[RFC PATCH 4/5] gdb/arm: Unwinding of secure procedure with cmse_nonsecure_entry attribute

[Tomas Vanek <vanekt@fbl.cz>]

This patch depends on pending:
"gdb/arm: PR 29738 Cache value for stack pointers for dwarf2 frames"

A secure procedure with cmse_nonsecure_entry attribute is compiled with
an epilogue ending by a return to the non-secure mode:
 bxns lr

When a non-secure context called such procedure, the dwarf2 unwinder
did not know about cmse_nonsecure_entry attribute, did not see 'bxns'
at the return and therefore assumed a normal return keeping the security
state unchanged. This caused incorrect unwinding of the frames following
this one as the secure stack was used instead of non-secure.

Detect a procedure with cmse_nonsecure_entry attribute when unwinding
a secure frame. Change the security state to non-secure and use
the proper stack if the cmse_nonsecure_entry was detected.

The detection of the cmse_nonsecure_entry attribute is based on the split
secure gateway veneer and the rest of procedure with the name prefixed
by '__acle_se_'. This is documented in
https://developer.arm.com/documentation/100748/0619/Security-features-supported-in-Arm-Compiler-for-Embedded/Overview-of-building-Secure-and-Non-secure-images-with-the-Armv8-M-Security-Extension
and GCC conforms this model too.

To choose main or process non-secure stack we need xPSR and SPSEL
bit of CONTROL_NS. For simplicity CONTROL_NS is not tracked for changes
in the inner frames, the CONTROL_NS value is passed unchanged from
the innermost frame.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
---
 gdb/arm-tdep.c | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 71 insertions(+), 8 deletions(-)

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 4180277..4fac09b 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -5125,6 +5125,7 @@ enum arm_vfp_cprc_base_type
 			   frame_info_ptr this_frame)
 {
   arm_gdbarch_tdep *tdep = gdbarch_tdep<arm_gdbarch_tdep> (gdbarch);
+  bool is_sp = (regnum == ARM_SP_REGNUM);
 
   if (is_pacbti_pseudo (gdbarch, regnum))
     {
@@ -5140,12 +5141,10 @@ enum arm_vfp_cprc_base_type
       reg->how = DWARF2_FRAME_REG_FN;
       reg->loc.fn = arm_dwarf2_prev_register;
     }
-  else if (regnum == ARM_SP_REGNUM)
-    reg->how = DWARF2_FRAME_REG_CFA;
-  else if (arm_is_alternative_sp_register (tdep, regnum))
+  else if (is_sp || arm_is_alternative_sp_register (tdep, regnum))
     {
       /* Identify what stack pointers that are synced with sp.  */
-      bool override_with_sp_value = false;
+      bool override_with_sp_value = is_sp;
 
       if (tdep->have_sec_ext)
 	{
@@ -5165,24 +5164,88 @@ enum arm_vfp_cprc_base_type
 	    = get_frame_register_unsigned (this_frame,
 					   tdep->m_profile_psp_ns_regnum);
 
+	  bool is_secure = (sp == msp_s || sp == psp_s);
+	  bool return_to_ns = false;
+	  if (is_secure)
+	    {
+	      CORE_ADDR func = get_frame_func (this_frame);
+	      struct bound_minimal_symbol sym
+		= lookup_minimal_symbol_by_pc (func);
+	      if (sym.minsym)
+		{
+		  const char *name = sym.minsym->natural_name ();
+		  arm_debug_printf ("ret to ns check minsym %s", name);
+		  return_to_ns = strncmp (name, "__acle_se_", 10) == 0;
+		}
+	    }
+
+	  bool ns_process_stack = false;
+	  if (return_to_ns &&
+	      (is_sp ||
+	       regnum == tdep->m_profile_msp_s_regnum ||
+	       regnum == tdep->m_profile_psp_s_regnum))
+	    {
+	      bool spsel = true;
+
+	      if (tdep->m_profile_control_ns_regnum >= 0)
+		{
+		  ULONGEST control_ns
+		    = get_frame_register_unsigned (this_frame,
+				tdep->m_profile_control_ns_regnum);
+		  spsel = (control_ns & (1 << 1)) != 0;
+		}
+
+	      if (spsel)
+		{
+		  ULONGEST xpsr = get_frame_register_unsigned (this_frame,
+							       ARM_PS_REGNUM);
+		  ns_process_stack = (xpsr & 0x1ff) == 0;
+		}
+
+	      if (is_sp)
+		{
+		  reg->how = DWARF2_FRAME_REG_SAVED_GDB_REG;
+		  reg->loc.reg = ns_process_stack ?
+				 tdep->m_profile_psp_ns_regnum :
+				 tdep->m_profile_msp_ns_regnum;
+		  return;
+		}
+	    }
+
+	  if (return_to_ns)
+	    {
+	      if (regnum == tdep->m_profile_msp_regnum)
+		{
+		  reg->how = DWARF2_FRAME_REG_SAVED_GDB_REG;
+		  reg->loc.reg = tdep->m_profile_msp_ns_regnum;
+		  return;
+		}
+	      else if (regnum == tdep->m_profile_psp_regnum)
+		{
+		  reg->how = DWARF2_FRAME_REG_SAVED_GDB_REG;
+		  reg->loc.reg = tdep->m_profile_psp_ns_regnum;
+		  return;
+		}
+	    }
+
 	  bool is_msp = (regnum == tdep->m_profile_msp_regnum)
 	    && (msp_s == sp || msp_ns == sp);
 	  bool is_msp_s = (regnum == tdep->m_profile_msp_s_regnum)
-	    && (msp_s == sp);
+	    && (msp_s == sp || (return_to_ns && !ns_process_stack));
 	  bool is_msp_ns = (regnum == tdep->m_profile_msp_ns_regnum)
 	    && (msp_ns == sp);
 	  bool is_psp = (regnum == tdep->m_profile_psp_regnum)
 	    && (psp_s == sp || psp_ns == sp);
 	  bool is_psp_s = (regnum == tdep->m_profile_psp_s_regnum)
-	    && (psp_s == sp);
+	    && (psp_s == sp || (return_to_ns && ns_process_stack));
 	  bool is_psp_ns = (regnum == tdep->m_profile_psp_ns_regnum)
 	    && (psp_ns == sp);
 
-	  override_with_sp_value = is_msp || is_msp_s || is_msp_ns
+	  override_with_sp_value = is_sp || is_msp || is_msp_s || is_msp_ns
 	    || is_psp || is_psp_s || is_psp_ns;
 
 	}
-      else if (tdep->is_m)
+      else if (tdep->is_m && !is_sp)
 	{
 	  CORE_ADDR sp
 	    = get_frame_register_unsigned (this_frame, ARM_SP_REGNUM);
-- 
1.9.1