Fix DataFlowSanitizer implementation of strchr() so that strchr(..., '\0') returns a pointer to '\0'.

[skerner]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=22392

[skerner]

The CI failure message from "clang-tidy linux" is:

/mnt/disks/ssd0/agent/workspace/amd64_debian_testing_clang8/compiler-rt/lib/dfsan/dfsan_custom.cpp:14:10: error: 'sanitizer_common/sanitizer_common.h' file not found [clang-diagnostic-error]
#include "sanitizer_common/sanitizer_common.h"

This file definitely exists, and is included successfully when compiling. I think this is a known bug in the clang-tidy configuration:

https://github.com/google/llvm-premerge-checks/issues/156

[morehouse]

LGTM

compiler-rt/lib/dfsan/dfsan_custom.cpp

SANITIZER_INTERFACE_ATTRIBUTE char *__dfsw_strchr(const char *s, int c,
   for (size_t i = 0;; ++i) {
     if (s[i] == c || s[i] == 0) {
       if (flags().strict_data_dependencies) {
         *ret_label = s_label;
       } else {
         *ret_label = dfsan_union(dfsan_read_label(s, i + 1),
                                  dfsan_union(s_label, c_label));
       }
-      return s[i] == 0 ? nullptr : const_cast<char *>(s+i);
+
+      // If s[i] is the \0 at the end of the string, and \0 is not the
+      // character we are searching for, then return null.
+      if (s[i] == 0 && c != 0) {
+        return nullptr;
+      }
+      return const_cast<char *>(s + i);
     }
   }
 }
 
 DECLARE_WEAK_INTERCEPTOR_HOOK(dfsan_weak_hook_memcmp, uptr caller_pc,
                               const void *s1, const void *s2, size_t n,
                               dfsan_label s1_label, dfsan_label s2_label,
                               dfsan_label n_label)

compiler-rt/test/dfsan/custom.cpp

#endif
 
   crv = strchr(str1, 'x');
   assert(!crv);
 #ifdef STRICT_DATA_DEPENDENCIES
   ASSERT_ZERO_LABEL(crv);
 #else
   ASSERT_LABEL(crv, i_label);
 #endif
+
+  // `man strchr` says:
+  // The terminating null byte is considered part of the string, so that if c
+  // is specified as '\0', these functions return a pointer to the terminator.
+  crv = strchr(str1, '\0');
+  assert(crv == &str1[4]);
+#ifdef STRICT_DATA_DEPENDENCIES
+  ASSERT_ZERO_LABEL(crv);
+#else
+  ASSERT_LABEL(crv, i_label);
+#endif
 }
 
 void test_calloc() {
   // With any luck this sequence of calls will cause calloc to return the same
   // pointer both times.  This is probably the best we can do to test this
   // function.
   char *crv = (char *) calloc(4096, 1);
   ASSERT_ZERO_LABEL(crv[0]);