From aaed67f78673d6fb213de5a58dd1ed08d2ab9db2 Mon Sep 17 00:00:00 2001
From: Edward Thomson <ethomson@edwardthomson.com>
Date: Thu, 18 Apr 2024 20:47:45 +0100
Subject: [PATCH] alloc: introduce debug allocators

Instead of tweaking the `stdalloc` allocator when
`GIT_DEBUG_STRICT_ALLOC` is defined, actually create a debugging
allocator. This allows us to ensure that we are strict about things like
not expecting `malloc(0)` to do something useful, but we can also
introduce an excessively pedantic `realloc` implementation that _always_
creates a new buffer, throws away its original `ptr`, and overwrites the
data that's there with garbage. This may be helpful to identify places
that make assumptions about realloc.
---
 src/util/alloc.c                 |  5 ++-
 src/util/allocators/debugalloc.c | 71 ++++++++++++++++++++++++++++++++
 src/util/allocators/debugalloc.h | 17 ++++++++
 src/util/allocators/stdalloc.c   | 10 -----
 4 files changed, 92 insertions(+), 11 deletions(-)
 create mode 100644 src/util/allocators/debugalloc.c
 create mode 100644 src/util/allocators/debugalloc.h

diff --git a/src/util/alloc.c b/src/util/alloc.c
index 6ec173d04..998b0aea1 100644
--- a/src/util/alloc.c
+++ b/src/util/alloc.c
@@ -8,8 +8,9 @@
 #include "alloc.h"
 #include "runtime.h"
 
-#include "allocators/failalloc.h"
 #include "allocators/stdalloc.h"
+#include "allocators/debugalloc.h"
+#include "allocators/failalloc.h"
 #include "allocators/win32_leakcheck.h"
 
 /* Fail any allocation until git_libgit2_init is called. */
@@ -88,6 +89,8 @@ static int setup_default_allocator(void)
 {
 #if defined(GIT_WIN32_LEAKCHECK)
 	return git_win32_leakcheck_init_allocator(&git__allocator);
+#elif defined(GIT_DEBUG_STRICT_ALLOC)
+	return git_debugalloc_init_allocator(&git__allocator);
 #else
 	return git_stdalloc_init_allocator(&git__allocator);
 #endif
diff --git a/src/util/allocators/debugalloc.c b/src/util/allocators/debugalloc.c
new file mode 100644
index 000000000..acb002dbb
--- /dev/null
+++ b/src/util/allocators/debugalloc.c
@@ -0,0 +1,71 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#include "debugalloc.h"
+
+static void *debugalloc__malloc(size_t len, const char *file, int line)
+{
+	void *ptr;
+	size_t total = len + sizeof(size_t);
+
+	GIT_UNUSED(file);
+	GIT_UNUSED(line);
+
+	if (!len || (ptr = malloc(total)) == NULL)
+		return NULL;
+
+	memcpy(ptr, &len, sizeof(size_t));
+	return ptr + sizeof(size_t);
+}
+
+static void *debugalloc__realloc(void *ptr, size_t len, const char *file, int line)
+{
+	void *newptr;
+	size_t original_len;
+	size_t total = len + sizeof(size_t);
+
+	GIT_UNUSED(file);
+	GIT_UNUSED(line);
+
+	if (!len && !ptr)
+		return NULL;
+
+	if (!len) {
+		free(ptr - sizeof(size_t));
+		return NULL;
+	}
+
+	if ((newptr = malloc(total)) == NULL)
+		return NULL;
+
+	if (ptr) {
+		memcpy(&original_len, ptr - sizeof(size_t), sizeof(size_t));
+		memcpy(newptr + sizeof(size_t), ptr, min(len, original_len));
+
+		memset(ptr - sizeof(size_t), 0xfd, original_len + sizeof(size_t));
+		free(ptr - sizeof(size_t));
+	}
+
+	memcpy(newptr, &len, sizeof(size_t));
+	return newptr + sizeof(size_t);
+}
+
+static void debugalloc__free(void *ptr)
+{
+	if (!ptr)
+		return;
+
+	free(ptr - sizeof(size_t));
+}
+
+int git_debugalloc_init_allocator(git_allocator *allocator)
+{
+	allocator->gmalloc = debugalloc__malloc;
+	allocator->grealloc = debugalloc__realloc;
+	allocator->gfree = debugalloc__free;
+	return 0;
+}
diff --git a/src/util/allocators/debugalloc.h b/src/util/allocators/debugalloc.h
new file mode 100644
index 000000000..dea0ca31c
--- /dev/null
+++ b/src/util/allocators/debugalloc.h
@@ -0,0 +1,17 @@
+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#ifndef INCLUDE_allocators_debugalloc_h__
+#define INCLUDE_allocators_debugalloc_h__
+
+#include "git2_util.h"
+
+#include "alloc.h"
+
+int git_debugalloc_init_allocator(git_allocator *allocator);
+
+#endif
diff --git a/src/util/allocators/stdalloc.c b/src/util/allocators/stdalloc.c
index f2d72a7e6..65ec40fbe 100644
--- a/src/util/allocators/stdalloc.c
+++ b/src/util/allocators/stdalloc.c
@@ -12,11 +12,6 @@ static void *stdalloc__malloc(size_t len, const char *file, int line)
 	GIT_UNUSED(file);
 	GIT_UNUSED(line);
 
-#ifdef GIT_DEBUG_STRICT_ALLOC
-	if (!len)
-		return NULL;
-#endif
-
 	return malloc(len);
 }
 
@@ -25,11 +20,6 @@ static void *stdalloc__realloc(void *ptr, size_t size, const char *file, int lin
 	GIT_UNUSED(file);
 	GIT_UNUSED(line);
 
-#ifdef GIT_DEBUG_STRICT_ALLOC
-	if (!size)
-		return NULL;
-#endif
-
 	return realloc(ptr, size);
 }