[libunwind] [libunwind] Add GCS support for AArch64 (PR #99335)
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Wed Jul 17 08:03:08 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libunwind
Author: John Brawn (john-brawn-arm)
<details>
<summary>Changes</summary>
AArch64 GCS (Guarded Control Stack) is similar enough to CET that we can re-use the existing code that is guarded by _LIBUNWIND_USE_CET, so long as we also add defines to locate the GCS stack and pop the entries from it. We also need the jumpto function to exit using br instead of ret, to prevent it from popping the GCS stack.
GCS support is enabled using the LIBUNWIND_ENABLE_GCS cmake option. This enables -mbranch-protection=standard, which enables GCS. For the places we need to use GCS instructions we use the target attribute, as there's not a command-line option to enable a specific architecture extension.
---
Full diff: https://github.com/llvm/llvm-project/pull/99335.diff
9 Files Affected:
- (modified) libunwind/CMakeLists.txt (+8)
- (modified) libunwind/src/Registers.hpp (+7)
- (modified) libunwind/src/UnwindLevel1.c (+28)
- (modified) libunwind/src/UnwindRegistersRestore.S (+1-1)
- (modified) libunwind/src/cet_unwind.h (+19)
- (modified) libunwind/test/CMakeLists.txt (+1)
- (modified) libunwind/test/configs/llvm-libunwind-merged.cfg.in (+3)
- (modified) libunwind/test/configs/llvm-libunwind-shared.cfg.in (+3)
- (modified) libunwind/test/configs/llvm-libunwind-static.cfg.in (+3)
``````````diff
diff --git a/libunwind/CMakeLists.txt b/libunwind/CMakeLists.txt
index b22ade0a7d71e..28d67b0fef92c 100644
--- a/libunwind/CMakeLists.txt
+++ b/libunwind/CMakeLists.txt
@@ -37,6 +37,7 @@ if (LIBUNWIND_BUILD_32_BITS)
endif()
option(LIBUNWIND_ENABLE_CET "Build libunwind with CET enabled." OFF)
+option(LIBUNWIND_ENABLE_GCS "Build libunwind with GCS enabled." OFF)
option(LIBUNWIND_ENABLE_ASSERTIONS "Enable assertions independent of build mode." ON)
option(LIBUNWIND_ENABLE_PEDANTIC "Compile with pedantic enabled." ON)
option(LIBUNWIND_ENABLE_WERROR "Fail and stop if a warning is triggered." OFF)
@@ -188,6 +189,13 @@ if (LIBUNWIND_ENABLE_CET)
endif()
endif()
+if (LIBUNWIND_ENABLE_GCS)
+ add_compile_flags_if_supported(-mbranch-protection=standard)
+ if (NOT CXX_SUPPORTS_MBRANCH_PROTECTION_EQ_STANDARD_FLAG)
+ message(SEND_ERROR "Compiler doesn't support GCS -mbranch-protection option!")
+ endif()
+endif()
+
if (WIN32)
# The headers lack matching dllexport attributes (_LIBUNWIND_EXPORT);
# silence the warning instead of cluttering the headers (which aren't
diff --git a/libunwind/src/Registers.hpp b/libunwind/src/Registers.hpp
index d11ddb3426d52..0fc2a683a7fa4 100644
--- a/libunwind/src/Registers.hpp
+++ b/libunwind/src/Registers.hpp
@@ -1815,6 +1815,13 @@ inline const char *Registers_ppc64::getRegisterName(int regNum) {
/// process.
