[Pal/Linux-SGX] Instruct GDB to bridge enclave-untrusted stacks

Before, GDB did not know about enclave stack frames when debugging
untrusted OCALL functions. This patch bridges untrusted stack to its
corresponding enclave stack by setting RBP to point to the first
enclave stack frame.

This functionality is only enabled if Graphene-SGX is built with
DEBUG flag. Without DEBUG, RBP is cleared to prevent leakage of the
address of enclave stack.

Pal/src/host/Linux-SGX/Makefile

@@ -23,6 +23,7 @@ all: $(host_files)
 ifeq ($(DEBUG),1)
 CC += -gdwarf-2 -g3
 CFLAGS += -DDEBUG
+ASFLAGS += -DDEBUG
 export DEBUG
 endif
 

Pal/src/host/Linux-SGX/enclave_entry.S

@@ -254,6 +254,19 @@ enclave_entry:
 	#     %rdi, %rsi: (optional) arguments to untrusted code.
 .Lclear_and_eexit:
 
+#ifdef DEBUG
+	# Enclave and untrusted stacks are split (segmented). GDB refuses to
+	# unwind such stacks because it looks like stack frames "jump" back
+	# and forth. Luckily, GDB special-cases stack frames for a function
+	# with hardcoded name "__morestack". Declare this dummy function
+	# to make GDB happy.
+
+	.global __morestack
+	.type __morestack, @function
+__morestack:
+#endif
+
+	.cfi_startproc
 	# CFA is away from RBP by ret_addr + saved_rbp + GPR context except RAX
 	.cfi_def_cfa %rbp, SGX_CONTEXT_SIZE - 8 + 16
 	.cfi_offset %rbp, -16
@@ -280,7 +293,15 @@ enclave_entry:
 	# %rcx is set to AEP by EEXIT
 	xorq %rdx, %rdx
 	# %rsi, %rdi are arguments to the untrusted code
+
+#ifdef DEBUG
+.Lfor_cfa_debug_info:
+	# Leave %rbp pointing to OCALL function on trusted stack.
+#else
+	# In non-debug mode, clear %rbp to not leak trusted stack address.
 	xorq %rbp, %rbp
+#endif
+
 	# %rsp points to untrusted stack
 	xorq %r8, %r8
 	xorq %r9, %r9
@@ -295,6 +316,7 @@ enclave_entry:
 	ENCLU
 
 	ud2 # We should never get here.
+	.cfi_endproc
 
 	# fxsave/xsave area to reset extended state.
 	#
@@ -376,6 +398,13 @@ sgx_ocall:
 	movq %gs:SGX_USTACK, %rsp
 	andq $STACK_ALIGN, %rsp
 
+#ifdef DEBUG
+	# Push %rip of some code inside __morestack() on untrusted stack.
+	# At sgx_entry(), GDB deduces saved_rip by looking at CFA-8 = %rsp.
+	leaq .Lfor_cfa_debug_info(%rip), %r8
+	pushq %r8
+#endif
+
 	movq %gs:SGX_EXIT_TARGET, %rbx
 	jmp .Lclear_and_eexit
 	.cfi_endproc