lld/ELF/ScriptLexer.cpp

//===- ScriptLexer.cpp ----------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file defines a lexer for the linker script.
//
// The linker script's grammar is not complex but ambiguous due to the
// lack of the formal specification of the language. What we are trying to
// do in this and other files in LLD is to make a "reasonable" linker
// script processor.
//
// Among simplicity, compatibility and efficiency, we put the most
// emphasis on simplicity when we wrote this lexer. Compatibility with the
// GNU linkers is important, but we did not try to clone every tiny corner
// case of their lexers, as even ld.bfd and ld.gold are subtly different
// in various corner cases. We do not care much about efficiency because
// the time spent in parsing linker scripts is usually negligible.
//
// Our grammar of the linker script is LL(2), meaning that it needs at
// most two-token lookahead to parse. The only place we need two-token
// lookahead is labels in version scripts, where we need to parse "local :"
// as if "local:".
//
// Overall, this lexer works fine for most linker scripts. There might
// be room for improving compatibility, but that's probably not at the
// top of our todo list.
//
//===----------------------------------------------------------------------===//

#include "ScriptLexer.h"
#include "lld/Common/ErrorHandler.h"
#include "llvm/ADT/Twine.h"

using namespace llvm;

namespace lld {
namespace elf {
// Returns a whole line containing the current token.
StringRef ScriptLexer::getLine() {
  StringRef s = getCurrentMB().getBuffer();
  StringRef tok = tokens[pos - 1];

  size_t pos = s.rfind('\n', tok.data() - s.data());
  if (pos != StringRef::npos)
    s = s.substr(pos + 1);
  return s.substr(0, s.find_first_of("\r\n"));
}

// Returns 1-based line number of the current token.
size_t ScriptLexer::getLineNumber() {
  if (pos == 0)
    return 1;
  StringRef s = getCurrentMB().getBuffer();
  StringRef tok = tokens[pos - 1];
  return s.substr(0, tok.data() - s.data()).count('\n') + 1;
}

// Returns 0-based column number of the current token.
size_t ScriptLexer::getColumnNumber() {
  StringRef tok = tokens[pos - 1];
  return tok.data() - getLine().data();
}

std::string ScriptLexer::getCurrentLocation() {
  std::string filename = getCurrentMB().getBufferIdentifier();
  return (filename + ":" + Twine(getLineNumber())).str();
}

ScriptLexer::ScriptLexer(MemoryBufferRef mb) { tokenize(mb); }

// We don't want to record cascading errors. Keep only the first one.
void ScriptLexer::setError(const Twine &msg) {
  if (errorCount())
    return;

  std::string s = (getCurrentLocation() + ": " + msg).str();
  if (pos)
    s += "\n>>> " + getLine().str() + "\n>>> " +
         std::string(getColumnNumber(), ' ') + "^";
  error(s);
}

// Split S into linker script tokens.
void ScriptLexer::tokenize(MemoryBufferRef mb) {
  std::vector<StringRef> vec;
  mbs.push_back(mb);
  StringRef s = mb.getBuffer();
  StringRef begin = s;

  for (;;) {
    s = skipSpace(s);
    if (s.empty())
      break;

    // Quoted token. Note that double-quote characters are parts of a token
    // because, in a glob match context, only unquoted tokens are interpreted
    // as glob patterns. Double-quoted tokens are literal patterns in that
    // context.
    if (s.startswith("\"")) {
      size_t e = s.find("\"", 1);
      if (e == StringRef::npos) {
        StringRef filename = mb.getBufferIdentifier();
        size_t lineno = begin.substr(0, s.data() - begin.data()).count('\n');
        error(filename + ":" + Twine(lineno + 1) + ": unclosed quote");
        return;
      }

      vec.push_back(s.take_front(e + 1));
      s = s.substr(e + 1);
      continue;
    }

    // ">foo" is parsed to ">" and "foo", but ">>" is parsed to ">>".
    // "|", "||", "&" and "&&" are different operators.
    if (s.startswith("<<") || s.startswith("<=") || s.startswith(">>") ||
        s.startswith(">=") || s.startswith("||") || s.startswith("&&")) {
      vec.push_back(s.substr(0, 2));
      s = s.substr(2);
      continue;
    }

    // Unquoted token. This is more relaxed than tokens in C-like language,
    // so that you can write "file-name.cpp" as one bare token, for example.
    size_t pos = s.find_first_not_of(
        "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
        "0123456789_.$/\\~=+[]*?-!^:");

