[clang] dfc1e31 - Produce warning for performing pointer arithmetic on a null pointer.

[Jamie Schmeiser via cfe-commits]

Author: Jamie Schmeiser
Date: 2021-05-11T11:29:50-04:00
New Revision: dfc1e31d49fe1380c9bab43373995df5fed15e6d

URL: https://github.com/llvm/llvm-project/commit/dfc1e31d49fe1380c9bab43373995df5fed15e6d
DIFF: https://github.com/llvm/llvm-project/commit/dfc1e31d49fe1380c9bab43373995df5fed15e6d.diff

LOG: Produce warning for performing pointer arithmetic on a null pointer.

Summary:
Test and produce warning for subtracting a pointer from null or subtracting
null from a pointer.  Reuse existing warning that this is undefined
behaviour.  Also add unit test for both warnings.

Reformat to satisfy clang-format.

Respond to review comments:  add additional test.

Respond to review comments:  Do not issue warning for nullptr - nullptr
in C++.

Fix indenting to satisfy clang-format.

Respond to review comments:  Add C++ tests.

Author: Jamie Schmeiser <schmeise at ca.ibm.com>
Reviewed By: efriedma (Eli Friedman), nickdesaulniers (Nick Desaulniers)
Differential Revision: https://reviews.llvm.org/D98798

Added: 
    clang/test/Sema/pointer-addition.cpp

Modified: 
    clang/lib/Sema/SemaExpr.cpp
    clang/test/Sema/pointer-addition.c

Removed: 
    


################################################################################
diff  --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index 73d7675eb189f..1390c17de9ece 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -10779,7 +10779,17 @@ QualType Sema::CheckSubtractionOperands(ExprResult &LHS, ExprResult &RHS,
                                                LHS.get(), RHS.get()))
         return QualType();
 
-      // FIXME: Add warnings for nullptr - ptr.
+      bool LHSIsNullPtr = LHS.get()->IgnoreParenCasts()->isNullPointerConstant(
+          Context, Expr::NPC_ValueDependentIsNotNull);
+      bool RHSIsNullPtr = RHS.get()->IgnoreParenCasts()->isNullPointerConstant(
+          Context, Expr::NPC_ValueDependentIsNotNull);
+
+      // Subtracting nullptr or from nullptr should produce
+      // a warning expect nullptr - nullptr is valid in C++ [expr.add]p7
+      if (LHSIsNullPtr && (!getLangOpts().CPlusPlus || !RHSIsNullPtr))
+        diagnoseArithmeticOnNullPointer(*this, Loc, LHS.get(), false);
+      if (RHSIsNullPtr && (!getLangOpts().CPlusPlus || !LHSIsNullPtr))
+        diagnoseArithmeticOnNullPointer(*this, Loc, RHS.get(), false);
 
       // The pointee type may have zero size.  As an extension, a structure or
       // union may have zero size or an array may have zero length.  In this

diff  --git a/clang/test/Sema/pointer-addition.c b/clang/test/Sema/pointer-addition.c
index 562f05340f7cb..ab641bd132363 100644
--- a/clang/test/Sema/pointer-addition.c
+++ b/clang/test/Sema/pointer-addition.c
@@ -29,4 +29,7 @@ void a(S* b, void* c) {
   // Cases that don't match the GNU inttoptr idiom get a 
diff erent warning.
   f = (char*)0 - i; // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
   int *g = (int*)0 + i; // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)((char*)0 - f); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)(f - (char*)0); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)((char*)0 - (char*)0); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}} expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
 }

diff  --git a/clang/test/Sema/pointer-addition.cpp b/clang/test/Sema/pointer-addition.cpp
new file mode 100644
index 0000000000000..5e09034ed9651
--- /dev/null
+++ b/clang/test/Sema/pointer-addition.cpp
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 %s -fsyntax-only -verify -pedantic -Wextra -std=c++11
+
+void a() {
+  char *f = (char*)0;
+  f = (char*)((char*)0 - f); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)(f - (char*)0); // expected-warning {{performing pointer arithmetic on a null pointer has undefined behavior}}
+  f = (char*)((char*)0 - (char*)0); // valid in C++
+}