AArch64 pauth: Indicate unmasked addresses in backtrace

Armv8.3-a Pointer Authentication causes the function return address to be
obfuscated on entry to some functions. GDB must unmask the link register in
order to produce a backtrace.

The following patch adds markers of [PAC] to the bracktrace, to indicate
which addresses needed unmasking.  This includes the backtrace when using MI.

For example, consider the following backtrace:

(gdb) bt
0  0x0000000000400490 in puts@plt ()
1  0x00000000004005dc in foo ("hello") at cbreak-lib.c:6
2  0x0000000000400604 [PAC] in bar () at cbreak-lib.c:12
3  0x0000000000400620 [PAC] in main2 () at cbreak.c:17
4  0x00000000004005b4 in main () at cbreak-3.c:10

The functions in cbreak-lib use pointer auth, which masks the return address
to the previous function, causing the addresses of bar (in the library) and main2
(in the main binary) to require unmasking in order to unwind the backtrace.

An extra bool is added alongside the prev_pc in the frame structure.  At the
point at which the link register is unmasked, the AArch64 port calls into frame
to sets the bool.  This is the most efficient way of doing it.

The marker is also added to the python frame printer, which is always printed if
set.  The marker is not explicitly exposed to the python code.

I expect this will potentially cause issues with some tests in the testsuite
when Armv8.3 pointer authentication is used.  This should be fixed up in the
the future once real hardware is available for full testsuite testing.

gdb/ChangeLog:

        * NEWS: Expand the Pointer Authentication entry.
        * aarch64-tdep.c (aarch64_frame_unmask_address): Rename from this.
        (aarch64_frame_unmask_lr): ... to this.
        (aarch64_prologue_prev_register, aarch64_dwarf2_prev_register):
        Call aarch64_frame_unmask_lr.
        * frame.c (struct frame_info): Add "masked" variable.
        (frame_set_previous_pc_masked) (frame_get_pc_masked): New functions.
        (fprint_frame): Check for masked pc.
        * frame.h (frame_set_previous_pc_masked) (frame_get_pc_masked): New
        declarations.
	* python/py-framefilter.c (py_print_frame): Check for masked pc.
        * stack.c (print_frame): Check for masked pc.

gdb/doc/ChangeLog:

        * gdb.texinfo (AArch64 Pointer Authentication): New subsection.

9 files changed, 82 insertions, 11 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 82e0819..bacf5d7 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,18 @@
+2019-08-07  Alan Hayward  <alan.hayward@arm.com>
+
+	* NEWS: Expand the Pointer Authentication entry.
+	* aarch64-tdep.c (aarch64_frame_unmask_address): Rename from this.
+	(aarch64_frame_unmask_lr): ... to this.
+	(aarch64_prologue_prev_register, aarch64_dwarf2_prev_register):
+	Call aarch64_frame_unmask_lr.
+	* frame.c (struct frame_info): Add "masked" variable.
+	(frame_set_previous_pc_masked) (frame_get_pc_masked): New functions.
+	(fprint_frame): Check for masked pc.
+	* frame.h (frame_set_previous_pc_masked) (frame_get_pc_masked): New
+	declarations.
+	* python/py-framefilter.c (py_print_frame): Check for masked pc.
+	* stack.c (print_frame): Check for masked pc.
+
 2019-08-06  Tom Tromey  <tom@tromey.com>
 
 	* stabsread.c (patch_block_stabs, read_one_struct_field)
diff --git a/gdb/NEWS b/gdb/NEWS
index b4c59e4..fa01adf 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -16,7 +16,9 @@
   architectures require kernel changes.  TLS is not yet supported for
   amd64 and i386 process core dumps.
 
-* Support for Pointer Authentication on AArch64 Linux.
+* Support for Pointer Authentication (PAC) on AArch64 Linux.  Return
+  addresses that required unmasking are shown in the backtrace with the
+  postfix [PAC].
 
