[PATCH] [dfsan] Introduce dfsan_read_label runtime function.

[Peter Collingbourne]

    - Add a test for dfsan_read_label unioning

Hi eugenis,

http://llvm-reviews.chandlerc.com/D1349

CHANGE SINCE LAST DIFF
  http://llvm-reviews.chandlerc.com/D1349?vs=3341&id=3399#toc

Files:
  include/sanitizer/dfsan_interface.h
  lib/dfsan/dfsan.cc
  lib/dfsan/lit_tests/basic.c
  lib/dfsan/lit_tests/propagate.c

Index: include/sanitizer/dfsan_interface.h
===================================================================
--- include/sanitizer/dfsan_interface.h
+++ include/sanitizer/dfsan_interface.h
@@ -61,6 +61,9 @@
 /// value.
 dfsan_label dfsan_get_label(long data);
 
+/// Retrieves the label associated with the data at the given address.
+dfsan_label dfsan_read_label(const void *addr, size_t size);
+
 /// Retrieves a pointer to the dfsan_label_info struct for the given label.
 const struct dfsan_label_info *dfsan_get_label_info(dfsan_label label);
 
Index: lib/dfsan/dfsan.cc
===================================================================
--- lib/dfsan/dfsan.cc
+++ lib/dfsan/dfsan.cc
@@ -120,7 +120,7 @@
 }
 
 extern "C" SANITIZER_INTERFACE_ATTRIBUTE
-dfsan_label __dfsan_union_load(dfsan_label *ls, size_t n) {
+dfsan_label __dfsan_union_load(const dfsan_label *ls, size_t n) {
   dfsan_label label = ls[0];
   for (size_t i = 1; i != n; ++i) {
     dfsan_label next_label = ls[i];
@@ -178,6 +178,13 @@
   return __dfsan_arg_tls[0];
 }
 
+SANITIZER_INTERFACE_ATTRIBUTE dfsan_label
+dfsan_read_label(const void *addr, size_t size) {
+  if (size == 0)
+    return 0;
+  return __dfsan_union_load(shadow_for(addr), size);
+}
+
 SANITIZER_INTERFACE_ATTRIBUTE
 const struct dfsan_label_info *dfsan_get_label_info(dfsan_label label) {
   __dfsan_retval_tls = 0;  // Ensures return value is unlabelled in the caller.
Index: lib/dfsan/lit_tests/basic.c
===================================================================
--- lib/dfsan/lit_tests/basic.c
+++ lib/dfsan/lit_tests/basic.c
@@ -13,5 +13,8 @@
   dfsan_label new_label = dfsan_get_label(i);
   assert(i_label == new_label);
 
+  dfsan_label read_label = dfsan_read_label(&i, sizeof(i));
+  assert(i_label == read_label);
+
   return 0;
 }
Index: lib/dfsan/lit_tests/propagate.c
===================================================================
--- lib/dfsan/lit_tests/propagate.c
+++ lib/dfsan/lit_tests/propagate.c
@@ -34,5 +34,8 @@
   assert(dfsan_has_label(ijk_label, j_label));
   assert(dfsan_has_label(ijk_label, k_label));
 
+  struct { int i, j; } s = { i, j };
+  assert(dfsan_read_label(&s, sizeof(s)) == ij_label);
+
   return 0;
 }
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