[libFuzzer] Link libFuzzer's own interceptors when other compiler runtimes are not linked.
Summary: libFuzzer intercepts certain library functions such as memcmp/strcmp by defining weak hooks. Weak hooks, however, are called only when other runtimes such as ASan is linked. This patch defines libFuzzer's own interceptors, which is linked into the libFuzzer executable when other runtimes are not linked, i.e., when -fsanitize=fuzzer is given, but not others.
Reviewers: kcc, morehouse, hctim
Reviewed By: morehouse, hctim
Subscribers: krytarowski, mgorny, cfe-commits, #sanitizers
Tags: #clang, #sanitizers
Differential Revision: https://reviews.llvm.org/D83494
GitOrigin-RevId: f78d9fceea736d431e9e3cbca291e3909e3aa46d
diff --git a/FuzzerInterceptors.cpp b/FuzzerInterceptors.cpp
new file mode 100644
index 0000000..cb55b4a
--- /dev/null
+++ b/FuzzerInterceptors.cpp
@@ -0,0 +1,170 @@
+//===-- FuzzerInterceptors.cpp --------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+// Intercept certain libc functions to aid fuzzing.
+// Linked only when other RTs that define their own interceptors are not linked.
+//===----------------------------------------------------------------------===//
+
+#include "FuzzerPlatform.h"
+
+#if LIBFUZZER_LINUX
+
+#define GET_CALLER_PC() __builtin_return_address(0)
+
+#define PTR_TO_REAL(x) real_##x
+#define REAL(x) __interception::PTR_TO_REAL(x)
+#define FUNC_TYPE(x) x##_type
+#define DEFINE_REAL(ret_type, func, ...) \
+ typedef ret_type (*FUNC_TYPE(func))(__VA_ARGS__); \
+ namespace __interception { \
+ FUNC_TYPE(func) PTR_TO_REAL(func); \
+ }
+
+#include <cassert>
+#include <cstdint>
+#include <dlfcn.h> // for dlsym()
+#include <sanitizer/common_interface_defs.h>
+
+static void *getFuncAddr(const char *name, uintptr_t wrapper_addr) {
+ void *addr = dlsym(RTLD_NEXT, name);
+ if (!addr) {
+ // If the lookup using RTLD_NEXT failed, the sanitizer runtime library is
+ // later in the library search order than the DSO that we are trying to
+ // intercept, which means that we cannot intercept this function. We still
+ // want the address of the real definition, though, so look it up using
+ // RTLD_DEFAULT.
+ addr = dlsym(RTLD_DEFAULT, name);
+
+ // In case `name' is not loaded, dlsym ends up finding the actual wrapper.
+ // We don't want to intercept the wrapper and have it point to itself.
+ if (reinterpret_cast<uintptr_t>(addr) == wrapper_addr)
+ addr = nullptr;
+ }
+ return addr;
+}
+
+static int FuzzerInited = 0;
+static bool FuzzerInitIsRunning;
+
+static void fuzzerInit();
+
+static void ensureFuzzerInited() {
+ assert(!FuzzerInitIsRunning);
+ if (!FuzzerInited) {
+ fuzzerInit();
+ }
+}
+
+extern "C" {
+
+DEFINE_REAL(int, memcmp, const void *, const void *, size_t)
+DEFINE_REAL(int, strncmp, const char *, const char *, size_t)
+DEFINE_REAL(int, strcmp, const char *, const char *)
+DEFINE_REAL(int, strncasecmp, const char *, const char *, size_t)
+DEFINE_REAL(int, strcasecmp, const char *, const char *)
+DEFINE_REAL(char *, strstr, const char *, const char *)
+DEFINE_REAL(char *, strcasestr, const char *, const char *)
+DEFINE_REAL(void *, memmem, const void *, size_t, const void *, size_t)
