[PATCH for_v2? v2 13/14] selftests/x86/sgx: Add check to verify exit handler stack alignment

[Sean Christopherson <sean.j.christopherson@xxxxxxxxx>]

Add an assembly trampoline to the basic exit handler to snapshot the
pre-CALL %rsp. Use the snapshot to verify that the stack is 16-byte
aligned as required by the x86_64 ABI.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
---
 tools/testing/selftests/x86/sgx/defines.h |  1 +
 tools/testing/selftests/x86/sgx/main.c    | 20 +++++++++++++++++---
 2 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/x86/sgx/defines.h b/tools/testing/selftests/x86/sgx/defines.h
index b303bcaeb8dd..c4d8b88f3d8a 100644
--- a/tools/testing/selftests/x86/sgx/defines.h
+++ b/tools/testing/selftests/x86/sgx/defines.h
@@ -15,6 +15,7 @@ typedef uint64_t u64;
 
 #define __aligned(x) __attribute__((__aligned__(x)))
 #define __packed __attribute__((packed))
+#define __used __attribute__((__used__))
 
 /* Derived from asm-generic/bitsperlong.h. */
 #if __x86_64__
diff --git a/tools/testing/selftests/x86/sgx/main.c b/tools/testing/selftests/x86/sgx/main.c
index 0a4716cef486..5ebe25dc877c 100644
--- a/tools/testing/selftests/x86/sgx/main.c
+++ b/tools/testing/selftests/x86/sgx/main.c
@@ -342,14 +342,28 @@ static void test_sgx_vdso(struct sgx_secs *secs)
 	EXPECT_EQ(exception.leaf, ENCLU_EENTER);
 }
 
-static int basic_exit_handler(long rdi, long rsi, long rdx, long ursp,
-			      long r8, long r9, void *tcs, int ret,
-			      struct sgx_enclave_exception *e)
+static int __used __basic_exit_handler(long rdi, long rsi, long rdx, long ursp,
+				       long r8, long r9, void *tcs, int ret,
+				       struct sgx_enclave_exception *e)
 {
+	ASSERT_RAW(!(r9 & 0xf), "Pre-CALL RSP not 16-byte aligned: %lx\n", r9);
 	ASSERT_EQ(ret, 0);
 	return 0;
 }
 
+extern void *basic_exit_handler;
+
+static void __used basic_exit_handler_trampoline(void)
+{
+	/* Load the pre-CALL %rsp into %r9 to verify correct alignment. */
+	asm volatile("1:\n\t"
+		     "lea 0x8(%%rsp), %%r9\n\t"
+		     "jmp __basic_exit_handler\n\t"
+		     "basic_exit_handler: .quad 1b\n\t"
+		     ".global basic_exit_handler"
+		     ::: "memory");
+}
+
 /*
  * Test the vDSO API, __vdso_sgx_enter_enclave(), with an exit handler.
  */
-- 
2.22.0