[cfi] Don't pass a uint16_t to memset. Make sure the 16-bit constant is appropriate for us.

Reviewers: eugenis, pcc, kcc

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D49252

llvm-svn: 337170

1 files changed, 4 insertions, 1 deletions

diff --git a/compiler-rt/lib/cfi/cfi.cc b/compiler-rt/lib/cfi/cfi.cc
index 52f504bd281..5ee8365697a 100644
--- a/compiler-rt/lib/cfi/cfi.cc
+++ b/compiler-rt/lib/cfi/cfi.cc
@@ -132,7 +132,10 @@ void ShadowBuilder::Start() {
 void ShadowBuilder::AddUnchecked(uptr begin, uptr end) {
   uint16_t *shadow_begin = MemToShadow(begin, shadow_);
   uint16_t *shadow_end = MemToShadow(end - 1, shadow_) + 1;
-  memset(shadow_begin, kUncheckedShadow,
+  // memset takes a byte, so our unchecked shadow value requires both bytes to
+  // be the same. Make sure we're ok during compilation.
+  static_assert(kUncheckedShadow & 0xff == ((kUncheckedShadow >> 8) & 0xff));
+  memset(shadow_begin, kUncheckedShadow & 0xff,
          (shadow_end - shadow_begin) * sizeof(*shadow_begin));
 }