[PATCH] Add dfsan_weak_hook_memcmp

[Kostya Serebryany]

Hi pcc,

Add a weak hook to be called from dfsan's custom memcmp.
The primary user will be lib/Fuzzer.
If this works well we'll add more hooks (strcmp, etc).

http://reviews.llvm.org/D9541

Files:
  include/sanitizer/dfsan_interface.h
  lib/dfsan/dfsan_custom.cc

Index: include/sanitizer/dfsan_interface.h
===================================================================
--- include/sanitizer/dfsan_interface.h
+++ include/sanitizer/dfsan_interface.h
@@ -91,6 +91,16 @@
 /// <label> <parent label 1> <parent label 2> <label description if any>
 void dfsan_dump_labels(int fd);
 
+/// Whenever a dfsan's custom function is called the corresponding
+/// hook is called it non-zero. The hooks should be defined by the user.
+/// The primary use case is taint-guided fuzzing, where the fuzzer
+/// needs to see the parameters of the function and the labels.
+/// FIXME: implement more hooks.
+
+/// memcmp hook.
+void dfsan_weak_hook_memcmp(const void *s1, const void *s2, size_t n,
+                            dfsan_label s1_label, dfsan_label s2_label,
+                            dfsan_label n_label);
 #ifdef __cplusplus
 }  // extern "C"
 
Index: lib/dfsan/dfsan_custom.cc
===================================================================
--- lib/dfsan/dfsan_custom.cc
+++ lib/dfsan/dfsan_custom.cc
@@ -82,11 +82,18 @@
   }
 }
 
+SANITIZER_INTERFACE_ATTRIBUTE SANITIZER_WEAK_ATTRIBUTE
+void dfsan_weak_hook_memcmp(const void *s1, const void *s2, size_t n,
+                            dfsan_label s1_label, dfsan_label s2_label,
+                            dfsan_label n_label);
+
 SANITIZER_INTERFACE_ATTRIBUTE int __dfsw_memcmp(const void *s1, const void *s2,
                                                 size_t n, dfsan_label s1_label,
                                                 dfsan_label s2_label,
                                                 dfsan_label n_label,
                                                 dfsan_label *ret_label) {
+  if (dfsan_weak_hook_memcmp)
+    dfsan_weak_hook_memcmp(s1, s2, n, s1_label, s2_label, n_label);
   const char *cs1 = (const char *) s1, *cs2 = (const char *) s2;
   for (size_t i = 0; i != n; ++i) {
     if (cs1[i] != cs2[i]) {

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