commit 7d0aa69421e5de41bdaa6bf16b722f57fdb3c1c8
Author: doyle <doylet@protonmail.com>
Date:   Mon Aug 28 00:33:11 2023 +1000

    Initial commit

diff --git a/build.bat b/build.bat
new file mode 100644
index 0000000..8cdd910
--- /dev/null
+++ b/build.bat
@@ -0,0 +1,2 @@
+@echo off
+cl main.cpp -Z7 -fsanitize=address -nologo -link || exit /b 1
diff --git a/main.cpp b/main.cpp
new file mode 100644
index 0000000..8abc67f
--- /dev/null
+++ b/main.cpp
@@ -0,0 +1,182 @@
+
+#define WIN32_MEAN_AND_LEAN
+#include <Windows.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <assert.h>
+#include <sanitizer/asan_interface.h>
+
+#define IsPowerOfTwo(value, pot) ((((size_t)value) & (((size_t)pot) - 1)) == 0)
+#define AsanAlignment 8
+
+#define PrintByteSpacing(space) \
+    if (index) \
+        printf(space); \
+    if (index && (index % 8 == 0)) \
+        printf("| ")
+
+static void PrintByteArray(char const *array, size_t array_size)
+{
+    printf("   Byte Array                    ");
+    for (size_t index = 0 ; index < array_size; index++) {
+        PrintByteSpacing(" ");
+        printf("%02zu", index);
+    }
+    printf("\n");
+}
+
+static void PrintPoisonedBytes(int step, char const *array, size_t array_size)
+{
+    printf("%d. __asan_address_is_poisoned    ", step);
+    for (size_t index = 0 ; index < array_size; index++) {
+        PrintByteSpacing("  ");
+        printf("%c", __asan_address_is_poisoned(array + index) ? 'x' : ' ');
+    }
+    printf("\n");
+}
+
+static void PrintPoisonMemoryIndex(int step, char const *array, size_t array_size, size_t poison_index)
+{
+    printf("%d. __asan_poison_memory_region   ", step);
+    for (size_t index = 0; index < array_size; index++) {
+        PrintByteSpacing(" ");
+        printf("%c ", index == poison_index ? 'x' : ' ');
+    }
+    printf("\n");
+}
+
+static void PrintPoisonMemoryRegion(int step, char const *array, size_t array_size, size_t poison_start_index, size_t poison_end_index)
+{
+    printf("%d. __asan_poison_memory_region   ", step);
+    for (size_t index = 0; index < array_size; index++) {
+        PrintByteSpacing(" ");
+        printf("%c ", index >= poison_start_index && index <= poison_end_index ? 'x' : ' ');
+    }
+    printf("\n");
+}
+
+int main()
+{
+    printf("Here we demonstrate that ASAN poison-ing will only poison the\n"
+           "byte region if the region meets an 8 byte boundary. It will only\n"
+           "poison bytes upto the 8 byte boundary, any bytes that straddle\n"
+           "the boundary that do not hit the next 8 byte boundary are not\n"
+           "poison-ed.\n\n");
+
+    uint32_t const ASAN_ALIGNMENT = 8;
+    uint32_t const REGION_WINDOW  = 7;
+    char array[16]                = {};
+    for (size_t poison_index = 0; poison_index < sizeof(array) - (REGION_WINDOW - 1); poison_index++) {
+        assert(IsPowerOfTwo(array, ASAN_ALIGNMENT));
+
+        // NOTE: Print byte array ==================================================================
+
+        PrintByteArray(array, sizeof(array));
+
+        // NOTE: Poison the array ==================================================================
+
+        __asan_poison_memory_region(&array[poison_index], REGION_WINDOW);
+        PrintPoisonMemoryRegion(1 /*step*/, array, sizeof(array), poison_index, poison_index + (REGION_WINDOW - 1));
+
+        // NOTE: Print the poison-ed bytes =========================================================
+
+        PrintPoisonedBytes(2 /*step*/, array, sizeof(array));
+
+        // NOTE: Cleanup ===========================================================================
+
+        __asan_unpoison_memory_region(array, sizeof(array));
+        printf("\n");
+    }
+
+    printf(
+        "Now we demonstrate that unpoison-ing 1 byte in the 8 byte window\n"