class _LIBUNWIND_HIDDEN Registers_arm64;
extern "C" void __libunwind_Registers_arm64_jumpto(Registers_arm64 *);
+
+#if defined(_LIBUNWIND_USE_CET)
+extern "C" void *__libunwind_cet_get_jump_target() {
+ return reinterpret_cast<void *>(&__libunwind_Registers_arm64_jumpto);
+}
+#endif
+
class _LIBUNWIND_HIDDEN Registers_arm64 {
public:
Registers_arm64();
diff --git a/libunwind/src/UnwindLevel1.c b/libunwind/src/UnwindLevel1.c
index 48e7bc3b9e00e..67a324947cbf1 100644
--- a/libunwind/src/UnwindLevel1.c
+++ b/libunwind/src/UnwindLevel1.c
@@ -72,6 +72,19 @@
__asm__ volatile("jmpq *%%rdx\n\t" :: "D"(cetRegContext), \
"d"(cetJumpAddress)); \
} while (0)
+#elif defined(_LIBUNWIND_TARGET_AARCH64)
+#define __cet_ss_step_size 8
+#define __unw_phase2_resume(cursor, fn) \
+ do { \
+ _LIBUNWIND_POP_CET_SSP((fn)); \
+ void *cetRegContext = __libunwind_cet_get_registers((cursor)); \
+ void *cetJumpAddress = __libunwind_cet_get_jump_target(); \
+ __asm__ volatile("mov x0, %0\n\t" \
+ "br %1\n\t" \
+ : \
+ : "r"(cetRegContext), "r"(cetJumpAddress) \
+ : "x0"); \
+ } while (0)
#endif
static _Unwind_Reason_Code
@@ -170,6 +183,10 @@ unwind_phase1(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
}
extern int __unw_step_stage2(unw_cursor_t *);
+#if defined(_LIBUNWIND_USE_CET) && defined(_LIBUNWIND_TARGET_AARCH64)
+// Enable the GCS target feature to permit GCS instructions to be used.
+__attribute__((target("gcs")))
+#endif
static _Unwind_Reason_Code
unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
__unw_init_local(cursor, uc);
@@ -181,7 +198,14 @@ unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
// frame walked is unwind_phase2.
unsigned framesWalked = 1;
#ifdef _LIBUNWIND_USE_CET
+#if defined(_LIBUNWIND_TARGET_I386) || defined(_LIBUNWIND_TARGET_X86_64)
unsigned long shadowStackTop = _get_ssp();
+#elif defined(_LIBUNWIND_TARGET_AARCH64)
+ unsigned long shadowStackTop = 0;
+ if (__chkfeat(_CHKFEAT_GCS)) {
+ shadowStackTop = (unsigned long)__gcspr();
+ }
+#endif
#endif
// Walk each frame until we reach where search phase said to stop.
while (true) {
@@ -306,6 +330,10 @@ unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
return _URC_FATAL_PHASE2_ERROR;
}
+#if defined(_LIBUNWIND_USE_CET) && defined(_LIBUNWIND_TARGET_AARCH64)
+// Enable the GCS target feature to permit GCS instructions to be used.
+__attribute__((target("gcs")))
+#endif
static _Unwind_Reason_Code
unwind_phase2_forced(unw_context_t *uc, unw_cursor_t *cursor,
_Unwind_Exception *exception_object,
diff --git a/libunwind/src/UnwindRegistersRestore.S b/libunwind/src/UnwindRegistersRestore.S
index 67d9e05711898..e1d6e17549880 100644
--- a/libunwind/src/UnwindRegistersRestore.S
+++ b/libunwind/src/UnwindRegistersRestore.S
@@ -680,7 +680,7 @@ DEFINE_LIBUNWIND_FUNCTION(__libunwind_Registers_arm64_jumpto)
ldr x16, [x0, #0x0F8]
ldp x0, x1, [x0, #0x000] // restore x0,x1
mov sp,x16 // restore sp
- ret x30 // jump to pc
+ br x30 // jump to pc
#elif defined(__arm__) && !defined(__APPLE__)
diff --git a/libunwind/src/cet_unwind.h b/libunwind/src/cet_unwind.h
index c364ed3e12feb..404f94551d78e 100644
--- a/libunwind/src/cet_unwind.h
+++ b/libunwind/src/cet_unwind.h
@@ -35,6 +35,25 @@
} while (0)
#endif
+// On AArch64 we use _LIBUNWIND_USE_CET to indicate that GCS is supported. We
+// need to guard any use of GCS instructions with __chkfeat though, as GCS may
+// not be enabled.
