arm64: add pac mask to better support gdb stack unwind

Currently, gdb passthroughs of 'bt', 'frame', 'up', 'down',
'info, locals' don't work on arm64 machine enabled pauth.
This is because gdb does not know the lr register actual values
to unwind the stack frames.

Without the patch:
    crash> gdb bt
    #0  __switch_to (prev=0xffffff8001af92c0, next=0xffffff889da7a580) at /proc/self/cwd/common/arch/arm64/kernel/process.c:569
    #1  0x9fc5c5d3602132c0 in ?? ()
    Backtrace stopped: previous frame identical to this frame (corrupt stack?)

With the patch:
    crash> gdb bt
    #0  __switch_to (prev=prev@entry=0xffffff8001af92c0, next=next@entry=0xffffff889da7a580) at /proc/self/cwd/common/arch/arm64/kernel/process.c:569
    #1  0xffffffd3602132c0 in context_switch (rq=0xffffff8a7295a080, prev=0xffffff8001af92c0, next=0xffffff889da7a580, rf=<optimized out>) at /proc/self/cwd/common/kernel/sched/core.c:5515
    #2  __schedule (sched_mode=<optimized out>, sched_mode@entry=2147859424) at /proc/self/cwd/common/kernel/sched/core.c:6843
    #3  0xffffffd3602136d8 in schedule () at /proc/self/cwd/common/kernel/sched/core.c:6917
    ...

Signed-off-by: Guanyou.Chen <[email protected]>

Author: Guanyou.Chen

gdb-10.2.patch

@@ -16216,3 +16216,26 @@ exit 0
  	    printf_filtered (_("Backtrace stopped: %s\n"),
  			     frame_stop_reason_string (trailing));
  	}
+--- gdb-10.2/gdb/frame.c.orig
++++ gdb-10.2/gdb/frame.c
+@@ -944,6 +944,10 @@ frame_find_by_id (struct frame_id id)
+   return NULL;
+ }
+
++#ifdef CRASH_MERGE
++extern "C" void crash_decode_ptrauth_pc(ulong* pc);
++#endif
++
+ static CORE_ADDR
+ frame_unwind_pc (struct frame_info *this_frame)
+ {
+@@ -974,6 +978,9 @@ frame_unwind_pc (struct frame_info *this_frame)
+       try
+	{
+	  pc = gdbarch_unwind_pc (prev_gdbarch, this_frame);
++#ifdef CRASH_MERGE
++	  crash_decode_ptrauth_pc(&pc);
++#endif
+	  pc_p = true;
+	}
+       catch (const gdb_exception_error &ex)

gdb_interface.c

@@ -1083,3 +1083,13 @@ int crash_get_current_task_reg (int regno, const char *regname,
 	return machdep->get_current_task_reg(regno, regname, regsize, value);
 }
 
+/* arm64 kernel lr maybe has patuh */
+void crash_decode_ptrauth_pc(ulong *pc);
+void crash_decode_ptrauth_pc(ulong *pc)
+{
+#ifdef ARM64
+	struct machine_specific *ms = machdep->machspec;
+	if (is_kernel_text(*pc | ms->CONFIG_ARM64_KERNELPACMASK))
+		*pc |= ms->CONFIG_ARM64_KERNELPACMASK;
+#endif /* !ARM64 */
+}