[libc-commits] [libc] [libc] fortify jmp buffer for x86-64 (PR #112769)

[Nick Desaulniers via libc-commits]

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@@ -0,0 +1,38 @@
+//===-- Implementation for jmpbuf checksum ----------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/setjmp/checksum.h"
+#include "src/__support/OSUtil/io.h"
+#include "src/stdlib/abort.h"
+#include <sys/syscall.h>
+
+namespace LIBC_NAMESPACE_DECL {
+namespace jmpbuf {
+// random bytes from https://www.random.org/cgi-bin/randbyte?nbytes=8&format=h
+// the cookie should not be zero otherwise it will be a bad seed as a multiplier
+__UINTPTR_TYPE__ value_mask =
+    static_cast<__UINTPTR_TYPE__>(0x3899'f0d3'5005'd953ull);
+__UINTPTR_TYPE__ checksum_cookie =
+    static_cast<__UINTPTR_TYPE__>(0xc7d9'd341'6afc'33f2ull);
+
+// initialize the checksum state
+void initialize() {
+  __UINTPTR_TYPE__ entropy[2];
+  syscall_impl<long>(SYS_getrandom, entropy, sizeof(entropy), 0);
+  // add in additional entropy
+  jmpbuf::value_mask ^= entropy[0];
+  jmpbuf::checksum_cookie ^= entropy[0];
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nickdesaulniers wrote:

Did a previous version of this PR use `entropy[1]`? Otherwise why is `entropy` a 2 element array?

https://github.com/llvm/llvm-project/pull/112769