    // A character that cannot start a word (which is usually a
    // punctuation) forms a single character token.
    if (pos == 0)
      pos = 1;
    vec.push_back(s.substr(0, pos));
    s = s.substr(pos);
  }

  tokens.insert(tokens.begin() + pos, vec.begin(), vec.end());
}

// Skip leading whitespace characters or comments.
StringRef ScriptLexer::skipSpace(StringRef s) {
  for (;;) {
    if (s.startswith("/*")) {
      size_t e = s.find("*/", 2);
      if (e == StringRef::npos) {
        error("unclosed comment in a linker script");
        return "";
      }
      s = s.substr(e + 2);
      continue;
    }
    if (s.startswith("#")) {
      size_t e = s.find('\n', 1);
      if (e == StringRef::npos)
        e = s.size() - 1;
      s = s.substr(e + 1);
      continue;
    }
    size_t size = s.size();
    s = s.ltrim();
    if (s.size() == size)
      return s;
  }
}

// An erroneous token is handled as if it were the last token before EOF.
bool ScriptLexer::atEOF() { return errorCount() || tokens.size() == pos; }

// Split a given string as an expression.
// This function returns "3", "*" and "5" for "3*5" for example.
static std::vector<StringRef> tokenizeExpr(StringRef s) {
  StringRef ops = "+-*/:!~=<>"; // List of operators

  // Quoted strings are literal strings, so we don't want to split it.
  if (s.startswith("\""))
    return {s};

  // Split S with operators as separators.
  std::vector<StringRef> ret;
  while (!s.empty()) {
    size_t e = s.find_first_of(ops);

    // No need to split if there is no operator.
    if (e == StringRef::npos) {
      ret.push_back(s);
      break;
    }

    // Get a token before the opreator.
    if (e != 0)
      ret.push_back(s.substr(0, e));

    // Get the operator as a token.
    // Keep !=, ==, >=, <=, << and >> operators as a single tokens.
    if (s.substr(e).startswith("!=") || s.substr(e).startswith("==") ||
        s.substr(e).startswith(">=") || s.substr(e).startswith("<=") ||
        s.substr(e).startswith("<<") || s.substr(e).startswith(">>")) {
      ret.push_back(s.substr(e, 2));
      s = s.substr(e + 2);
    } else {
      ret.push_back(s.substr(e, 1));
      s = s.substr(e + 1);
    }
  }
  return ret;
}

// In contexts where expressions are expected, the lexer should apply
// different tokenization rules than the default one. By default,
// arithmetic operator characters are regular characters, but in the
// expression context, they should be independent tokens.
//
// For example, "foo*3" should be tokenized to "foo", "*" and "3" only
// in the expression context.
//
// This function may split the current token into multiple tokens.
void ScriptLexer::maybeSplitExpr() {
  if (!inExpr || errorCount() || atEOF())
    return;

  std::vector<StringRef> v = tokenizeExpr(tokens[pos]);
  if (v.size() == 1)
    return;
  tokens.erase(tokens.begin() + pos);
  tokens.insert(tokens.begin() + pos, v.begin(), v.end());
}

StringRef ScriptLexer::next() {
  maybeSplitExpr();

  if (errorCount())
    return "";
  if (atEOF()) {
    setError("unexpected EOF");
    return "";
  }
  return tokens[pos++];
}

StringRef ScriptLexer::peek() {
  StringRef tok = next();
  if (errorCount())
    return "";
  pos = pos - 1;
  return tok;
}

StringRef ScriptLexer::peek2() {
  skip();
  StringRef tok = next();
  if (errorCount())
    return "";
  pos = pos - 2;
  return tok;
}

bool ScriptLexer::consume(StringRef tok) {
  if (peek() == tok) {
    skip();
    return true;
  }
  return false;
}

// Consumes Tok followed by ":". Space is allowed between Tok and ":".
bool ScriptLexer::consumeLabel(StringRef tok) {
  if (consume((tok + ":").str()))
    return true;
  if (tokens.size() >= pos + 2 && tokens[pos] == tok &&
      tokens[pos + 1] == ":") {
    pos += 2;
    return true;
  }
  return false;
}

void ScriptLexer::skip() { (void)next(); }

void ScriptLexer::expect(StringRef expect) {
  if (errorCount())
    return;
  StringRef tok = next();
  if (tok != expect)
    setError(expect + " expected, but got " + tok);
}

// Returns true if S encloses T.
static bool encloses(StringRef s, StringRef t) {
  return s.bytes_begin() <= t.bytes_begin() && t.bytes_end() <= s.bytes_end();
}

MemoryBufferRef ScriptLexer::getCurrentMB() {
  // Find input buffer containing the current token.
  assert(!mbs.empty());
  if (pos == 0)
    return mbs.back();
  for (MemoryBufferRef mb : mbs)
    if (encloses(mb.getBuffer(), tokens[pos - 1]))
      return mb;
  llvm_unreachable("getCurrentMB: failed to find a token");
}

} // namespace elf
} // namespace lld