dwarf: Push correct CFA onto stack for dwarf expression evaluation. (#93)

dwarf: Push correct CFA onto stack for dwarf expression evaluation.

This change fixes a bug where stale CFAs were pushed onto the dwarf
expression stack before expression evaluation. Some optimising compilers
emit CFI which relies on this being correct.

1 files changed, 78 insertions, 0 deletions

diff --git a/tests/x64-test-dwarf-expressions.S b/tests/x64-test-dwarf-expressions.S
new file mode 100644
index 00000000..f275625d
--- /dev/null
+++ b/tests/x64-test-dwarf-expressions.S
@@ -0,0 +1,78 @@
+.global DW_CFA_expression_testcase
+
+.extern recover_register
+
+.text
+
+# CFI expressions were added in DWARF v3 to allow compilers to specify memory
+# locations or register values using DWARF programs. These programs are simple
+# stack-based operations which allow the compiler to encode integer mathematics
+# and other complex logic. CFI expressions are therefore more powerful than the
+# conventional register + offset schemes.
+#
+# These tests capture a bug we have fixed in libunwind. CFI expression programs
+# always start with the current CFA pushed onto the stack. This file contains a
+# pair of routines which test CFI expression parsing. Specifically they test
+# DW_CFA_expression logic, which uses DWARF expressions to compute the address
+# where a non-volatile register was stored.
+#
+# Main calls DW_CFA_expression_testcase, which sets up known state in a
+# non-volatile (caller-saved) register. We use r12 for this purpose. After this
+# DW_CFA_expression_testcase then calls DW_CFA_expression_inner, which clobbers
+# r12 after stashing its value on the stack. This routine contains a DWARF3 CFI
+# expression to restore the value of r12 on unwind which should allow libunwind
+# to recover clobbered state. DW_CFA_expression_inner calls recover_register to
+# retrieve the cached register value. This function recovers the register value
+# by using libunwind to unwind the stack through DW_CFA_expression_inner and up
+# to the call site in DW_CFA_expression_testcase. If our expression is correct,
+# libunwind will be able to restore r12 from the stack.
+#
+# BE CAREFUL WITH rdi, rsi, rax HERE! The arguments to recover_register are
+# passed in via rdi, rsi and I just let them flow through unchanged. Similarly
+# RAX flows back unchanged. Adding any function calls to the below may clobber
+# these registers and cause this test to fail mysteriously.
+
+
+########################################################
+# Test: Restoring a register using a DW_CFA_expression #
+# which uses implicit CFA pushed onto stack.           #
+########################################################
+
+.type DW_CFA_expression_testcase STT_FUNC
+DW_CFA_expression_testcase:
+  .cfi_startproc
+  push %r12
+  .cfi_adjust_cfa_offset 8
+  # Move our sentinel (known) value into non-volatile (Callee-saved) r12
+  mov $111222333, %r12
+  .cfi_rel_offset %r12, 0
+  call DW_CFA_expression_inner
+  pop %r12
+  .cfi_restore %r12
+  .cfi_adjust_cfa_offset -8
+  ret
+  .cfi_endproc
+.size DW_CFA_expression_testcase,.-DW_CFA_expression_testcase
+
+.type DW_CFA_expression_inner STT_FUNC
+DW_CFA_expression_inner:
+  .cfi_startproc
+  push %r12
+  .cfi_adjust_cfa_offset 8
+  # !! IMPORTANT BIT !! The test is all about how we parse the following bytes.
+  # Now we use an expression to describe where our sentinel value is stored:
+  # DW_CFA_expression(0x10), r12(0x0c), Length(0x02),        (preamble)
+  # DW_OP_lit16(0x40), DW_OP_minus(0x1c)                     (instructions)
+  # Parsing starts with the CFA on the stack, then pushes 16, then does a minus
+  # which is eqivalent to a=pop(), b=pop(), push(b-a), leaving us with a value
+  # of cfa-16 (cfa points at old rsp, cfa-8 is our rip, so we stored r12 at
+  # cfa-16).
+  xor %r12, %r12                             # Trash r12
+  .cfi_escape 0x10, 0x0c, 0x2, 0x40, 0x1c   # DW_CFA_expression for recovery
+  call recover_register
+  pop %r12
+  .cfi_restore %r12
+  .cfi_adjust_cfa_offset -8
+  ret
+  .cfi_endproc
+.size DW_CFA_expression_inner,.-DW_CFA_expression_inner