 * Two new convenience functions $_cimag and $_creal that extract the
   imaginary and real parts respectively from complex numbers.
diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index e23cc3f..9b6324f 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -250,13 +250,13 @@ class instruction_reader : public abstract_instruction_reader
 
 } // namespace
 
-/* If address signing is enabled, mask off the signature bits from ADDR, using
-   the register values in THIS_FRAME.  */
+/* If address signing is enabled, mask off the signature bits from the link
+   register, which is passed by value in ADDR, using the register values in
+   THIS_FRAME.  */
 
 static CORE_ADDR
-aarch64_frame_unmask_address (struct gdbarch_tdep *tdep,
-			      struct frame_info *this_frame,
-			      CORE_ADDR addr)
+aarch64_frame_unmask_lr (struct gdbarch_tdep *tdep,
+			 struct frame_info *this_frame, CORE_ADDR addr)
 {
   if (tdep->has_pauth ()
       && frame_unwind_register_unsigned (this_frame,
@@ -265,6 +265,9 @@ aarch64_frame_unmask_address (struct gdbarch_tdep *tdep,
       int cmask_num = AARCH64_PAUTH_CMASK_REGNUM (tdep->pauth_reg_base);
       CORE_ADDR cmask = frame_unwind_register_unsigned (this_frame, cmask_num);
       addr = addr & ~cmask;
+
+      /* Record in the frame that the link register required unmasking.  */
+      set_frame_previous_pc_masked (this_frame);
     }
 
   return addr;
@@ -952,7 +955,7 @@ aarch64_prologue_prev_register (struct frame_info *this_frame,
       if (tdep->has_pauth ()
 	  && trad_frame_value_p (cache->saved_regs,
 				 tdep->pauth_ra_state_regnum))
-	lr = aarch64_frame_unmask_address (tdep, this_frame, lr);
+	lr = aarch64_frame_unmask_lr (tdep, this_frame, lr);
 
       return frame_unwind_got_constant (this_frame, prev_regnum, lr);
     }
@@ -1119,7 +1122,7 @@ aarch64_dwarf2_prev_register (struct frame_info *this_frame,
     {
     case AARCH64_PC_REGNUM:
       lr = frame_unwind_register_unsigned (this_frame, AARCH64_LR_REGNUM);
-      lr = aarch64_frame_unmask_address (tdep, this_frame, lr);
+      lr = aarch64_frame_unmask_lr (tdep, this_frame, lr);
       return frame_unwind_got_constant (this_frame, regnum, lr);
 
     default:
diff --git a/gdb/doc/ChangeLog b/gdb/doc/ChangeLog
index 6faf5f3..702bb7c 100644
--- a/gdb/doc/ChangeLog
+++ b/gdb/doc/ChangeLog
@@ -1,3 +1,7 @@
+2019-08-07  Alan Hayward  <alan.hayward@arm.com>
+
+	* gdb.texinfo (AArch64 Pointer Authentication): New subsection.
+
 2019-08-05  Christian Biesinger  <cbiesinger@google.com>
 
 	* python.texi (Blocks In Python): Document dictionary access on blocks.
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 89b1eda..7f8c0af 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -24390,6 +24390,14 @@ but the lengths of the @code{z} and @code{p} registers will not change.  This
 is a known limitation of @value{GDBN} and does not affect the execution of the
 target process.
 
+@subsubsection AArch64 Pointer Authentication.
+@cindex AArch64 Pointer Authentication.
+
+When @value{GDBN} is debugging the AArch64 architecture, and the program is
+using the v8.3-A feature Pointer Authentication (PAC), then whenever the link
+register @code{$lr} is pointing to an PAC function it's value will be masked.
+When GDB prints a backtrace, any addresses that required unmasking will be
+postfixed with the marker [PAC].
 