+#if defined(_LIBUNWIND_TARGET_AARCH64) && defined(__ARM_FEATURE_GCS_DEFAULT)
+#define _LIBUNWIND_USE_CET 1
+#include <arm_acle.h>
+
+#define _LIBUNWIND_POP_CET_SSP(x) \
+ do { \
+ if (__chkfeat(_CHKFEAT_GCS)) { \
+ unsigned int tmp = (x); \
+ while (tmp--) { \
+ __gcspopm(); \
+ } \
+ } \
+ } while (0)
+
+#endif
+
extern void *__libunwind_cet_get_registers(unw_cursor_t *);
extern void *__libunwind_cet_get_jump_target(void);
diff --git a/libunwind/test/CMakeLists.txt b/libunwind/test/CMakeLists.txt
index 19f055f6f93ff..c7b1b3d01d8c7 100644
--- a/libunwind/test/CMakeLists.txt
+++ b/libunwind/test/CMakeLists.txt
@@ -9,6 +9,7 @@ macro(pythonize_bool var)
endmacro()
pythonize_bool(LIBUNWIND_ENABLE_CET)
+pythonize_bool(LIBUNWIND_ENABLE_GCS)
pythonize_bool(LIBUNWIND_ENABLE_THREADS)
pythonize_bool(LIBUNWIND_USES_ARM_EHABI)
diff --git a/libunwind/test/configs/llvm-libunwind-merged.cfg.in b/libunwind/test/configs/llvm-libunwind-merged.cfg.in
index 38b79840c9fe2..fafe12962b428 100644
--- a/libunwind/test/configs/llvm-libunwind-merged.cfg.in
+++ b/libunwind/test/configs/llvm-libunwind-merged.cfg.in
@@ -11,6 +11,9 @@ link_flags = []
if @LIBUNWIND_ENABLE_CET@:
compile_flags.append('-fcf-protection=full')
+if @LIBUNWIND_ENABLE_GCS@:
+ compile_flags.append('-mbranch-protection=standard')
+
# On ELF platforms, link tests with -Wl,--export-dynamic if supported by the linker.
if len('@CMAKE_EXE_EXPORTS_CXX_FLAG@'):
link_flags.append('@CMAKE_EXE_EXPORTS_CXX_FLAG@')
diff --git a/libunwind/test/configs/llvm-libunwind-shared.cfg.in b/libunwind/test/configs/llvm-libunwind-shared.cfg.in
index 13896aeb13bc4..f3e40928b525d 100644
--- a/libunwind/test/configs/llvm-libunwind-shared.cfg.in
+++ b/libunwind/test/configs/llvm-libunwind-shared.cfg.in
@@ -10,6 +10,9 @@ link_flags = []
if @LIBUNWIND_ENABLE_CET@:
compile_flags.append('-fcf-protection=full')
+if @LIBUNWIND_ENABLE_GCS@:
+ compile_flags.append('-mbranch-protection=standard')
+
# On ELF platforms, link tests with -Wl,--export-dynamic if supported by the linker.
if len('@CMAKE_EXE_EXPORTS_CXX_FLAG@'):
link_flags.append('@CMAKE_EXE_EXPORTS_CXX_FLAG@')
diff --git a/libunwind/test/configs/llvm-libunwind-static.cfg.in b/libunwind/test/configs/llvm-libunwind-static.cfg.in
index 50b64dc665a5a..a3a65ae82591b 100644
--- a/libunwind/test/configs/llvm-libunwind-static.cfg.in
+++ b/libunwind/test/configs/llvm-libunwind-static.cfg.in
@@ -13,6 +13,9 @@ if @LIBUNWIND_ENABLE_THREADS@:
if @LIBUNWIND_ENABLE_CET@:
compile_flags.append('-fcf-protection=full')
+if @LIBUNWIND_ENABLE_GCS@:
+ compile_flags.append('-mbranch-protection=standard')
+
# On ELF platforms, link tests with -Wl,--export-dynamic if supported by the linker.
if len('@CMAKE_EXE_EXPORTS_CXX_FLAG@'):
link_flags.append('@CMAKE_EXE_EXPORTS_CXX_FLAG@')
``````````
</details>
https://github.com/llvm/llvm-project/pull/99335
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