[PATCH v2] gdb/arm: Fix M-profile EXC_RETURN exception_domain_is_secure logic

[Tomas Vanek <vanekt@fbl.cz>]

Arm v8-M Architecture Reference Manual,
D1.2.95 EXC_RETURN, Exception Return Payload
describes ES bit:

"ES, bit [0]
     Exception Secure. The security domain the exception was taken to.
     The possible values of this bit are:
       0 Non-secure.
       1 Secure"

arm-tdep.c:3443, arm_m_exception_cache () function tests this bit:

  exception_domain_is_secure = (bit (lr, 0) == 0);

The test is negated!

Later on line 3553, the condition evaluates if an additional state
context is stacked:

  /* With the Security extension, the hardware saves R4..R11 too.  */
  if (tdep->have_sec_ext && secure_stack_used
      && (!default_callee_register_stacking || exception_domain_is_secure))

RM, B3.19 Exception entry, context stacking
reads:
RPLHM "In a PE with the Security Extension, on taking an exception,
the PE hardware:
  ...
  2. If exception entry requires a transition from Secure state to
     Non-secure state, the PE hardware extends the stack frame and also
     saves additional state context."

So we should test for !exception_domain_is_secure instead of non-negated
value!
These two bugs compensate each other so unstacking works correctly.

But another test of exception_domain_is_secure (negated due to the
first bug) prevents arm_unwind_secure_frames to work as expected:

  /* Unwinding from non-secure to secure can trip security
     measures.  In order to avoid the debugger being
     intrusive, rely on the user to configure the requested
     mode.  */
  if (secure_stack_used && !exception_domain_is_secure
      && !arm_unwind_secure_frames)

Test with GNU gdb (GDB) 13.0.50.20221016-git.
Stopped in a non-secure handler:

 (gdb) set arm unwind-secure-frames 0
 (gdb) bt
 #0  HAL_SYSTICK_Callback () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/NonSecure/Src/nsmain.c:490
 #1  0x0804081c in SysTick_Handler ()
     at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/NonSecure/Src/nsstm32l5xx_it.c:134
 #2  <signal handler called>
 #3  HAL_GPIO_ReadPin (GPIOx=0x52020800, GPIO_Pin=8192)
     at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Drivers/STM32L5xx_HAL_Driver/Src/stm32l5xx_hal_gpio.c:386
 #4  0x0c000338 in SECURE_Mode () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/main.c:86
 #5  0x080403f2 in main () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/NonSecure/Src/nsmain.c:278
 Backtrace stopped: previous frame inner to this frame (corrupt stack?)

The frames #3 and #4 are secure. backtrace should stop before #3.

Stopped in a secure handler:

 (gdb) bt
 #0  HAL_SYSTICK_Callback () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/main.c:425
 #1  0x0c000b6a in SysTick_Handler ()
     at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/stm32l5xx_it.c:234
 warning: Non-secure to secure stack unwinding disabled.
 #2  <signal handler called>

The exception from secure to secure erroneously stops unwinding. It should
continue as far as the security unlimited backtrace:

 (gdb) set arm unwind-secure-frames 1
 (gdb) si <-- used to rebuild frame cache after change of unwind-secure-frames
 0x0c0008e6      425       if (SecureTimingDelay != 0U)
 (gdb) bt
 #0  0x0c0008e6 in HAL_SYSTICK_Callback () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/main.c:425
 #1  0x0c000b6a in SysTick_Handler ()
     at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/stm32l5xx_it.c:234
 #2  <signal handler called>
 #3  0x0c000328 in SECURE_Mode () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/Secure/Src/main.c:88
 #4  0x080403f2 in main () at C:/dvl/stm32l5trustzone/GPIO_IOToggle_TrustZone/NonSecure/Src/nsmain.c:278

 Backtrace stopped: previous frame inner to this frame (corrupt stack?)

Set exception_domain_is_secure to the value expected by its name.
Fix exception_domain_is_secure usage in the additional state context
stacking condition.

v2: Corrected backtrace logs in commit message

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
---
 gdb/arm-tdep.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 55295e1..20b6f3f 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -3496,7 +3496,7 @@ struct frame_unwind arm_stub_unwind = {
 	{
 	  secure_stack_used = (bit (lr, 6) != 0);
 	  default_callee_register_stacking = (bit (lr, 5) != 0);
-	  exception_domain_is_secure = (bit (lr, 0) == 0);
+	  exception_domain_is_secure = (bit (lr, 0) != 0);
 
 	  /* Unwinding from non-secure to secure can trip security
 	     measures.  In order to avoid the debugger being
@@ -3606,7 +3606,7 @@ struct frame_unwind arm_stub_unwind = {
 
       /* With the Security extension, the hardware saves R4..R11 too.  */
       if (tdep->have_sec_ext && secure_stack_used
-	  && (!default_callee_register_stacking || exception_domain_is_secure))
+	  && (!default_callee_register_stacking || !exception_domain_is_secure))
 	{
 	  /* Read R4..R11 from the integer callee registers.  */
 	  cache->saved_regs[4].set_addr (unwound_sp + 0x08);
-- 
1.9.1