[-Wunsafe-buffer-usage] Fold the expression "cond ? E1 : E2" when checking safe patterns, if "cond" is a constant

[ziqingluo-90]

In -Wunsafe-buffer-usage, many safe pattern checks can benefit from constant folding. This commit improves null-terminated pointer checks by folding conditional expressions.

rdar://159374822

[llvmbot]

@llvm/pr-subscribers-clang-analysis

@llvm/pr-subscribers-clang

Author: Ziqing Luo (ziqingluo-90)

Changes

In -Wunsafe-buffer-usage, many safe pattern checks can benefit from constant folding. This commit improves null-terminated pointer checks by folding conditional expressions.

rdar://159374822

---

Full diff: https://github.com/llvm/llvm-project/pull/167989.diff

2 Files Affected:

• (modified) clang/lib/Analysis/UnsafeBufferUsage.cpp (+21-5)
• (added) clang/test/SemaCXX/warn-unsafe-buffer-usage-fold-conditional.cpp (+33)

diff --git a/clang/lib/Analysis/UnsafeBufferUsage.cpp b/clang/lib/Analysis/UnsafeBufferUsage.cpp index f5a368636c43d..1203a27e7dae5 100644 --- a/clang/lib/Analysis/UnsafeBufferUsage.cpp +++ b/clang/lib/Analysis/UnsafeBufferUsage.cpp @@ -781,9 +781,25 @@ struct LibcFunNamePrefixSuffixParser { } }; +// Constant fold a conditional expression 'cond ? A : B' to +// - 'A', if 'cond' has constant true value; +// - 'B', if 'cond' has constant false value. +static const Expr *tryConstantFoldConditionalExpr(const Expr *E, + const ASTContext &Ctx) { + // FIXME: more places can use this function + if (const auto *CE = dyn_cast<ConditionalOperator>(E)) { + bool CondEval; + + if (CE->getCond()->EvaluateAsBooleanCondition(CondEval, Ctx)) + return CondEval ? CE->getLHS() : CE->getRHS(); + } + return E; +} + // A pointer type expression is known to be null-terminated, if it has the // form: E.c_str(), for any expression E of `std::string` type. -static bool isNullTermPointer(const Expr *Ptr) { +static bool isNullTermPointer(const Expr *Ptr, ASTContext &Ctx) { + Ptr = tryConstantFoldConditionalExpr(Ptr, Ctx); if (isa<clang::StringLiteral>(Ptr->IgnoreParenImpCasts())) return true; if (isa<PredefinedExpr>(Ptr->IgnoreParenImpCasts())) @@ -874,7 +890,7 @@ static bool hasUnsafeFormatOrSArg(const CallExpr *Call, const Expr *&UnsafeArg, const Expr *Arg = Call->getArg(ArgIdx); - if (isNullTermPointer(Arg)) + if (isNullTermPointer(Arg, Ctx)) // If Arg is a null-terminated pointer, it is safe anyway. return true; // continue parsing @@ -922,8 +938,8 @@ static bool hasUnsafeFormatOrSArg(const CallExpr *Call, const Expr *&UnsafeArg, // (including the format argument) is unsafe pointer. return llvm::any_of( llvm::make_range(Call->arg_begin() + FmtArgIdx, Call->arg_end()), - [&UnsafeArg](const Expr *Arg) -> bool { - if (Arg->getType()->isPointerType() && !isNullTermPointer(Arg)) { + [&UnsafeArg, &Ctx](const Expr *Arg) -> bool { + if (Arg->getType()->isPointerType() && !isNullTermPointer(Arg, Ctx)) { UnsafeArg = Arg; return true; } @@ -1175,7 +1191,7 @@ static bool hasUnsafePrintfStringArg(const CallExpr &Node, ASTContext &Ctx, // We don't really recognize this "normal" printf, the only thing we // can do is to require all pointers to be null-terminated: for (const auto *Arg : Node.arguments()) - if (Arg->getType()->isPointerType() && !isNullTermPointer(Arg)) { + if (Arg->getType()->isPointerType() && !isNullTermPointer(Arg, Ctx)) { Result.addNode(Tag, DynTypedNode::create(*Arg)); return true; } diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-fold-conditional.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-fold-conditional.cpp new file mode 100644 index 0000000000000..edb71c299708a --- /dev/null +++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-fold-conditional.cpp @@ -0,0 +1,33 @@ +// RUN: %clang_cc1 -fsyntax-only -std=c++20 -Wno-all -Wunsafe-buffer-usage -verify %s +// RUN: %clang_cc1 -fsyntax-only -x c -Wno-all -Wunsafe-buffer-usage -verify %s +// expected-no-diagnostics + +#include <ptrcheck.h> + +typedef struct {} FILE; +int fprintf( FILE* stream, const char* format, ... ); +FILE * stderr; + +#define DEBUG_ASSERT_MESSAGE(name, assertion, label, message, file, line, value) \ + fprintf(stderr, "AssertMacros: %s, %s file: %s, line: %d, value: %lld\n", \ + assertion, (message!=0) ? message : "", file, line, (long long) (value)); + + +#define Require(assertion, exceptionLabel) \ + do \ + { \ + if ( __builtin_expect(!(assertion), 0) ) { \ + DEBUG_ASSERT_MESSAGE( \ + "DEBUG_ASSERT_COMPONENT_NAME_STRING", \ + #assertion, #exceptionLabel, 0, __FILE__, __LINE__, 0); \ +goto exceptionLabel; \ + }	\ + } while ( 0 ) + + +void f(int x, int y) { + Require(x == y, L1); + L1: + return; +} + 

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[steakhal]

To me the change looks good FWIW.

[ziqingluo-90]

@steakhal Thanks for the rapid review!

[github-actions]

🐧 Linux x64 Test Results

• 111277 tests passed
• 4414 tests skipped