os/arch/arm/src/armv7-m: Preserve EXC_RETURN across trampoline

[seokhun-eom24]

The common-vector register contains REG_EXC_RETURN in both flat and protected builds.
Signal delivery temporarily redirects the saved context to up_sigdeliver, so it must save the original EXC_RETURN, force the trampoline to return through privileged Thread mode, and restore the original value after the handler completes.
Move the EXC_RETURN save, update, and restore outside the protected-build guard while keeping LR handling protected-only.
Always select EXC_RETURN_PRIVTHR for a non-running target instead of only converting handler-mode contexts.
This prevents a blocked task from entering the kernel signal trampoline with an incompatible handler or unprivileged return state and avoids exception-return integrity faults.

In CONFIG_ARM_CMNVECTOR is not set and CONFIG_BUILD_PROTECTED is not set case we're setting EXC_RETURN_PRIVTHR in up_lazyexception.S

 #ifdef CONFIG_BUILD_PROTECTED
	ldmia	r0, {r2-r11,r14}		/* Recover R4-R11, r14 + 2 temp values */
 #else
	ldmia	r0, {r2-r11}			/* Recover R4-R11 + 2 temp values */
	ldr	r14, =EXC_RETURN_PRIVTHR	/* Load the special value */
 #endif

[seokhun-eom24]

Similar PR: #7385

[rish-sg]

LGTM

[namanjain7]

we have verified it on mediatek_2.0 . It's working fine.
However, below additional changes needed:

1. in irq.h , since we want to save current priority level of task. We will need to add saved_exec_ret with ARM_CMNVECTOR. Like below:
   #if defined(CONFIG_BUILD_PROTECTED) || defined(CONFIG_ARM_CMNVECTOR)
   uint32_t saved_lr;
   uint32_t saved_exec_ret;
   #endif

2. Add ARM_CMNVECTOR in REG_EXC_RETURN in declaration.
   #if defined(CONFIG_BUILD_PROTECTED) || defined(CONFIG_ARM_CMNVECTOR)
   #define REG_EXC_RETURN (10) /* EXC_RETURN */

After these changes, all build issues will be resolved when we enable CONFIG_ARM_CMNVECTOR in Mediatek_2.0 with PR#7456

[namanjain7]

LGTM

[eunwoo-nam]

Could you please explan why CONFIG_ARM_CMNVECTOR checking is added?

[seokhun-eom24]

REG_EXC_RETURN is only available when CONFIG_ARM_CMNVECTOR or CONFIG_BUILD_PROTECTED is enabled.
Therefore, the usage should be guarded with #if defined(CONFIG_ARM_CMNVECTOR) || defined(CONFIG_BUILD_PROTECTED) to handle the case where both options are disabled like qemu/build_test.

[seokhun-eom24]

ARM_CMNVECTOR	BUILD_PROTECTED	Context Layout Header	REG_EXC_RETURN	saved_exc_ret	Reason
❌	❌	irq_lazyfpu.h	❌	❌	Standard lazy-FPU / flat context does not have a dedicated slot for storing EXC_RETURN.
❌	⭕	irq_lazyfpu.h	⭕	⭕	In protected builds, EXC_RETURN must be saved in slot 10 to manage privilege return states.
⭕	❌	irq_cmnvector.h	⭕	⭕	Common-vector context always preserves EXC_RETURN, even in flat builds.
⭕	⭕	irq_cmnvector.h	⭕	⭕	Applies both common-vector requirements and protected-build privilege tracking.

[seokhun-eom24]

#if defined(CONFIG_ARM_CMNVECTOR) || defined(CONFIG_BUILD_PROTECTED)
	/* All tasks start via a stub function in kernel space.  So all
	 * tasks must start in privileged thread mode.  If CONFIG_BUILD_PROTECTED
	 * is defined, then that stub function will switch to unprivileged
	 * mode before transferring control to the user task.
	 */

	xcp->regs[REG_EXC_RETURN] = EXC_RETURN_PRIVTHR;

#endif							/* CONFIG_ARM_CMNVECTOR || CONFIG_BUILD_PROTECTED */

https://github.com/Samsung/TizenRT/blob/master/os/arch/arm/src/armv7-m/up_initialstate.c#L150

This is why we need to use #if defined(CONFIG_ARM_CMNVECTOR) || defined(CONFIG_BUILD_PROTECTED)