[llvm] Fix crash lowering stack guard on OpenBSD/aarch64. (PR #125416)

[via llvm-commits]

https://github.com/3405691582 created https://github.com/llvm/llvm-project/pull/125416

TargetLoweringBase::getIRStackGuard refers to a platform-specific guard variable. Before this change, TargetLoweringBase::getSDagStackGuard only referred to a different variable.

This means that SelectionDAGBuilder's getLoadStackGuard does not get memory operands. However, AArch64InstrInfo::expandPostRAPseudo assumes that the passed MachineInstr has nonzero memoperands, causing a segfault.

We have two possible options here: either disabling the LOAD_STACK_GUARD node entirely in AArch64TargetLowering::useLoadStackGuardNode or just making the platform-specific values match across TargetLoweringBase. Here, we try the latter.

>From 2aa48cf2c31d40d3c6bf3b13243f71d89191f934 Mon Sep 17 00:00:00 2001
From: 3405691582 <dsk at google.com>
Date: Sun, 2 Feb 2025 10:51:03 -0500
Subject: [PATCH] Fix crash lowering stack guard on OpenBSD/aarch64.

TargetLoweringBase::getIRStackGuard refers to a platform-specific guard
variable. Before this change, TargetLoweringBase::getSDagStackGuard
only referred to a different variable.

This means that SelectionDAGBuilder's getLoadStackGuard does not get
memory operands. However, AArch64InstrInfo::expandPostRAPseudo assumes
that the passed MachineInstr has nonzero memoperands, causing a
segfault.

We have two possible options here: either disabling the LOAD_STACK_GUARD
node entirely in AArch64TargetLowering::useLoadStackGuardNode or just
making the platform-specific values match across TargetLoweringBase.
Here, we try the latter.
---
 llvm/lib/CodeGen/TargetLoweringBase.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index 9c56912aa6ba031..411f59e714b0efe 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -1987,6 +1987,9 @@ void TargetLoweringBase::insertSSPDeclarations(Module &M) const {
 // Currently only support "standard" __stack_chk_guard.
 // TODO: add LOAD_STACK_GUARD support.
 Value *TargetLoweringBase::getSDagStackGuard(const Module &M) const {
+  if (getTargetMachine().getTargetTriple().isOSOpenBSD()) {
+    return M.getNamedValue("__guard_local");
+  }
   return M.getNamedValue("__stack_chk_guard");
 }