[clang] [llvm] De-support SafeStack on non-x86 Fuchsia (PR #164855)

[Roland McGrath via llvm-commits]

https://github.com/frobtech created https://github.com/llvm/llvm-project/pull/164855

The Fuchsia Compiler ABI will no longer provide the unsafe stack
for SafeStack on machines other than x86-64.  The x86_64-fuchsia
target still both supports -fsanitize=safe-stack and defaults to
it.  Fuchsia targets that default to -fsanitize=shadow-call-stack
do not need SafeStack also to be available.


>From 110f3e50d5355101aa9630109c72b97b308edf37 Mon Sep 17 00:00:00 2001
From: Roland McGrath <mcgrathr at google.com>
Date: Thu, 23 Oct 2025 10:29:30 -0700
Subject: [PATCH] De-support SafeStack on non-x86 Fuchsia

The Fuchsia Compiler ABI will no longer provide the unsafe stack
for SafeStack on machines other than x86-64.  The x86_64-fuchsia
target still both supports -fsanitize=safe-stack and defaults to
it.  Fuchsia targets that default to -fsanitize=shadow-call-stack
do not need SafeStack also to be available.
---
 clang/lib/Driver/ToolChains/Fuchsia.cpp           |  4 +++-
 clang/test/Driver/fuchsia.c                       | 10 ++++++++++
 llvm/lib/Target/AArch64/AArch64ISelLowering.cpp   |  5 -----
 llvm/test/Transforms/SafeStack/AArch64/abi_ssp.ll |  3 ---
 4 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/clang/lib/Driver/ToolChains/Fuchsia.cpp b/clang/lib/Driver/ToolChains/Fuchsia.cpp
index 146dc8bbd5313..31c2f3f7e1be4 100644
--- a/clang/lib/Driver/ToolChains/Fuchsia.cpp
+++ b/clang/lib/Driver/ToolChains/Fuchsia.cpp
@@ -481,9 +481,11 @@ SanitizerMask Fuchsia::getSupportedSanitizers() const {
   Res |= SanitizerKind::Fuzzer;
   Res |= SanitizerKind::FuzzerNoLink;
   Res |= SanitizerKind::Leak;
-  Res |= SanitizerKind::SafeStack;
   Res |= SanitizerKind::Scudo;
   Res |= SanitizerKind::Thread;
+  if (getTriple().getArch() == llvm::Triple::x86_64) {
+    Res |= SanitizerKind::SafeStack;
+  }
   return Res;
 }
 
diff --git a/clang/test/Driver/fuchsia.c b/clang/test/Driver/fuchsia.c
index 3fb2a94124cd1..d0fec18e13a20 100644
--- a/clang/test/Driver/fuchsia.c
+++ b/clang/test/Driver/fuchsia.c
@@ -312,3 +312,13 @@
 // RUN:     | FileCheck %s -check-prefix=CHECK-NOSTDLIB-NOLIBC
 // CHECK-NOSTDLIB-NOLIBC-NOT: "warning:"
 // CHECK-NOSTDLIB-NOLIBC-NOT: "error:"
+
+// RUN: not %clang -### %s --target=aarch64-unknown-fuchsia \
+// RUN:     -fsanitize=safe-stack 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     | FileCheck %s -check-prefix=CHECK-NONX86-SAFESTACK
+// RUN: not %clang -### %s --target=riscv64-unknown-fuchsia \
+// RUN:     -fsanitize=safe-stack 2>&1 \
+// RUN:     -resource-dir=%S/Inputs/resource_dir_with_per_target_subdir \
+// RUN:     | FileCheck %s -check-prefix=CHECK-NONX86-SAFESTACK
+// CHECK-NONX86-SAFESTACK: error: unsupported option '-fsanitize=safe-stack' for target '{{.*}}'
diff --git a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
index a81de5c5adc34..edd8e41f13663 100644
--- a/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
@@ -29447,11 +29447,6 @@ AArch64TargetLowering::getSafeStackPointerLocation(IRBuilderBase &IRB) const {
   if (Subtarget->isTargetAndroid())
     return UseTlsOffset(IRB, 0x48);
 
-  // Fuchsia is similar.
-  // <zircon/tls.h> defines ZX_TLS_UNSAFE_SP_OFFSET with this value.
-  if (Subtarget->isTargetFuchsia())
-    return UseTlsOffset(IRB, -0x8);
-
   return TargetLowering::getSafeStackPointerLocation(IRB);
 }
 
diff --git a/llvm/test/Transforms/SafeStack/AArch64/abi_ssp.ll b/llvm/test/Transforms/SafeStack/AArch64/abi_ssp.ll
index 43fb2605ff646..d981626894ec3 100644
--- a/llvm/test/Transforms/SafeStack/AArch64/abi_ssp.ll
+++ b/llvm/test/Transforms/SafeStack/AArch64/abi_ssp.ll
@@ -1,7 +1,5 @@
 ; RUN: opt -safe-stack -S -mtriple=aarch64-linux-android < %s -o - | FileCheck --check-prefixes=TLS,ANDROID %s
-; RUN: opt -safe-stack -S -mtriple=aarch64-unknown-fuchsia < %s -o - | FileCheck --check-prefixes=TLS,FUCHSIA %s
 ; RUN: opt -passes=safe-stack -S -mtriple=aarch64-linux-android < %s -o - | FileCheck --check-prefixes=TLS,ANDROID %s
-; RUN: opt -passes=safe-stack -S -mtriple=aarch64-unknown-fuchsia < %s -o - | FileCheck --check-prefixes=TLS,FUCHSIA %s
 
 define void @foo() nounwind uwtable safestack sspreq {
 entry:
@@ -10,7 +8,6 @@ entry:
 
 ; TLS: %[[TP2:.*]] = call ptr @llvm.thread.pointer.p0()
 ; ANDROID: %[[B:.*]] = getelementptr i8, ptr %[[TP2]], i32 40
-; FUCHSIA: %[[B:.*]] = getelementptr i8, ptr %[[TP2]], i32 -16
 ; TLS: %[[StackGuard:.*]] = load ptr, ptr %[[B]]
 ; TLS: store ptr %[[StackGuard]], ptr %[[StackGuardSlot:.*]]
   %a = alloca i128, align 16