 @node i386
 @subsection x86 Architecture-specific Issues
diff --git a/gdb/frame.c b/gdb/frame.c
index adac24f..b62fd5c 100644
--- a/gdb/frame.c
+++ b/gdb/frame.c
@@ -123,6 +123,8 @@ struct frame_info
   /* Cached copy of the previous frame's resume address.  */
   struct {
     enum cached_copy_status status;
+    /* Did VALUE require unmasking when being read.  */
+    bool masked;
     CORE_ADDR value;
   } prev_pc;
   
@@ -161,6 +163,25 @@ struct frame_info
   const char *stop_string;
 };
 
+/* See frame.h.  */
+
+void
+set_frame_previous_pc_masked (struct frame_info *frame)
+{
+  frame->prev_pc.masked = true;
+}
+
+/* See frame.h.  */
+
+bool
+get_frame_pc_masked (const struct frame_info *frame)
+{
+  gdb_assert (frame->next != nullptr);
+  gdb_assert (frame->next->prev_pc.status == CC_VALUE);
+
+  return frame->next->prev_pc.masked;
+}
+
 /* A frame stash used to speed up frame lookups.  Create a hash table
    to stash frames previously accessed from the frame cache for
    quicker subsequent retrieval.  The hash table is emptied whenever
@@ -429,8 +450,11 @@ fprint_frame (struct ui_file *file, struct frame_info *fi)
   if (fi->next == NULL || fi->next->prev_pc.status == CC_UNKNOWN)
     fprintf_unfiltered (file, "<unknown>");
   else if (fi->next->prev_pc.status == CC_VALUE)
-    fprintf_unfiltered (file, "%s",
-			hex_string (fi->next->prev_pc.value));
+    {
+      fprintf_unfiltered (file, "%s", hex_string (fi->next->prev_pc.value));
+      if (fi->next->prev_pc.masked)
+	fprintf_unfiltered (file, "[PAC]");
+    }
   else if (fi->next->prev_pc.status == CC_NOT_SAVED)
     val_print_not_saved (file);
   else if (fi->next->prev_pc.status == CC_UNAVAILABLE)
diff --git a/gdb/frame.h b/gdb/frame.h
index ccc2850..fdb401d 100644
--- a/gdb/frame.h
+++ b/gdb/frame.h
@@ -949,4 +949,13 @@ extern const gdb::option::option_def set_backtrace_option_defs[2];
 /* The values behind the global "set backtrace ..." settings.  */
 extern set_backtrace_options user_set_backtrace_options;
 
+/* Mark that the PC value is masked for the previous frame.  */
+
+extern void set_frame_previous_pc_masked (struct frame_info *frame);
+
+/* Get whether the PC value is masked for the given frame.  */
+
+extern bool get_frame_pc_masked (const struct frame_info *frame);
+
+
 #endif /* !defined (FRAME_H)  */
diff --git a/gdb/python/py-framefilter.c b/gdb/python/py-framefilter.c
index a2a96ac..d805ec6 100644
--- a/gdb/python/py-framefilter.c
+++ b/gdb/python/py-framefilter.c
@@ -901,6 +901,8 @@ py_print_frame (PyObject *filter, frame_filter_flags flags,
 	    {
 	      annotate_frame_address ();
 	      out->field_core_addr ("addr", gdbarch, address);
+	      if (get_frame_pc_masked (frame))
+		out->field_string ("pac", " [PAC]");
 	      annotate_frame_address_end ();
 	      out->text (" in ");
 	    }
diff --git a/gdb/stack.c b/gdb/stack.c
index c68b387..0859815 100644
--- a/gdb/stack.c
+++ b/gdb/stack.c
@@ -1298,7 +1298,11 @@ print_frame (const frame_print_options &fp_opts,
 	{
 	  annotate_frame_address ();
 	  if (pc_p)
-	    uiout->field_core_addr ("addr", gdbarch, pc);
+	    {
+	      uiout->field_core_addr ("addr", gdbarch, pc);
+	      if (get_frame_pc_masked (frame))
+		uiout->field_string ("pac", " [PAC]");
+	    }
 	  else
 	    uiout->field_string ("addr", "<unavailable>",
 				 ui_out_style_kind::ADDRESS);