SemaChecking.cpp

//===--- SemaChecking.cpp - Extra Semantic Checking -----------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by Ted Kremenek and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
//  This file implements extra semantic analysis beyond what is enforced 
//  by the C type system.
//
//===----------------------------------------------------------------------===//

#include "Sema.h"
#include "clang/AST/ASTContext.h"
#include "clang/AST/Decl.h"
#include "clang/AST/Expr.h"
#include "clang/Lex/Preprocessor.h"
#include "clang/Lex/LiteralSupport.h"
#include "clang/Basic/SourceManager.h"
#include "clang/Basic/Diagnostic.h"
#include "clang/Basic/LangOptions.h"
#include "clang/Basic/TargetInfo.h"
#include "llvm/ADT/SmallString.h"
#include "llvm/ADT/StringExtras.h"
using namespace clang;

/// CheckFunctionCall - Check a direct function call for various correctness
/// and safety properties not strictly enforced by the C type system.
void
Sema::CheckFunctionCall(Expr *Fn,
                        SourceLocation LParenLoc, SourceLocation RParenLoc,
                        FunctionDecl *FDecl,
                        Expr** Args, unsigned NumArgsInCall) {
                        
  // Get the IdentifierInfo* for the called function.
  IdentifierInfo *FnInfo = FDecl->getIdentifier();
  
  // Search the KnownFunctionIDs for the identifier.
  unsigned i = 0, e = id_num_known_functions;
  for (; i != e; ++i) { if (KnownFunctionIDs[i] == FnInfo) break; }
  if (i == e) return;
  
  // Printf checking.
  if (i <= id_vprintf) {
    // Retrieve the index of the format string parameter and determine
    // if the function is passed a va_arg argument.
    unsigned format_idx = 0;
    bool HasVAListArg = false;
    
    switch (i) {
      default: assert(false && "No format string argument index.");
      case id_printf:    format_idx = 0; break;
      case id_fprintf:   format_idx = 1; break;
      case id_sprintf:   format_idx = 1; break;
      case id_snprintf:  format_idx = 2; break;
      case id_asprintf:  format_idx = 1; HasVAListArg = true; break;
      case id_vsnprintf: format_idx = 2; HasVAListArg = true; break;
      case id_vasprintf: format_idx = 1; HasVAListArg = true; break;
      case id_vfprintf:  format_idx = 1; HasVAListArg = true; break;
      case id_vsprintf:  format_idx = 1; HasVAListArg = true; break;
      case id_vprintf:   format_idx = 0; HasVAListArg = true; break;
    }
    
    CheckPrintfArguments(Fn, LParenLoc, RParenLoc, HasVAListArg,
			 FDecl, format_idx, Args, NumArgsInCall);
  }
}

/// CheckPrintfArguments - Check calls to printf (and similar functions) for
/// correct use of format strings.  
///
///  HasVAListArg - A predicate indicating whether the printf-like
///    function is passed an explicit va_arg argument (e.g., vprintf)
///
///  format_idx - The index into Args for the format string.
///
/// Improper format strings to functions in the printf family can be
/// the source of bizarre bugs and very serious security holes.  A
/// good source of information is available in the following paper
/// (which includes additional references):
///
///  FormatGuard: Automatic Protection From printf Format String
///  Vulnerabilities, Proceedings of the 10th USENIX Security Symposium, 2001.
///
/// Functionality implemented:
///
///  We can statically check the following properties for string
///  literal format strings for non v.*printf functions (where the
///  arguments are passed directly):
//
///  (1) Are the number of format conversions equal to the number of
///      data arguments?
///
///  (2) Does each format conversion correctly match the type of the
///      corresponding data argument?  (TODO)
///
/// Moreover, for all printf functions we can:
///
///  (3) Check for a missing format string (when not caught by type checking).
///
///  (4) Check for no-operation flags; e.g. using "#" with format
///      conversion 'c'  (TODO)
///
///  (5) Check the use of '%n', a major source of security holes.
///
///  (6) Check for malformed format conversions that don't specify anything.
///
///  (7) Check for empty format strings.  e.g: printf("");
///
///  (8) Check that the format string is a wide literal.
///
/// All of these checks can be done by parsing the format string.
///
/// For now, we ONLY do (1), (3), (5), (6), (7), and (8).
void
Sema::CheckPrintfArguments(Expr *Fn, 
                           SourceLocation LParenLoc, SourceLocation RParenLoc,
                           bool HasVAListArg, FunctionDecl *FDecl,
                           unsigned format_idx, Expr** Args, 
                           unsigned NumArgsInCall) {
  // CHECK: printf-like function is called with no format string.  
  if (format_idx >= NumArgsInCall) {
    Diag(RParenLoc, diag::warn_printf_missing_format_string, 
         Fn->getSourceRange());
    return;
  }
  
  // CHECK: format string is not a string literal.
  // 
  // Dynamically generated format strings are difficult to
  // automatically vet at compile time.  Requiring that format strings
  // are string literals: (1) permits the checking of format strings by
  // the compiler and thereby (2) can practically remove the source of
  // many format string exploits.
  StringLiteral *FExpr = dyn_cast<StringLiteral>(Args[format_idx]);
  
  if (FExpr == NULL) {
    Diag(Args[format_idx]->getLocStart(), 
         diag::warn_printf_not_string_constant, Fn->getSourceRange());
    return;
  }

  // CHECK: is the format string a wide literal?
  if (FExpr->isWide()) {
    Diag(Args[format_idx]->getLocStart(),
         diag::warn_printf_format_string_is_wide_literal,
         Fn->getSourceRange());
    return;
  }