+        "will unpoison all bytes prior to it up until the start of the window\n"
+        "until the previous 8 byte boundary.\n\n");
+
+    for (size_t unpoison_index = 0; unpoison_index < sizeof(array); unpoison_index++) {
+        assert(IsPowerOfTwo(array, ASAN_ALIGNMENT));
+
+        // NOTE: Print byte array ==================================================================
+
+        PrintByteArray(array, sizeof(array));
+
+        // NOTE: Poison the array ==================================================================
+
+        __asan_poison_memory_region(array, sizeof(array));
+        PrintPoisonMemoryRegion(1 /*step*/, array, sizeof(array), 0, sizeof(array) - 1);
+
+        // NOTE: Print the poison-ed bytes =========================================================
+
+        PrintPoisonedBytes(2 /*step*/, array, sizeof(array));
+
+        // NOTE: Unpoison byte at each position separately =========================================
+
+        printf("3. __asan_unpoison_memory_region ");
+        __asan_unpoison_memory_region(array + unpoison_index, 1);
+
+        for (size_t index = 0 ; index < sizeof(array); index++) {
+            PrintByteSpacing("  ");
+            printf("%c", index == unpoison_index ? 'x' : ' ');
+        }
+        printf("\n");
+
+        // NOTE: Print the poison-ed bytes =========================================================
+
+        printf("4. __asan_address_is_poisoned    ");
+        for (size_t index = 0 ; index < sizeof(array); index++) {
+            PrintByteSpacing("  ");
+            printf("%c", __asan_address_is_poisoned(array + index) ? 'x' : ' ');
+        }
+        printf("\n\n");
+
+        // NOTE: Cleanup ===========================================================================
+
+        __asan_unpoison_memory_region(array, sizeof(array));
+    }
+
+    printf(
+        "Unpoison-ing across 8 byte boundaries may lead to undesired\n"
+        "behaviour, with all bytes on the left side of the boundary being\n"
+        "unpoisoned\n\n");
+    {
+        // NOTE: Print byte array ==================================================================
+
+        char array[16] = {};
+        PrintByteArray(array, sizeof(array));
+
+        // NOTE: Poison the array ==================================================================
+
+        __asan_poison_memory_region(array, sizeof(array));
+        PrintPoisonMemoryRegion(1 /*step*/, array, sizeof(array), 0, sizeof(array) - 1);
+
+        // NOTE: Print the poison-ed bytes =========================================================
+
+        PrintPoisonedBytes(2 /*step*/, array, sizeof(array));
+
+        // NOTE: Unpoison across the 8 byte boundary ===============================================
+
+        printf("3. __asan_unpoison_memory_region ");
+        uint32_t const start_poison_index = 7;
+        uint32_t const bytes_to_unpoison  = 2;
+        uint32_t const end_poison_index   = start_poison_index + (bytes_to_unpoison - 1);
+        __asan_unpoison_memory_region(array + start_poison_index, bytes_to_unpoison);
+
+        for (size_t index = 0 ; index < sizeof(array); index++) {
+            PrintByteSpacing("  ");
+            printf("%c", index >= start_poison_index && index <= end_poison_index ? 'x' : ' ');
+        }
+        printf("\n");
+
+        // NOTE: Print the poison-ed bytes =========================================================
+
+        PrintPoisonedBytes(4 /*step*/, array, sizeof(array));
+    }
+
+    printf("\n\nTLDR:\n"
+           "  __asan_poison_memory_region(ptr, size)\n"
+           "  Poisons the byte region [ptr, AlignDown(ptr+size, 8))\n\n"
+           "  __asan_unpoison_memory_region(ptr, size)\n"
+           "  Unpoisons the byte region [AlignDown(ptr, 8), ptr+size)\n\n"
+          );
+    return 0;
+}