[msan][asan] Add runtime flag intercept_strcmp

[vitalybuka]

Summary

Can be used to disable interceptor to workaround issues of
non-instrumented code.

[eugenis]

LGTM

[morehouse]

LGTM

compiler-rt/lib/sanitizer_common/sanitizer_common_interceptors.inc

INTERCEPTOR(int, strcmp, const char *s1, const char *s2) {
   COMMON_INTERCEPTOR_ENTER(ctx, strcmp, s1, s2);
   unsigned char c1, c2;
   uptr i;
   for (i = 0;; i++) {
     c1 = (unsigned char)s1[i];
     c2 = (unsigned char)s2[i];
     if (c1 != c2 || c1 == '\0') break;
   }
+  if (common_flags()->intercept_strcmp) {
     COMMON_INTERCEPTOR_READ_STRING(ctx, s1, i + 1);
     COMMON_INTERCEPTOR_READ_STRING(ctx, s2, i + 1);
+  }
   int result = CharCmpX(c1, c2);
   CALL_WEAK_INTERCEPTOR_HOOK(__sanitizer_weak_hook_strcmp, GET_CALLER_PC(), s1,
                              s2, result);
   return result;
 }
 
 DECLARE_WEAK_INTERCEPTOR_HOOK(__sanitizer_weak_hook_strncmp, uptr called_pc,
                               const char *s1, const char *s2, uptr n,

compiler-rt/lib/sanitizer_common/sanitizer_flags.inc

COMMON_FLAG(bool, intercept_strspn, true,
             "If set, uses custom wrappers for strspn and strcspn function "
             "to find more errors.")
 COMMON_FLAG(bool, intercept_strtok, true,
             "If set, uses a custom wrapper for the strtok function "
             "to find more errors.")
 COMMON_FLAG(bool, intercept_strpbrk, true,
             "If set, uses custom wrappers for strpbrk function "
             "to find more errors.")
+COMMON_FLAG(
+    bool, intercept_strcmp, true,
+    "If set, uses custom wrappers for strcmp functions to find more errors.")
 COMMON_FLAG(bool, intercept_strlen, true,
             "If set, uses custom wrappers for strlen and strnlen functions "
             "to find more errors.")
 COMMON_FLAG(bool, intercept_strndup, true,
             "If set, uses custom wrappers for strndup functions "
             "to find more errors.")
 COMMON_FLAG(bool, intercept_strchr, true,
             "If set, uses custom wrappers for strchr, strchrnul, and strrchr "

compiler-rt/test/asan/TestCases/strcmp.c

+// RUN: %clang_asan %s -o %t
+// RUN: %env_asan_opts=intercept_strcmp=false %run %t 2>&1
+// RUN: %env_asan_opts=intercept_strcmp=true not %run %t 2>&1 | FileCheck %s
+// RUN:                                      not %run %t 2>&1 | FileCheck %s
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main(int argc, char **argv) {
+  char s1[] = "abcd";
+  char s2[] = "1234";
+  assert(strcmp(s1, s2) > 0);
+  assert(strcmp(s1 - 1, s2));
+
+  // CHECK: {{.*ERROR: AddressSanitizer: stack-buffer-underflow on address}}
+  // CHECK: READ of size 1
+  return 0;
+}

compiler-rt/test/msan/strcmp.c

+// RUN: %clang_msan %s -o %t
+// RUN: MSAN_OPTIONS=intercept_strcmp=false %run %t 2>&1
+// RUN: MSAN_OPTIONS=intercept_strcmp=true not %run %t 2>&1 | FileCheck %s
+// RUN:                                    not %run %t 2>&1 | FileCheck %s
+
+#include <assert.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main(int argc, char **argv) {
+  char undef;
+  char s1[] = "abcd";
+  char s2[] = "1234";
+  assert(strcmp(s1, s2) > 0);
+  s2[0] = undef;
+  assert(strcmp(s1, s2));
+
+  // CHECK: {{.*WARNING: MemorySanitizer: use-of-uninitialized-value}}
+  return 0;
+}