  // Str - The format string.  NOTE: this is NOT null-terminated!
  const char * const Str = FExpr->getStrData();

  // CHECK: empty format string?
  const unsigned StrLen = FExpr->getByteLength();
  
  if (StrLen == 0) {
    Diag(Args[format_idx]->getLocStart(),
         diag::warn_printf_empty_format_string, Fn->getSourceRange());
    return;
  }

  // We process the format string using a binary state machine.  The
  // current state is stored in CurrentState.
  enum {
    state_OrdChr,
    state_Conversion
  } CurrentState = state_OrdChr;
  
  // numConversions - The number of conversions seen so far.  This is
  //  incremented as we traverse the format string.
  unsigned numConversions = 0;

  // numDataArgs - The number of data arguments after the format
  //  string.  This can only be determined for non vprintf-like
  //  functions.  For those functions, this value is 1 (the sole
  //  va_arg argument).
  unsigned numDataArgs = NumArgsInCall-(format_idx+1);

  // Inspect the format string.
  unsigned StrIdx = 0;
  
  // LastConversionIdx - Index within the format string where we last saw
  //  a '%' character that starts a new format conversion.
  unsigned LastConversionIdx = 0;
  
  for ( ; StrIdx < StrLen ; ++StrIdx ) {

    // Is the number of detected conversion conversions greater than
    // the number of matching data arguments?  If so, stop.
    if (!HasVAListArg && numConversions > numDataArgs) break;
    
    // Handle "\0"
    if(Str[StrIdx] == '\0' ) {
      // The string returned by getStrData() is not null-terminated,
      // so the presence of a null character is likely an error.
    
      SourceLocation Loc =
      PP.AdvanceToTokenCharacter(Args[format_idx]->getLocStart(),StrIdx+1);
    
      Diag(Loc, diag::warn_printf_format_string_contains_null_char,
           Fn->getSourceRange());
    
      return;
    }
    
    // Ordinary characters (not processing a format conversion).
    if (CurrentState == state_OrdChr) {
      if (Str[StrIdx] == '%') {
        CurrentState = state_Conversion;
        LastConversionIdx = StrIdx;
      }
      continue;
    }

    // Seen '%'.  Now processing a format conversion.
    switch (Str[StrIdx]) {
      // Characters which can terminate a format conversion
      // (e.g. "%d").  Characters that specify length modifiers or
      // other flags are handled by the default case below.
      //
      // TODO: additional checks will go into the following cases.
      case 'i':
      case 'd':
      case 'o': 
      case 'u': 
      case 'x':
      case 'X':
      case 'D':
      case 'O':
      case 'U':
      case 'e':
      case 'E':
      case 'f':
      case 'F':
      case 'g':
      case 'G':
      case 'a':
      case 'A':
      case 'c':
      case 'C':
      case 'S':
      case 's':
      case 'P': 
        ++numConversions;
        CurrentState = state_OrdChr;
        break;

      // CHECK: Are we using "%n"?  Issue a warning.
      case 'n': {
        ++numConversions;
        CurrentState = state_OrdChr;
        SourceLocation Loc = 
          PP.AdvanceToTokenCharacter(Args[format_idx]->getLocStart(),
                                     LastConversionIdx+1);
                                     
        Diag(Loc, diag::warn_printf_write_back, Fn->getSourceRange());
        break;
      }
                    
      // Handle "%%"
      case '%':
        // Sanity check: Was the first "%" character the previous one?
        // If not, we will assume that we have a malformed format
        // conversion, and that the current "%" character is the start
        // of a new conversion.
        if (StrIdx - LastConversionIdx == 1)
          CurrentState = state_OrdChr; 
        else {
          // Issue a warning: invalid format conversion.
          SourceLocation Loc =
            PP.AdvanceToTokenCharacter(Args[format_idx]->getLocStart(),
                                       LastConversionIdx+1);
              
          Diag(Loc, diag::warn_printf_invalid_conversion, 
	       std::string(Str+LastConversionIdx, Str+StrIdx),
               Fn->getSourceRange());
               
          // This conversion is broken.  Advance to the next format
          // conversion.
          LastConversionIdx = StrIdx;
          ++numConversions;
        }
        
        break;
                
      default:
        // This case catches all other characters: flags, widths, etc.
        // We should eventually process those as well.
        break;
    }
  }

  if (CurrentState == state_Conversion) {
    // Issue a warning: invalid format conversion.
    SourceLocation Loc =
      PP.AdvanceToTokenCharacter(Args[format_idx]->getLocStart(),
                                 LastConversionIdx+1);
    
    Diag(Loc, diag::warn_printf_invalid_conversion,
	 std::string(Str+LastConversionIdx, Str+StrIdx),
         Fn->getSourceRange());
    return;
  }
  
  if (!HasVAListArg) {
    // CHECK: Does the number of format conversions exceed the number
    //        of data arguments?
    if (numConversions > numDataArgs) {
      SourceLocation Loc =
        PP.AdvanceToTokenCharacter(Args[format_idx]->getLocStart(),
                                   LastConversionIdx);
                                   
      Diag(Loc, diag::warn_printf_insufficient_data_args,
           Fn->getSourceRange());
    }
    // CHECK: Does the number of data arguments exceed the number of
    //        format conversions in the format string?
    else if (numConversions < numDataArgs)
      Diag(Args[format_idx+numConversions+1]->getLocStart(),
           diag::warn_printf_too_many_data_args, Fn->getSourceRange());
  }
}