[libunwind] [libunwind] Fix problems caused by combining BTI and GCS (PR #102322)

[John Brawn via cfe-commits]

https://github.com/john-brawn-arm created https://github.com/llvm/llvm-project/pull/102322

The libunwind assembly files need adjustment in order to work correctly when both BTI and GCS are both enabled (which will be the case when using -mbranch-protection=standard):
 * __libunwind_Registers_arm64_jumpto can't use br to jump to the return location, instead we need to use gcspush then ret.
 * Because we indirectly call __libunwind_Registers_arm64_jumpto it needs to start with bti jc.
 * We need to set the GCS GNU property bit when it's enabled.

>From 1c6cbe68b3d2698cf5958daf84ec25496bb7589a Mon Sep 17 00:00:00 2001
From: John Brawn <john.brawn at arm.com>
Date: Wed, 7 Aug 2024 14:25:29 +0100
Subject: [PATCH] [libunwind] Fix problems caused by combining BTI and GCS

The libunwind assembly files need adjustment in order to work
correctly when both BTI and GCS are both enabled (which will be the
case when using -mbranch-protection=standard):
 * __libunwind_Registers_arm64_jumpto can't use br to jump to the
   return location, instead we need to use gcspush then ret.
 * Because we indirectly call __libunwind_Registers_arm64_jumpto it
   needs to start with bit jc.
 * We need to set the GCS GNU property bit when it's enabled.
---
 libunwind/src/UnwindRegistersRestore.S | 16 +++++++++++++++-
 libunwind/src/assembly.h               | 25 ++++++++++++++++++++-----
 2 files changed, 35 insertions(+), 6 deletions(-)

diff --git a/libunwind/src/UnwindRegistersRestore.S b/libunwind/src/UnwindRegistersRestore.S
index b407221ca45f8..7783c94088f68 100644
--- a/libunwind/src/UnwindRegistersRestore.S
+++ b/libunwind/src/UnwindRegistersRestore.S
@@ -629,6 +629,10 @@ Lnovec:
 
 #elif defined(__aarch64__)
 
+#if defined(__ARM_FEATURE_GCS_DEFAULT)
+.arch_extension gcs
+#endif
+
 //
 // extern "C" void __libunwind_Registers_arm64_jumpto(Registers_arm64 *);
 //
@@ -680,7 +684,17 @@ DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_jumpto)
   ldr    x16,     [x0, #0x0F8]
   ldp    x0, x1,  [x0, #0x000]  // restore x0,x1
   mov    sp,x16                 // restore sp
-  br     x30                    // jump to pc
+#if defined(__ARM_FEATURE_GCS_DEFAULT)
+  // If GCS is enabled we need to push the address we're returning to onto the
+  // GCS stack. We can't just return using br, as there won't be a BTI landing
+  // pad instruction at the destination.
+  mov      x16, #1
+  chkfeat  x16
+  cbnz     x16, Lnogcs
+  gcspushm x30
+Lnogcs:
+#endif
+  ret                           // jump to pc
 
 #elif defined(__arm__) && !defined(__APPLE__)
 
diff --git a/libunwind/src/assembly.h b/libunwind/src/assembly.h
index fb07d04071af3..f8e83e138eff5 100644
--- a/libunwind/src/assembly.h
+++ b/libunwind/src/assembly.h
@@ -82,7 +82,22 @@
 #define PPC64_OPD2
 #endif
 
-#if defined(__aarch64__) && defined(__ARM_FEATURE_BTI_DEFAULT)
+#if defined(__aarch64__)
+#if defined(__ARM_FEATURE_GCS_DEFAULT) && defined(__ARM_FEATURE_BTI_DEFAULT)
+// Set BTI, PAC, and GCS gnu property bits
+#define GNU_PROPERTY 7
+// We indirectly branch to __libunwind_Registers_arm64_jumpto from
+// __unw_phase2_resume, so we need to use bti jc.
+#define AARCH64_BTI bti jc
+#elif defined(__ARM_FEATURE_GCS_DEFAULT)
+// Set GCS gnu property bit
+#define GNU_PROPERTY 4
+#elif defined(__ARM_FEATURE_BTI_DEFAULT)
+// Set BTI and PAC gnu property bits
+#define GNU_PROPERTY 3
+#define AARCH64_BTI bti c
+#endif
+#ifdef GNU_PROPERTY
   .pushsection ".note.gnu.property", "a" SEPARATOR                             \
   .balign 8 SEPARATOR                                                          \
   .long 4 SEPARATOR                                                            \
@@ -91,12 +106,12 @@
   .asciz "GNU" SEPARATOR                                                       \
   .long 0xc0000000 SEPARATOR /* GNU_PROPERTY_AARCH64_FEATURE_1_AND */          \
   .long 4 SEPARATOR                                                            \
-  .long 3 SEPARATOR /* GNU_PROPERTY_AARCH64_FEATURE_1_BTI AND */               \
-                    /* GNU_PROPERTY_AARCH64_FEATURE_1_PAC */                   \
+  .long GNU_PROPERTY SEPARATOR                                                 \
   .long 0 SEPARATOR                                                            \
   .popsection SEPARATOR
-#define AARCH64_BTI  bti c
-#else
+#endif
+#endif
+#if !defined(AARCH64_BTI)
 #define AARCH64_BTI
 #endif