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//== CStringSyntaxChecker.cpp - CoreFoundation containers API *- C++ -*-==//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// An AST checker that looks for common pitfalls when using C string APIs.
// - Identifies erroneous patterns in the last argument to strncat - the number
// of bytes to copy.
//
//===----------------------------------------------------------------------===//
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/AST/Expr.h"
#include "clang/AST/OperationKinds.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/Analysis/AnalysisDeclContext.h"
#include "clang/Basic/TargetInfo.h"
#include "clang/Basic/TypeTraits.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugReporter.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/AnalysisManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/Support/raw_ostream.h"
using namespace clang;
using namespace ento;
namespace {
class WalkAST: public StmtVisitor<WalkAST> {
const CheckerBase *Checker;
BugReporter &BR;
AnalysisDeclContext* AC;
/// Check if two expressions refer to the same declaration.
bool sameDecl(const Expr *A1, const Expr *A2) {
if (const auto *D1 = dyn_cast<DeclRefExpr>(A1->IgnoreParenCasts()))
if (const auto *D2 = dyn_cast<DeclRefExpr>(A2->IgnoreParenCasts()))
return D1->getDecl() == D2->getDecl();
return false;
}
/// Check if the expression E is a sizeof(WithArg).
bool isSizeof(const Expr *E, const Expr *WithArg) {
if (const auto *UE = dyn_cast<UnaryExprOrTypeTraitExpr>(E))
if (UE->getKind() == UETT_SizeOf && !UE->isArgumentType())
return sameDecl(UE->getArgumentExpr(), WithArg);
return false;
}
/// Check if the expression E is a strlen(WithArg).
bool isStrlen(const Expr *E, const Expr *WithArg) {
if (const auto *CE = dyn_cast<CallExpr>(E)) {
const FunctionDecl *FD = CE->getDirectCallee();
if (!FD)
return false;
return (CheckerContext::isCLibraryFunction(FD, "strlen") &&
sameDecl(CE->getArg(0), WithArg));
}
return false;
}
/// Check if the expression is an integer literal with value 1.
bool isOne(const Expr *E) {
if (const auto *IL = dyn_cast<IntegerLiteral>(E))
return (IL->getValue().isIntN(1));
return false;
}
StringRef getPrintableName(const Expr *E) {
if (const auto *D = dyn_cast<DeclRefExpr>(E->IgnoreParenCasts()))
return D->getDecl()->getName();
return StringRef();
}
/// Identify erroneous patterns in the last argument to strncat - the number
/// of bytes to copy.
bool containsBadStrncatPattern(const CallExpr *CE);
/// Identify erroneous patterns in the last argument to strlcpy - the number
/// of bytes to copy.
/// The bad pattern checked is when the size is known
/// to be larger than the destination can handle.
/// char dst[2];
/// size_t cpy = 4;
/// strlcpy(dst, "abcd", sizeof("abcd") - 1);
/// strlcpy(dst, "abcd", 4);
/// strlcpy(dst + 3, "abcd", 2);
/// strlcpy(dst, "abcd", cpy);
/// Identify erroneous patterns in the last argument to strlcat - the number
/// of bytes to copy.
/// The bad pattern checked is when the last argument is basically
/// pointing to the destination buffer size or argument larger or
/// equal to.
/// char dst[2];
/// strlcat(dst, src2, sizeof(dst));
/// strlcat(dst, src2, 2);
/// strlcat(dst, src2, 10);
bool containsBadStrlcpyStrlcatPattern(const CallExpr *CE);
public:
WalkAST(const CheckerBase *Checker, BugReporter &BR, AnalysisDeclContext *AC)
: Checker(Checker), BR(BR), AC(AC) {}
// Statement visitor methods.
void VisitChildren(Stmt *S);
void VisitStmt(Stmt *S) {
VisitChildren(S);
}
void VisitCallExpr(CallExpr *CE);
};
} // end anonymous namespace
// The correct size argument should look like following:
// strncat(dst, src, sizeof(dst) - strlen(dest) - 1);
// We look for the following anti-patterns:
// - strncat(dst, src, sizeof(dst) - strlen(dst));
// - strncat(dst, src, sizeof(dst) - 1);
// - strncat(dst, src, sizeof(dst));
bool WalkAST::containsBadStrncatPattern(const CallExpr *CE) {
if (CE->getNumArgs() != 3)
return false;
const Expr *DstArg = CE->getArg(0);
const Expr *SrcArg = CE->getArg(1);
const Expr *LenArg = CE->getArg(2);
// Identify wrong size expressions, which are commonly used instead.
