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

[Nick Desaulniers via libc-commits]

================
@@ -0,0 +1,102 @@
+//===-- Implementation of longjmp (64-bit) --------------------------------===//
+//
+// 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 "include/llvm-libc-macros/offsetof-macro.h"
+#include "src/__support/common.h"
+#include "src/__support/macros/config.h"
+#include "src/setjmp/longjmp.h"
+
+#if !defined(LIBC_TARGET_ARCH_IS_X86_64) || !LIBC_COPT_SETJMP_FORTIFICATION
+#error "Invalid file include"
+#endif
+
+#include "src/setjmp/checksum.h"
+
+namespace LIBC_NAMESPACE_DECL {
+
+LLVM_LIBC_FUNCTION(void, longjmp, (jmp_buf, int)) {
+  asm(R"(
+      mov %[cookie], %%rdx
+
+      mov %c[rbx](%%rdi), %%rbx
+      xor %%rbx, %%rdx
+      xor %[mask], %%rbx
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[rbp](%%rdi), %%rbp
+      xor %%rbp, %%rdx
+      xor %[mask], %%rbp
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[r12](%%rdi), %%r12
+      xor %%r12, %%rdx
+      xor %[mask], %%r12
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[r13](%%rdi), %%r13
+      xor %%r13, %%rdx
+      xor %[mask], %%r13
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[r14](%%rdi), %%r14
+      xor %%r14, %%rdx
+      xor %[mask], %%r14
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[r15](%%rdi), %%r15
+      xor %%r15, %%rdx
+      xor %[mask], %%r15
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[rsp](%%rdi), %%rsp
+      xor %%rsp, %%rdx
+      xor %[mask], %%rsp
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      mov %c[rip](%%rdi), %%rcx
+      xor %%rcx, %%rdx
+      xor %[mask], %%rcx
+      mov $%c[multiple], %%rax
+      mul %%rdx
+      xor %%rax, %%rdx
+
+      cmp %c[chksum](%%rdi), %%rdx
+      jne __libc_jmpbuf_corruption
+
+      cmpl $0x1, %%esi
+      adcl $0x0, %%esi
+      movq %%rsi, %%rax
+
+      jmpq *%%rcx
+      )"
+      : [cookie] "=m"(jmpbuf::checksum_cookie), [mask] "=m"(jmpbuf::value_mask)
----------------
nickdesaulniers wrote:

For example, you explicitly `mov` `cookie` into `rdx`. If you use `"d"` rather than `"m"`, then the register allocator will setup to correct load into `rdx` before the inline asm blob.

https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html

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