[Bug tdep/29716] New: Arm v8 M-profile FNC_RETURN unwinder uses wrong stack

["tomas.vanek at fbl dot cz" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29716

            Bug ID: 29716
           Summary: Arm v8 M-profile FNC_RETURN unwinder uses wrong stack
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: minor
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: tomas.vanek at fbl dot cz
  Target Milestone: ---

Discovered on STM32L552 with Cortex-M33, should be same for all ARMv8-M devices
with the security extension.

A secure code runs in thread mode and uses MSP_S because CONTROL_S bit SPSEL =
0
A non-secure function is called from secure code.
Command 'backtrace' does not show the correct stack frames:

 (gdb) bt
 #0  NonSecureFc ()
 #1  <signal handler called>
 #2  0x00000000 in ?? ()
 Backtrace stopped: previous frame identical to this frame (corrupt stack?)

The problem is in arm-tdep.c:3394, function arm_m_exception_cache (),
block if (fnc_return):

  ULONGEST xpsr = get_frame_register_unsigned (this_frame, ARM_PS_REGNUM);
  if ((xpsr & 0xff) != 0)
    /* Handler mode: This is the mode that exceptions are handled in.  */
    arm_cache_switch_prev_sp (cache, tdep, tdep->m_profile_msp_s_regnum);
  else
    /* Thread mode: This is the normal mode that programs run in.  */
    arm_cache_switch_prev_sp (cache, tdep, tdep->m_profile_psp_s_regnum);

This code does not comply
Arm®v8-M Architecture Reference Manual
B3.8 Stack pointer
IDMLS "In Thread mode, CONTROL.SPSEL determines whether the PE uses the main or
process stack"

and uses process stack regardless of SPSEL.

The test should check SPSEL bit of CONTROL_S register.
Unfortunately the current arm-tdep is not aware of this register, we need to
look up the register number in arm_gdbarch_init().

Moreover stack unwinders probably do not keep track of CONTROL_S updates.
Even if we neglect usually very seldom updates of SPSEL and use the value from
the innermost frame we have better chance to see correct stack frames.

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