if (const auto *BE = dyn_cast<BinaryOperator>(LenArg->IgnoreParenCasts())) {
// - sizeof(dst) - strlen(dst)
if (BE->getOpcode() == BO_Sub) {
const Expr *L = BE->getLHS();
const Expr *R = BE->getRHS();
if (isSizeof(L, DstArg) && isStrlen(R, DstArg))
return true;
// - sizeof(dst) - 1
if (isSizeof(L, DstArg) && isOne(R->IgnoreParenCasts()))
return true;
}
}
// - sizeof(dst)
if (isSizeof(LenArg, DstArg))
return true;
// - sizeof(src)
if (isSizeof(LenArg, SrcArg))
return true;
return false;
}
bool WalkAST::containsBadStrlcpyStrlcatPattern(const CallExpr *CE) {
if (CE->getNumArgs() != 3)
return false;
const Expr *DstArg = CE->getArg(0);
const Expr *LenArg = CE->getArg(2);
const auto *DstArgDRE = dyn_cast<DeclRefExpr>(DstArg->IgnoreParenImpCasts());
const auto *LenArgDRE =
dyn_cast<DeclRefExpr>(LenArg->IgnoreParenLValueCasts());
uint64_t DstOff = 0;
if (isSizeof(LenArg, DstArg))
return false;
// - size_t dstlen = sizeof(dst)
if (LenArgDRE) {
const auto *LenArgVal = dyn_cast<VarDecl>(LenArgDRE->getDecl());
// If it's an EnumConstantDecl instead, then we're missing out on something.
if (!LenArgVal) {
assert(isa<EnumConstantDecl>(LenArgDRE->getDecl()));
return false;
}
if (LenArgVal->getInit())
LenArg = LenArgVal->getInit();
}
// - integral value
// We try to figure out if the last argument is possibly longer
// than the destination can possibly handle if its size can be defined.
if (const auto *IL = dyn_cast<IntegerLiteral>(LenArg->IgnoreParenImpCasts())) {
uint64_t ILRawVal = IL->getValue().getZExtValue();
// Case when there is pointer arithmetic on the destination buffer
// especially when we offset from the base decreasing the
// buffer length accordingly.
if (!DstArgDRE) {
if (const auto *BE =
dyn_cast<BinaryOperator>(DstArg->IgnoreParenImpCasts())) {
DstArgDRE = dyn_cast<DeclRefExpr>(BE->getLHS()->IgnoreParenImpCasts());
if (BE->getOpcode() == BO_Add) {
if ((IL = dyn_cast<IntegerLiteral>(BE->getRHS()->IgnoreParenImpCasts()))) {
DstOff = IL->getValue().getZExtValue();
}
}
}
}
if (DstArgDRE) {
if (const auto *Buffer =
dyn_cast<ConstantArrayType>(DstArgDRE->getType())) {
ASTContext &C = BR.getContext();
uint64_t BufferLen = C.getTypeSize(Buffer) / 8;
auto RemainingBufferLen = BufferLen - DstOff;
if (RemainingBufferLen < ILRawVal)
return true;
}
}
}
return false;
}
void WalkAST::VisitCallExpr(CallExpr *CE) {
const FunctionDecl *FD = CE->getDirectCallee();
if (!FD)
return;
if (CheckerContext::isCLibraryFunction(FD, "strncat")) {
if (containsBadStrncatPattern(CE)) {
const Expr *DstArg = CE->getArg(0);
const Expr *LenArg = CE->getArg(2);
PathDiagnosticLocation Loc =
PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
StringRef DstName = getPrintableName(DstArg);
SmallString<256> S;
llvm::raw_svector_ostream os(S);
os << "Potential buffer overflow. ";
if (!DstName.empty()) {
os << "Replace with 'sizeof(" << DstName << ") "
"- strlen(" << DstName <<") - 1'";
os << " or u";
} else
os << "U";
os << "se a safer 'strlcat' API";
BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
"C String API", os.str(), Loc,
LenArg->getSourceRange());
}
} else if (CheckerContext::isCLibraryFunction(FD, "strlcpy") ||
CheckerContext::isCLibraryFunction(FD, "strlcat")) {
if (containsBadStrlcpyStrlcatPattern(CE)) {
const Expr *DstArg = CE->getArg(0);
const Expr *LenArg = CE->getArg(2);
PathDiagnosticLocation Loc =
PathDiagnosticLocation::createBegin(LenArg, BR.getSourceManager(), AC);
StringRef DstName = getPrintableName(DstArg);
SmallString<256> S;
llvm::raw_svector_ostream os(S);
os << "The third argument allows to potentially copy more bytes than it should. ";
os << "Replace with the value ";
if (!DstName.empty())
os << "sizeof(" << DstName << ")";
else
os << "sizeof(<destination buffer>)";
os << " or lower";
BR.EmitBasicReport(FD, Checker, "Anti-pattern in the argument",
"C String API", os.str(), Loc,
LenArg->getSourceRange());
}
}
// Recurse and check children.
VisitChildren(CE);
}
void WalkAST::VisitChildren(Stmt *S) {
for (Stmt *Child : S->children())
if (Child)
Visit(Child);
}
namespace {
class CStringSyntaxChecker: public Checker<check::ASTCodeBody> {
public:
void checkASTCodeBody(const Decl *D, AnalysisManager& Mgr,
BugReporter &BR) const {
WalkAST walker(this, BR, Mgr.getAnalysisDeclContext(D));
walker.Visit(D->getBody());
}
};
}
void ento::registerCStringSyntaxChecker(CheckerManager &mgr) {
mgr.registerChecker<CStringSyntaxChecker>();
}
bool ento::shouldRegisterCStringSyntaxChecker(const LangOptions &LO) {
return true;
}