+
+ATTRIBUTE_INTERFACE int memcmp(const void *s1, const void *s2, size_t n) {
+ ensureFuzzerInited();
+ int result = REAL(memcmp)(s1, s2, n);
+ __sanitizer_weak_hook_memcmp(GET_CALLER_PC(), s1, s2, n, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE int strncmp(const char *s1, const char *s2, size_t n) {
+ ensureFuzzerInited();
+ int result = REAL(strncmp)(s1, s2, n);
+ __sanitizer_weak_hook_strncmp(GET_CALLER_PC(), s1, s2, n, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE int strcmp(const char *s1, const char *s2) {
+ ensureFuzzerInited();
+ int result = REAL(strcmp)(s1, s2);
+ __sanitizer_weak_hook_strcmp(GET_CALLER_PC(), s1, s2, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE int strncasecmp(const char *s1, const char *s2, size_t n) {
+ ensureFuzzerInited();
+ int result = REAL(strncasecmp)(s1, s2, n);
+ __sanitizer_weak_hook_strncasecmp(GET_CALLER_PC(), s1, s2, n, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE int strcasecmp(const char *s1, const char *s2) {
+ ensureFuzzerInited();
+ int result = REAL(strcasecmp)(s1, s2);
+ __sanitizer_weak_hook_strcasecmp(GET_CALLER_PC(), s1, s2, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE char *strstr(const char *s1, const char *s2) {
+ ensureFuzzerInited();
+ char *result = REAL(strstr)(s1, s2);
+ __sanitizer_weak_hook_strstr(GET_CALLER_PC(), s1, s2, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE char *strcasestr(const char *s1, const char *s2) {
+ ensureFuzzerInited();
+ char *result = REAL(strcasestr)(s1, s2);
+ __sanitizer_weak_hook_strcasestr(GET_CALLER_PC(), s1, s2, result);
+
+ return result;
+}
+
+ATTRIBUTE_INTERFACE
+void *memmem(const void *s1, size_t len1, const void *s2, size_t len2) {
+ ensureFuzzerInited();
+ void *result = REAL(memmem)(s1, len1, s2, len2);
+ __sanitizer_weak_hook_memmem(GET_CALLER_PC(), s1, len1, s2, len2, result);
+
+ return result;
+}
+
+__attribute__((section(".preinit_array"),
+ used)) static void (*__local_fuzzer_preinit)(void) = fuzzerInit;
+
+} // extern "C"
+
+static void fuzzerInit() {
+ assert(!FuzzerInitIsRunning);
+ if (FuzzerInited)
+ return;
+ FuzzerInitIsRunning = true;
+
+ REAL(memcmp) = reinterpret_cast<memcmp_type>(
+ getFuncAddr("memcmp", reinterpret_cast<uintptr_t>(&memcmp)));
+ REAL(strncmp) = reinterpret_cast<strncmp_type>(
+ getFuncAddr("strncmp", reinterpret_cast<uintptr_t>(&strncmp)));
+ REAL(strcmp) = reinterpret_cast<strcmp_type>(
+ getFuncAddr("strcmp", reinterpret_cast<uintptr_t>(&strcmp)));
+ REAL(strncasecmp) = reinterpret_cast<strncasecmp_type>(
+ getFuncAddr("strncasecmp", reinterpret_cast<uintptr_t>(&strncasecmp)));
+ REAL(strcasecmp) = reinterpret_cast<strcasecmp_type>(
+ getFuncAddr("strcasecmp", reinterpret_cast<uintptr_t>(&strcasecmp)));
+ REAL(strstr) = reinterpret_cast<strstr_type>(
+ getFuncAddr("strstr", reinterpret_cast<uintptr_t>(&strstr)));
+ REAL(strcasestr) = reinterpret_cast<strcasestr_type>(
+ getFuncAddr("strcasestr", reinterpret_cast<uintptr_t>(&strcasestr)));
+ REAL(memmem) = reinterpret_cast<memmem_type>(
+ getFuncAddr("memmem", reinterpret_cast<uintptr_t>(&memmem)));
+
+ FuzzerInitIsRunning = false;
+ FuzzerInited = 1;
+}
+
+#endif
