LibIMAP: Make parsing of atom data fallible

We now return an error where `parse_atom()` would have previously
returned an empty StringView. This is consistent with RFC3501, which
says that an atom consists of one or more characters.

This prevents a few cases where parsing an invalid atom could lead to
an infinite loop.

Userland/Libraries/LibIMAP/Parser.cpp

@@ -147,7 +147,7 @@ ErrorOr<void> Parser::parse_untagged()
  auto number = try_parse_number();
  if (number.has_value()) {
  TRY(consume(" "sv));
- auto data_type = parse_atom();
+ auto data_type = TRY(parse_atom());
  if (data_type == "EXISTS"sv) {
  m_response.data().set_exists(number.value());
  TRY(consume("\r\n"sv));
@@ -180,7 +180,7 @@ ErrorOr<void> Parser::parse_untagged()
  } else if (consume_if("OK"sv)) {
  TRY(consume(" "sv));
  if (consume_if("["sv)) {
- auto actual_type = parse_atom();
+ auto actual_type = TRY(parse_atom());
  if (actual_type == "CLOSED"sv) {
  // No-op.
  } else if (actual_type == "UIDNEXT"sv) {
@@ -230,7 +230,7 @@ ErrorOr<void> Parser::parse_untagged()
  auto status_item = StatusItem();
  status_item.set_mailbox(mailbox);
  while (!consume_if(")"sv)) {
- auto status_att = parse_atom();
+ auto status_att = TRY(parse_atom());
  TRY(consume(" "sv));
  auto value = TRY(parse_number());
 
@@ -582,14 +582,13 @@ ErrorOr<void> Parser::parse_capability_response()
  auto capability = AK::Vector<DeprecatedString>();
  while (!consume_if("\r\n"sv)) {
  TRY(consume(" "sv));
- auto x = DeprecatedString(parse_atom());
- capability.append(x);
+ capability.append(TRY(parse_atom()));
  }
  m_response.data().add_capabilities(move(capability));
  return {};
 }
 
-StringView Parser::parse_atom()
+ErrorOr<StringView> Parser::parse_atom()
 {
  dbgln_if(IMAP_PARSER_DEBUG, "p: {}, parse_atom()", m_position);
  auto is_non_atom_char = [](u8 x) {
@@ -604,14 +603,17 @@ StringView Parser::parse_atom()
  m_position++;
  }
 
+ if (count == 0)
+ return Error::from_string_literal("Invalid atom value");
+
  StringView s = StringView(m_buffer.data() + start, count);
  dbgln_if(IMAP_PARSER_DEBUG, "p: {}, ret \"{}\"", m_position, s);
  return s;
 }
 
 ErrorOr<ResponseStatus> Parser::parse_status()
 {
- auto atom = parse_atom();
+ auto atom = TRY(parse_atom());
 
  if (atom == "OK"sv) {
  return ResponseStatus::OK;
@@ -733,7 +735,7 @@ ErrorOr<FetchCommand::DataItem> Parser::parse_fetch_data_item()
  data_item.section->parts->append(num.value());
  continue;
  }
- auto atom = parse_atom();
+ auto atom = TRY(parse_atom());
  if (atom.equals_ignoring_ascii_case("MIME"sv)) {
  data_item.section->ends_with_mime = true;
  continue;

Userland/Libraries/LibIMAP/Parser.h

@@ -41,7 +41,7 @@ class Parser {
  ErrorOr<void> parse_untagged();
  ErrorOr<void> parse_capability_response();
 
- StringView parse_atom();
+ ErrorOr<StringView> parse_atom();
  ErrorOr<StringView> parse_quoted_string();
  ErrorOr<StringView> parse_literal_string();
  ErrorOr<StringView> parse_string();