Kernel+Userland: Add symlink() syscall and add "-s" flag to /bin/ln.

It's now possible to create symbolic links! :^)

This exposed an issue in Ext2FS where we'd write uninitialized data past
the end of an inode's content. Fix this by zeroing out the tail end of
the last block in a file.

Kernel/Ext2FileSystem.cpp

@@ -13,6 +13,8 @@
 
 //#define EXT2_DEBUG
 
+static const ssize_t max_inline_symlink_length = 60;
+
 Retained<Ext2FS> Ext2FS::create(Retained<DiskDevice>&& device)
 {
     return adopt(*new Ext2FS(move(device)));
@@ -437,10 +439,9 @@ ssize_t Ext2FSInode::read_bytes(off_t offset, ssize_t count, byte* buffer, FileD
 
     // Symbolic links shorter than 60 characters are store inline inside the i_block array.
     // This avoids wasting an entire block on short links. (Most links are short.)
-    static const unsigned max_inline_symlink_length = 60;
     if (is_symlink() && size() < max_inline_symlink_length) {
         ssize_t nread = min((off_t)size() - offset, static_cast<off_t>(count));
-        memcpy(buffer, m_raw_inode.i_block + offset, (size_t)nread);
+        memcpy(buffer, ((byte*)m_raw_inode.i_block) + offset, (size_t)nread);
         return nread;
     }
 
@@ -495,13 +496,24 @@ ssize_t Ext2FSInode::read_bytes(off_t offset, ssize_t count, byte* buffer, FileD
 
 ssize_t Ext2FSInode::write_bytes(off_t offset, ssize_t count, const byte* data, FileDescriptor*)
 {
+    ASSERT(offset >= 0);
+    ASSERT(count >= 0);
+
     Locker inode_locker(m_lock);
     Locker fs_locker(fs().m_lock);
 
-    // FIXME: Support writing to symlink inodes.
-    ASSERT(!is_symlink());
-
-    ASSERT(offset >= 0);
+    if (is_symlink()) {
+        if ((offset + count) < max_inline_symlink_length) {
+#ifdef EXT2_DEBUG
+            dbgprintf("Ext2FSInode: write_bytes poking into i_block array for inline symlink '%s' (%u bytes)\n", String((const char*)data, count).characters(), count);
+#endif
+            memcpy(((byte*)m_raw_inode.i_block) + offset, data, (size_t)count);
+            if ((offset + count) > m_raw_inode.i_size)
+                m_raw_inode.i_size = offset + count;
+            set_metadata_dirty(true);
+            return count;
+        }
+    }
 
     const ssize_t block_size = fs().block_size();
     size_t old_size = size();
@@ -528,6 +540,8 @@ ssize_t Ext2FSInode::write_bytes(off_t offset, ssize_t count, const byte* data,
 
     dword offset_into_first_block = offset % block_size;
 
+    dword last_logical_block_index_in_file = size() / block_size;
+
     ssize_t nwritten = 0;
     size_t remaining_count = min((off_t)count, (off_t)new_size - offset);
     const byte* in = data;
@@ -542,7 +556,7 @@ ssize_t Ext2FSInode::write_bytes(off_t offset, ssize_t count, const byte* data,
         size_t num_bytes_to_copy = min((size_t)block_size - offset_into_block, remaining_count);
 
         ByteBuffer block;
-        if (offset_into_block != 0) {
+        if (offset_into_block != 0 || num_bytes_to_copy != block_size) {
             block = fs().read_block(block_list[bi]);
             if (!block) {
                 kprintf("Ext2FSInode::write_bytes: read_block(%u) failed (lbi: %u)\n", block_list[bi], bi);
@@ -552,8 +566,14 @@ ssize_t Ext2FSInode::write_bytes(off_t offset, ssize_t count, const byte* data,
             block = buffer_block;
 
         memcpy(block.pointer() + offset_into_block, in, num_bytes_to_copy);
-        if (offset_into_block == 0 && !num_bytes_to_copy)
-            memset(block.pointer() + num_bytes_to_copy, 0, block_size - num_bytes_to_copy);
+        if (bi == last_logical_block_index_in_file && num_bytes_to_copy < block_size) {
+            size_t padding_start = new_size % block_size;
+            size_t padding_bytes = block_size - padding_start;
+#ifdef EXT2_DEBUG
+            dbgprintf("Ext2FSInode::write_bytes padding last block of file with zero x %u (new_size=%u, offset_into_block=%u, num_bytes_to_copy=%u)\n", padding_bytes, new_size, offset_into_block, num_bytes_to_copy);
+#endif
+            memset(block.pointer() + padding_start, 0, padding_bytes);
+        }
 #ifdef EXT2_DEBUG
         dbgprintf("Ext2FSInode::write_bytes: writing block %u (offset_into_block: %u)\n", block_list[bi], offset_into_block);
 #endif

Kernel/Process.cpp

@@ -2144,6 +2144,15 @@ int Process::sys$unlink(const char* pathname)
     return VFS::the().unlink(String(pathname), cwd_inode());
 }
 
+int Process::sys$symlink(const char* target, const char* linkpath)
+{
+    if (!validate_read_str(target))
+        return -EFAULT;
+    if (!validate_read_str(linkpath))
+        return -EFAULT;
+    return VFS::the().symlink(String(target), String(linkpath), cwd_inode());
+}
+
 int Process::sys$rmdir(const char* pathname)
 {
     if (!validate_read_str(pathname))

