bpo-32962: Fix test_gdb failure in debug build with -mcet -fcf-protection -O0 (#6754)

When Python is built with the intel control-flow protection flags,
-mcet -fcf-protection, gdb is not able to read the stack without
actually jumping inside the function. This means an extra
'next' command is required to make the $pc (program counter)
enter the function and make the stack of the function exposed to gdb.

Author: Marcel Plch

Lib/test/test_gdb.py

@@ -162,7 +162,11 @@ def get_stack_trace(self, source=None, script=None,
             commands += ['set print entry-values no']
 
         if cmds_after_breakpoint:
-            commands += cmds_after_breakpoint
+            # bpo-32962: When Python is compiled with -mcet -fcf-protection,
+            # arguments are unusable before running the first instruction
+            # of the function entry point. The 'next' command makes the
+            # required first step.
+            commands += ['next'] + cmds_after_breakpoint
         else:
             commands += ['backtrace']
 
@@ -847,9 +851,12 @@ def __init__(self):
             id("first break point")
             l = MyList()
         ''')
+        # bpo-32962: same case as in get_stack_trace():
+        # we need an additional 'next' command in order to read
+        # arguments of the innermost function of the call stack.
         # Verify with "py-bt":
         gdb_output = self.get_stack_trace(cmd,
-                                          cmds_after_breakpoint=['break wrapper_call', 'continue', 'py-bt'])
+                                          cmds_after_breakpoint=['break wrapper_call', 'continue', 'next', 'py-bt'])
         self.assertRegex(gdb_output,
                          r"<method-wrapper u?'__init__' of MyList object at ")
 

Misc/NEWS.d/next/Tests/2018-05-10-16-59-15.bpo-32962.S-rcIN.rst

@@ -0,0 +1 @@
+Fixed test_gdb when Python is compiled with flags -mcet -fcf-protection -O0.