[clang] [analyzer][NFC] Add ArrayBound tests to document casting bug (PR #127062)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat updated https://github.com/llvm/llvm-project/pull/127062

>From c3dc63db8914d759b6c58611b7448d4e83c66752 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Don=C3=A1t=20Nagy?= <donat.nagy at ericsson.com>
Date: Thu, 13 Feb 2025 11:35:45 +0100
Subject: [PATCH 1/2] [analyzer][NFC] Add ArrayBound tests to document casting
 bug

Add a few security.ArrayBound testcases that document the false
positives caused the fact that the analyzer doesn't model a cast from
`signed char` to `unsigned char`.
---
 clang/test/Analysis/out-of-bounds.c | 31 +++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/clang/test/Analysis/out-of-bounds.c b/clang/test/Analysis/out-of-bounds.c
index 923797200d0b4..177c03500a0c3 100644
--- a/clang/test/Analysis/out-of-bounds.c
+++ b/clang/test/Analysis/out-of-bounds.c
@@ -194,3 +194,34 @@ char test_comparison_with_extent_symbol(struct incomplete *p) {
   return ((char *)p)[-1]; // no-warning
 }
 
+int table[256], small_table[128];
+int test_cast_to_unsigned(signed char x) {
+  unsigned char y = x;
+  if (x >= 0)
+    return x;
+  // FIXME: Here the analyzer ignores the signed -> unsigned cast, and manages to
+  // load a negative value from an unsigned variable. This causes an underflow
+  // report, which is an ugly false positive.
+  // The underlying issue is tracked by Github ticket #39492.
+  return table[y]; // expected-warning {{Out of bound access to memory preceding}}
+}
+
+int test_cast_to_unsigned_overflow(signed char x) {
+  unsigned char y = x;
+  if (x >= 0)
+    return x;
+  // A variant of 'test_cast_to_unsigned' where the correct behavior would be
+  // an overflow report (because the negative values are cast to `unsigned
+  // char` values that are too large).
+  // FIXME: See comment in 'test_cast_to_unsigned'.
+  return small_table[y]; // expected-warning {{Out of bound access to memory preceding}}
+}
+
+int test_negative_offset_with_unsigned_idx(void) {
+  // An example where the subscript operator uses an unsigned index, but the
+  // underflow report is still justified. (We should try to keep this if we
+  // silence false positives like the one in 'test_cast_to_unsigned'.)
+  int *p = table - 10;
+  unsigned idx = 2u;
+  return p[idx]; // expected-warning {{Out of bound access to memory preceding}}
+}

>From 0927d4de066484959b3272fd579390e29ca4ae91 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Don=C3=A1t=20Nagy?= <donat.nagy at ericsson.com>
Date: Thu, 13 Feb 2025 15:11:35 +0100
Subject: [PATCH 2/2] Dump the value of the signed unsigned char

---
 clang/test/Analysis/out-of-bounds.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/clang/test/Analysis/out-of-bounds.c b/clang/test/Analysis/out-of-bounds.c
index 177c03500a0c3..067ccbd58fff2 100644
--- a/clang/test/Analysis/out-of-bounds.c
+++ b/clang/test/Analysis/out-of-bounds.c
@@ -1,6 +1,7 @@
 // RUN: %clang_analyze_cc1 -Wno-array-bounds -analyzer-checker=core,security.ArrayBound,debug.ExprInspection -verify %s
 
 void clang_analyzer_eval(int);
+void clang_analyzer_value(int);
 
 // Tests doing an out-of-bounds access after the end of an array using:
 // - constant integer index
@@ -203,6 +204,7 @@ int test_cast_to_unsigned(signed char x) {
   // load a negative value from an unsigned variable. This causes an underflow
   // report, which is an ugly false positive.
   // The underlying issue is tracked by Github ticket #39492.
+  clang_analyzer_value(y); // expected-warning {{8s:{ [-128, -1] } }}
   return table[y]; // expected-warning {{Out of bound access to memory preceding}}
 }
 
@@ -214,6 +216,7 @@ int test_cast_to_unsigned_overflow(signed char x) {
   // an overflow report (because the negative values are cast to `unsigned
   // char` values that are too large).
   // FIXME: See comment in 'test_cast_to_unsigned'.
+  clang_analyzer_value(y); // expected-warning {{8s:{ [-128, -1] } }}
   return small_table[y]; // expected-warning {{Out of bound access to memory preceding}}
 }