build: Add a new build flag, use_cfi_icall.

This flag enables CFI for indirect calls via a function pointer.

BUG=701919
R=thakis@chromium.org

Review-Url: https://codereview.chromium.org/2749393002
Cr-Original-Commit-Position: refs/heads/master@{#457229}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a7bb0b59a2a4c5918ddfa951f91591ddee7d468e

config/sanitizers/BUILD.gn

@@ -385,6 +385,10 @@ config("cfi_flags") {
       ]
     }
 
+    if (use_cfi_icall) {
+      cflags += [ "-fsanitize=cfi-icall" ]
+    }
+
     if (use_cfi_diag) {
       cflags += [
         "-fno-sanitize-trap=cfi",

config/sanitizers/sanitizers.gni

@@ -62,6 +62,11 @@ declare_args() {
   # https://crbug.com/626794
   use_cfi_cast = false
 
+  # Enable checks for indirect function calls via a function pointer.
+  # TODO(pcc): remove this when we're ready to add these checks by default.
+  # https://crbug.com/701919
+  use_cfi_icall = false
+
   # By default, Control Flow Integrity will crash the program if it detects a
   # violation. Set this to true to print detailed diagnostics instead.
   use_cfi_diag = false