feat(fs): readlink/readlinkat and correct ioctl errno

kernel/arch/aarch64/syscall/linux_syscalls.h

@@ -27,6 +27,7 @@ constexpr uint64_t WRITE            = 64;
 constexpr uint64_t WRITEV           = 66;
 constexpr uint64_t PSELECT6         = 72;
 constexpr uint64_t PPOLL            = 73;
+constexpr uint64_t READLINKAT       = 78;
 constexpr uint64_t NEWFSTATAT       = 79;
 constexpr uint64_t FSTAT            = 80;
 constexpr uint64_t FSYNC            = 82;

kernel/arch/x86_64/syscall/linux_syscalls.h

@@ -54,6 +54,7 @@ constexpr uint64_t RENAME           = 82;
 constexpr uint64_t MKDIR            = 83;
 constexpr uint64_t RMDIR            = 84;
 constexpr uint64_t UNLINK           = 87;
+constexpr uint64_t READLINK         = 89;
 constexpr uint64_t GETTIMEOFDAY     = 96;
 constexpr uint64_t GETUID           = 102;
 constexpr uint64_t GETGID           = 104;
@@ -70,6 +71,7 @@ constexpr uint64_t MKDIRAT          = 258;
 constexpr uint64_t NEWFSTATAT       = 262;
 constexpr uint64_t UNLINKAT         = 263;
 constexpr uint64_t RENAMEAT         = 264;
+constexpr uint64_t READLINKAT       = 267;
 constexpr uint64_t FACCESSAT        = 269;
 constexpr uint64_t PSELECT6         = 270;
 constexpr uint64_t PPOLL            = 271;

kernel/syscall/handlers/sys_fd.cpp

@@ -1447,6 +1447,73 @@ DEFINE_SYSCALL2(access, pathname, mode) {
         pathname, mode, 0, 0, 0);
 }
 
+static int64_t do_readlinkat(int64_t dirfd, uint64_t pathname,
+                             uint64_t u_buf, uint64_t bufsize) {
+    if (bufsize == 0) {
+        return syscall::EINVAL;
+    }
+
+    char kpath[fs::PATH_MAX];
+    int32_t copy_rc = mm::uaccess::copy_cstr_from_user(
+        kpath, sizeof(kpath),
+        reinterpret_cast<const char*>(pathname));
+    if (copy_rc != mm::uaccess::OK) {
+        if (copy_rc == mm::uaccess::ERR_NAMETOOLONG) {
+            return syscall::ENAMETOOLONG;
+        }
+        return syscall::EFAULT;
+    }
+
+    if (kpath[0] == '\0') {
+        return syscall::ENOENT;
+    }
+
+    sched::task* task = sched::current();
+    if (!task) {
+        return syscall::EIO;
+    }
+
+    fs::node* node = nullptr;
+    int64_t lookup_rc = lookup_node_for_dirfd_path(task, dirfd, kpath, &node);
+    if (lookup_rc != 0) {
+        return lookup_rc;
+    }
+
+    // Only symbolic links have a target to read
+    if (node->type() != fs::node_type::symlink) {
+        release_node_ref(node);
+        return syscall::EINVAL;
+    }
+
+    // The input path is no longer needed, so reuse kpath for the target
+    size_t cap = bufsize < sizeof(kpath) ? bufsize : sizeof(kpath);
+    size_t target_len = 0;
+    int32_t fs_rc = node->readlink(kpath, cap, &target_len);
+    release_node_ref(node);
+    if (fs_rc != fs::OK) {
+        return syscall::error_map::map_fs_error(fs_rc);
+    }
+
+    // Guard against a provider reporting more than it wrote
+    if (target_len > cap) {
+        target_len = cap;
+    }
+
+    if (mm::uaccess::copy_to_user(reinterpret_cast<void*>(u_buf),
+                                  kpath, target_len) != mm::uaccess::OK) {
+        return syscall::EFAULT;
+    }
+    return static_cast<int64_t>(target_len);
+}
+
+DEFINE_SYSCALL4(readlinkat, dirfd, pathname, buf, bufsize) {
+    return do_readlinkat(static_cast<int64_t>(dirfd), pathname, buf, bufsize);
+}
+
+DEFINE_SYSCALL3(readlink, pathname, buf, bufsize) {
+    return do_readlinkat(static_cast<int64_t>(-100), pathname, buf, bufsize);
+}
+
 DEFINE_SYSCALL4(renameat, olddirfd, oldpath, newdirfd, newpath) {
     (void)olddirfd;
     (void)oldpath;

kernel/syscall/handlers/sys_fd.h

@@ -24,6 +24,8 @@ DECLARE_SYSCALL(mkdirat);
 DECLARE_SYSCALL(mkdir);
 DECLARE_SYSCALL(faccessat);
 DECLARE_SYSCALL(access);
+DECLARE_SYSCALL(readlinkat);
+DECLARE_SYSCALL(readlink);
 DECLARE_SYSCALL(renameat);
 DECLARE_SYSCALL(fsync);
 

kernel/syscall/handlers/sys_io.cpp

@@ -109,5 +109,8 @@ DEFINE_SYSCALL3(ioctl, fd, cmd, arg) {
         task, static_cast<resource::handle_t>(fd),
         static_cast<uint32_t>(cmd), arg);
     if (rc == resource::OK) return 0;
+
+    // POSIX: an unsupported request on any fd is ENOTTY, not ENOSYS
+    if (rc == resource::ERR_UNSUP) return syscall::ENOTTY;
     return map_resource_error(rc);
 }

kernel/syscall/syscall_table.cpp

@@ -99,12 +99,14 @@ __PRIVILEGED_CODE void init_syscall_table() {
     REGISTER_SYSCALL(linux_nr::UNLINKAT,    unlinkat);
     REGISTER_SYSCALL(linux_nr::FACCESSAT,   faccessat);
     REGISTER_SYSCALL(linux_nr::RENAMEAT,    renameat);
+    REGISTER_SYSCALL(linux_nr::READLINKAT,  readlinkat);
 #if defined(__x86_64__)
     REGISTER_SYSCALL(linux_nr::MKDIR,       mkdir);
     REGISTER_SYSCALL(linux_nr::UNLINK,      unlink);
     REGISTER_SYSCALL(linux_nr::RMDIR,       rmdir);
     REGISTER_SYSCALL(linux_nr::ACCESS,      access);
     REGISTER_SYSCALL(linux_nr::RENAME,      renameat);
+    REGISTER_SYSCALL(linux_nr::READLINK,    readlink);
 #endif
 
     REGISTER_SYSCALL(SYS_ELEVATE, elevate);