Kernel/Process.h

@@ -216,6 +216,7 @@ public:
     int sys$utime(const char* pathname, const struct utimbuf*);
     int sys$link(const char* old_path, const char* new_path);
     int sys$unlink(const char* pathname);
+    int sys$symlink(const char* target, const char* linkpath);
     int sys$rmdir(const char* pathname);
     int sys$read_tsc(dword* lsw, dword* msw);
     int sys$chmod(const char* pathname, mode_t);

Kernel/Syscall.cpp

@@ -193,6 +193,8 @@ static dword handle(RegisterDump& regs, dword function, dword arg1, dword arg2,
         return current->sys$link((const char*)arg1, (const char*)arg2);
     case Syscall::SC_unlink:
         return current->sys$unlink((const char*)arg1);
+    case Syscall::SC_symlink:
+        return current->sys$symlink((const char*)arg1, (const char*)arg2);
     case Syscall::SC_read_tsc:
         return current->sys$read_tsc((dword*)arg1, (dword*)arg2);
     case Syscall::SC_rmdir:

Kernel/Syscall.h

@@ -84,6 +84,7 @@
     __ENUMERATE_SYSCALL(link) \
     __ENUMERATE_SYSCALL(chown) \
     __ENUMERATE_SYSCALL(fchmod) \
+    __ENUMERATE_SYSCALL(symlink) \
 
 
 namespace Syscall {

Kernel/VirtualFileSystem.cpp

@@ -431,6 +431,31 @@ KResult VFS::unlink(const String& path, Inode& base)
     return parent_inode->remove_child(FileSystemPath(path).basename());
 }
 
+KResult VFS::symlink(const String& target, const String& linkpath, Inode& base)
+{
+    RetainPtr<Inode> parent_inode;
+    auto existing_file_or_error = resolve_path_to_inode(linkpath, base, &parent_inode);
+    if (!existing_file_or_error.is_error())
+        return KResult(-EEXIST);
+    if (!parent_inode)
+        return KResult(-ENOENT);
+    if (existing_file_or_error.error() != -ENOENT)
+        return existing_file_or_error.error();
+    if (!parent_inode->metadata().may_write(*current))
+        return KResult(-EACCES);
+
+    FileSystemPath p(linkpath);
+    dbgprintf("VFS::symlink: '%s' (-> '%s') in %u:%u\n", p.basename().characters(), target.characters(), parent_inode->fsid(), parent_inode->index());
+    int error;
+    auto new_file = parent_inode->fs().create_inode(parent_inode->identifier(), p.basename(), 0120644, 0, error);
+    if (!new_file)
+        return KResult(error);
+    ssize_t nwritten = new_file->write_bytes(0, target.length(), (const byte*)target.characters(), nullptr);
+    if (nwritten < 0)
+        return KResult(nwritten);
+    return KSuccess;
+}
+
 KResult VFS::rmdir(const String& path, Inode& base)
 {
     RetainPtr<Inode> parent_inode;

Kernel/VirtualFileSystem.h

@@ -68,6 +68,7 @@ public:
     KResult mkdir(const String& path, mode_t mode, Inode& base);
     KResult link(const String& old_path, const String& new_path, Inode& base);
     KResult unlink(const String& path, Inode& base);
+    KResult symlink(const String& target, const String& linkpath, Inode& base);
     KResult rmdir(const String& path, Inode& base);
     KResult chmod(const String& path, mode_t, Inode& base);
     KResult chmod(Inode&, mode_t);

LibC/unistd.cpp

@@ -247,6 +247,12 @@ int unlink(const char* pathname)
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
 
+int symlink(const char* target, const char* linkpath)
+{
+    int rc = syscall(SC_symlink, target, linkpath);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 int rmdir(const char* pathname)
 {
     int rc = syscall(SC_rmdir, pathname);

LibC/unistd.h

@@ -62,6 +62,7 @@ int ttyname_r(int fd, char* buffer, size_t);
 off_t lseek(int fd, off_t, int whence);
 int link(const char* oldpath, const char* newpath);
 int unlink(const char* pathname);
+int symlink(const char* target, const char* linkpath);
 int rmdir(const char* pathname);
 int getdtablesize();
 int dup(int old_fd);

Userland/ln.cpp

@@ -1,13 +1,41 @@
 #include <stdio.h>
 #include <errno.h>
 #include <unistd.h>
+#include <getopt.h>
+#include <stdlib.h>
+
+[[noreturn]] static void print_usage_and_exit()
+{
+    printf("usage: ln [-s] <target> <link-path>\n");
+    exit(0);
+}
 
 int main(int argc, char** argv)
 {
-    if (argc != 3) {
-        fprintf(stderr, "usage: ln <old-path> <new-path>\n");
+    bool flag_symlink = false;
+    int opt;
+    while ((opt = getopt(argc, argv, "s")) != -1) {
+        switch (opt) {
+        case 's':
+            flag_symlink = true;
+            break;
+        default:
+            print_usage_and_exit();
+        }
+    }
+
+    if ((optind + 1) >= argc)
+        print_usage_and_exit();
+
+    if (flag_symlink) {
+        int rc = symlink(argv[optind], argv[optind + 1]);
+        if (rc < 0) {
+            perror("symlink");
             return 1;
         }
+        return 0;
+    }
+
     int rc = link(argv[1], argv[2]);
     if (rc < 0) {
         perror("link");