Android support for SafeStack.

Add two new ways of accessing the unsafe stack pointer:

* At a fixed offset from the thread TLS base. This is very similar to
  StackProtector cookies, but we plan to extend it to other backends
  (ARM in particular) soon. Bionic-side implementation here:
  https://android-review.googlesource.com/170988.
* Via a function call, as a fallback for platforms that provide
  neither a fixed TLS slot, nor a reasonable TLS implementation (i.e.
  not emutls).

llvm-svn: 248357

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index fbead736ec0..d366e280739 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -2079,6 +2079,29 @@ bool X86TargetLowering::getStackCookieLocation(unsigned &AddressSpace,
   return true;
 }
 
+/// Android provides a fixed TLS slot for the SafeStack pointer.
+/// See the definition of TLS_SLOT_SAFESTACK in
+/// https://android.googlesource.com/platform/bionic/+/master/libc/private/bionic_tls.h
+bool X86TargetLowering::getSafeStackPointerLocation(unsigned &AddressSpace,
+                                                    unsigned &Offset) const {
+  if (!Subtarget->isTargetAndroid())
+    return false;
+
+  if (Subtarget->is64Bit()) {
+    // %fs:0x48, unless we're using a Kernel code model, in which case it's %gs:
+    Offset = 0x48;
+    if (getTargetMachine().getCodeModel() == CodeModel::Kernel)
+      AddressSpace = 256;
+    else
+      AddressSpace = 257;
+  } else {
+    // %gs:0x24 on i386
+    Offset = 0x24;
+    AddressSpace = 256;
+  }
+  return true;
+}
+
 bool X86TargetLowering::isNoopAddrSpaceCast(unsigned SrcAS,
                                             unsigned DestAS) const {
   assert(SrcAS != DestAS && "Expected different address spaces!");