//===--- SignalHandlerCheck.cpp - clang-tidy ------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//

#include "SignalHandlerCheck.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/RecursiveASTVisitor.h"
#include "clang/ASTMatchers/ASTMatchFinder.h"
#include "clang/Analysis/CallGraph.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include <iterator>
#include <queue>

using namespace clang::ast_matchers;

namespace clang {
namespace tidy {
namespace bugprone {

static bool isSystemCall(const FunctionDecl *FD) {
  // Find a possible redeclaration in system header.
  // FIXME: Looking at the canonical declaration is not the most exact way
  // to do this.

  // Most common case will be inclusion directly from a header.
  // This works fine by using canonical declaration.
  // a.c
  // #include <sysheader.h>

  // Next most common case will be extern declaration.
  // Can't catch this with either approach.
  // b.c
  // extern void sysfunc(void);

  // Canonical declaration is the first found declaration, so this works.
  // c.c
  // #include <sysheader.h>
  // extern void sysfunc(void); // redecl won't matter

  // This does not work with canonical declaration.
  // Probably this is not a frequently used case but may happen (the first
  // declaration can be in a non-system header for example).
  // d.c
  // extern void sysfunc(void); // Canonical declaration, not in system header.
  // #include <sysheader.h>

  return FD->getASTContext().getSourceManager().isInSystemHeader(
      FD->getCanonicalDecl()->getLocation());
}

AST_MATCHER(FunctionDecl, isSystemCall) { return isSystemCall(&Node); }

// This is the  minimal set of safe functions.
// FIXME: Add checker option to allow a POSIX compliant extended set.
llvm::StringSet<> SignalHandlerCheck::StrictConformingFunctions{
    "signal", "abort", "_Exit", "quick_exit"};

SignalHandlerCheck::SignalHandlerCheck(StringRef Name,
                                       ClangTidyContext *Context)
    : ClangTidyCheck(Name, Context) {}

bool SignalHandlerCheck::isLanguageVersionSupported(
    const LangOptions &LangOpts) const {
  // FIXME: Make the checker useful on C++ code.
  if (LangOpts.CPlusPlus)
    return false;

  return true;
}

void SignalHandlerCheck::registerMatchers(MatchFinder *Finder) {
  auto SignalFunction = functionDecl(hasAnyName("::signal", "::std::signal"),
                                     parameterCountIs(2), isSystemCall());
  auto HandlerExpr =
      declRefExpr(hasDeclaration(functionDecl().bind("handler_decl")),
                  unless(isExpandedFromMacro("SIG_IGN")),
                  unless(isExpandedFromMacro("SIG_DFL")))
          .bind("handler_expr");
  Finder->addMatcher(
      callExpr(callee(SignalFunction), hasArgument(1, HandlerExpr))
          .bind("register_call"),
      this);
}

void SignalHandlerCheck::check(const MatchFinder::MatchResult &Result) {
  const auto *SignalCall = Result.Nodes.getNodeAs<CallExpr>("register_call");
  const auto *HandlerDecl =
      Result.Nodes.getNodeAs<FunctionDecl>("handler_decl");
  const auto *HandlerExpr = Result.Nodes.getNodeAs<DeclRefExpr>("handler_expr");

  // Visit each function encountered in the callgraph only once.
  llvm::DenseSet<const FunctionDecl *> SeenFunctions;

  // The worklist of the callgraph visitation algorithm.
  std::deque<const CallExpr *> CalledFunctions;

  auto ProcessFunction = [&](const FunctionDecl *F, const Expr *CallOrRef) {
    // Ensure that canonical declaration is used.
    F = F->getCanonicalDecl();

    // Do not visit function if already encountered.
    if (!SeenFunctions.insert(F).second)
      return true;

    // Check if the call is allowed.
    // Non-system calls are not considered.
    if (isSystemCall(F)) {
      if (isSystemCallAllowed(F))
        return true;

      reportBug(F, CallOrRef, SignalCall, HandlerDecl);

      return false;
    }

    // Get the body of the encountered non-system call function.
    const FunctionDecl *FBody;
    if (!F->hasBody(FBody)) {
      reportBug(F, CallOrRef, SignalCall, HandlerDecl);
      return false;
    }

    // Collect all called functions.
    auto Matches = match(decl(forEachDescendant(callExpr().bind("call"))),
                         *FBody, FBody->getASTContext());
    for (const auto &Match : Matches) {
      const auto *CE = Match.getNodeAs<CallExpr>("call");
      if (isa<FunctionDecl>(CE->getCalleeDecl()))
        CalledFunctions.push_back(CE);
    }

    return true;
  };

  if (!ProcessFunction(HandlerDecl, HandlerExpr))
    return;

  // Visit the definition of every function referenced by the handler function.
  // Check for allowed function calls.
  while (!CalledFunctions.empty()) {
    const CallExpr *FunctionCall = CalledFunctions.front();
    CalledFunctions.pop_front();
    // At insertion we have already ensured that only function calls are there.
    const auto *F = cast<FunctionDecl>(FunctionCall->getCalleeDecl());

    if (!ProcessFunction(F, FunctionCall))
      break;
  }
}

bool SignalHandlerCheck::isSystemCallAllowed(const FunctionDecl *FD) const {
  const IdentifierInfo *II = FD->getIdentifier();
  // Unnamed functions are not explicitly allowed.
  if (!II)
    return false;

  // FIXME: Improve for C++ (check for namespace).
  if (StrictConformingFunctions.count(II->getName()))
    return true;

  return false;
}

void SignalHandlerCheck::reportBug(const FunctionDecl *CalledFunction,
                                   const Expr *CallOrRef,
                                   const CallExpr *SignalCall,
                                   const FunctionDecl *HandlerDecl) {
  diag(CallOrRef->getBeginLoc(),
       "%0 may not be asynchronous-safe; "
       "calling it from a signal handler may be dangerous")
      << CalledFunction;
  diag(SignalCall->getSourceRange().getBegin(),
       "signal handler registered here", DiagnosticIDs::Note);
  diag(HandlerDecl->getBeginLoc(), "handler function declared here",
       DiagnosticIDs::Note);
}

} // namespace bugprone
} // namespace tidy
} // namespace clang