(MemoryManager::strdup_from_user(args[1]));
+    int newdirfd = (int)args[2];
+    auto newpath = TRY(MemoryManager::strdup_from_user(args[3]));
+    int flags = (int)args[4];
+
+    auto* current = Scheduler::current();
+
+    auto parser = TRY(PathParser::create(newpath.chars()));
+    auto parent = TRY(parser.dirname());
+
+    // FIXME: Use AT_SYMLINK_FOLLOW.
+    auto target = TRY(current->resolve_atfile(olddirfd, oldpath, flags & AT_EMPTY_PATH, false));
+
+    if (target->type() == VFS::InodeType::Directory) return err(EPERM);
+
+    auto parent_inode = TRY(current->resolve_atfile(newdirfd, parent, false, true));
+
+    if (target->fs() != parent_inode->fs()) return err(EXDEV);
+
+    if (!VFS::can_write(parent_inode, current->auth)) return err(EACCES);
+
+    auto child_name = TRY(parser.basename());
+
+    TRY(VFS::validate_filename(child_name.view()));
+
+    TRY(parent_inode->add_entry(target, child_name.chars()));
+
+    return 0;
+}

kernel/src/sys/stat.cpp

@@ -38,7 +38,7 @@ Result<u64> sys_fstatat(Registers*, SyscallArgs args)
 
     kstat.st_ino = inode->inode_number();
     kstat.st_mode = make_mode(inode->mode(), inode->type());
-    kstat.st_nlink = 1; // FIXME: Count hard links to files.
+    kstat.st_nlink = inode->nlinks();
     kstat.st_uid = inode->uid();
     kstat.st_gid = inode->gid();
     kstat.st_size = inode->size();

libc/include/unistd.h

@@ -145,6 +145,12 @@ extern "C"
     /* Read the contents of a symbolic link relative to a file descriptor. */
     ssize_t readlinkat(int dirfd, const char* path, char* buf, size_t max);
 
+    /* Create a hard link. */
+    int link(const char* oldpath, const char* newpath);
+
+    /* Create a hard link relative to a file descriptor. */
+    int linkat(int olddirfd, const char* oldpath, int newdirfd, const char* newpath, int flags);
+
 #ifdef __cplusplus
 }
 #endif

libc/src/unistd.cpp

@@ -430,4 +430,16 @@ extern "C"
         long rc = syscall(SYS_readlinkat, dirfd, path, buf, max);
         __errno_return(rc, ssize_t);
     }
+
+    int link(const char* oldpath, const char* newpath)
+    {
+        long rc = syscall(SYS_linkat, AT_FDCWD, oldpath, AT_FDCWD, newpath, 0);
+        __errno_return(rc, int);
+    }
+
+    int linkat(int olddirfd, const char* oldpath, int newdirfd, const char* newpath, int flags)
+    {
+        long rc = syscall(SYS_linkat, olddirfd, oldpath, newdirfd, newpath, flags);
+        __errno_return(rc, int);
+    }
 }

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(symlinkat) _e(readlinkat) _e(umask)
+                    _e(umount) _e(pstat) _e(getrusage) _e(symlinkat) _e(readlinkat) _e(umask) _e(linkat)
 
 enum Syscalls
 {