armv7-m/armv8-m: honor SP modifications on exception return

arch/arm/src/armv7-m/Kconfig

@@ -151,3 +151,15 @@ config ARMV7M_SYSTICK_IRQ_THREAD_STACK_SIZE
 	default DEFAULT_TASK_STACKSIZE
 
 endif # ARMV7M_SYSTICK_IRQ_WQUEUE
+
+config ARMV7M_SP_CONTEXT_RESTORE
+	bool "Honor SP modifications in signal context"
+	default n
+	---help---
+		On ARMv7-M, the hardware exception return determines the
+		final SP from the physical location of the HW exception
+		frame, ignoring any software modification to REG_R13 in
+		the saved context. Enable this to relocate the HW frame
+		when the desired SP differs from the implied SP, allowing
+		signal handlers to modify SP (e.g., for managed runtime
+		stack unwinding).

arch/arm/src/armv7-m/arm_exception.S

@@ -221,6 +221,79 @@ exception_common:
 	addne		r0, #(4*SW_FPU_REGS)
 #endif
 
+#ifdef CONFIG_ARMV7M_SP_CONTEXT_RESTORE
+	/* Check if the desired SP (r2) differs from the implied SP.
+	 * The implied SP after exception return = r0 + hw_frame_size,
+	 * where hw_frame_size depends on whether the frame includes FPU
+	 * state (determined by EXC_RETURN bit 4 in r14).
+	 * If they differ, we must relocate the HW frame so that the final
+	 * SP after hardware pop equals the desired value (r2).
+	 */
+
+#ifdef CONFIG_ARCH_FPU
+	/* Determine actual HW frame size from EXC_RETURN:
+	 *   bit 4 set (STD_CONTEXT): 8 words = 32 bytes (no FPU)
+	 *   bit 4 clear:            26 words = 104 bytes (with FPU)
+	 */
+
+	tst		r14, #EXC_RETURN_STD_CONTEXT
+	ite		ne
+	movne		r1, #(4*HW_INT_REGS)			/* Standard frame: 32 bytes */
+	moveq		r1, #HW_XCPT_SIZE			/* Extended frame: 104 bytes */
+#else
+	mov		r1, #HW_XCPT_SIZE			/* Always 32 bytes without FPU */
+#endif
+	/* r1 = actual HW frame size in bytes */
+
+	add		r1, r0, r1				/* r1 = implied SP = r0 + frame_size */
+	cmp		r2, r1					/* desired SP == implied SP? */
+	beq		.Lno_sp_relocate			/* Yes, skip relocation */
+
+	/* Relocate HW exception frame from r0 to (r2 - frame_size).
+	 * frame_size = r1 - r0. We have r0 (source), r2 (desired SP).
+	 * r4-r11 are already restored. Available scratch: r1, r2, r3.
+	 * r3 holds basepri which we need to preserve — push to MSP.
+	 *
+	 * Copy direction matters for overlapping regions:
+	 *   dest < source: forward copy (low to high)
+	 *   dest > source: backward copy (high to low)
+	 */
+
+	push		{r3}					/* Save basepri on MSP */
+	sub		r3, r1, r0				/* r3 = frame_size = implied_SP - src */
+	sub		r2, r2, r3				/* r2 = dest = desired_SP - frame_size */
+	cmp		r2, r0					/* dest vs source? */
+	bhs		.Lsp_copy_backward			/* dest >= source: copy backward */
+
+	/* Forward copy (dest < source) */
+
+	push		{r2}					/* Save dest start for later */
+.Lsp_copy_fwd:
+	ldr		r1, [r0], #4
+	str		r1, [r2], #4
+	subs		r3, r3, #4
+	bne		.Lsp_copy_fwd
+	pop		{r0}					/* r0 = dest start = relocated frame */
+	pop		{r3}					/* Restore basepri */
+	b		.Lno_sp_relocate
+
+.Lsp_copy_backward:
+	/* Backward copy (dest >= source) */
+
+	push		{r2}					/* Save dest start for later */
+	add		r0, r0, r3				/* r0 = source end (one past) */
+	add		r2, r2, r3				/* r2 = dest end (one past) */
+.Lsp_copy_bwd:
+	ldr		r1, [r0, #-4]!				/* Pre-decrement, load */
+	str		r1, [r2, #-4]!				/* Pre-decrement, store */
+	subs		r3, r3, #4
+	bne		.Lsp_copy_bwd
+	pop		{r0}					/* r0 = dest start = relocated frame */
+	pop		{r3}					/* Restore basepri */
+
+.Lno_sp_relocate:
+#endif /* CONFIG_ARMV7M_SP_CONTEXT_RESTORE */
+
 	/* The EXC_RETURN value tells us whether we are returning on the MSP or PSP
 	 */
 

arch/arm/src/armv8-m/Kconfig

@@ -155,3 +155,15 @@ config ARMV8M_CMSE
 	bool "ARMv8-M Security Extensions"
 	---help---
 		Enable ARMv8-M Security Extensions.
+
+config ARMV8M_SP_CONTEXT_RESTORE
+	bool "Honor SP modifications in signal context"
+	default n
+	---help---
+		On ARMv8-M, the hardware exception return determines the
+		final SP from the physical location of the HW exception
+		frame, ignoring any software modification to REG_R13 in
+		the saved context. Enable this to relocate the HW frame
+		when the desired SP differs from the implied SP, allowing
+		signal handlers to modify SP (e.g., for managed runtime
+		stack unwinding).

arch/arm/src/armv8-m/arm_exception.S

@@ -235,6 +235,78 @@ exception_common:
 	ldmia		r0!, {r1}		/* Get psplim/msplim */
 #endif
 
+#ifdef CONFIG_ARMV8M_SP_CONTEXT_RESTORE
+	/* Check if the desired SP (r2) differs from the implied SP.
+	 * The implied SP after exception return = r0 + hw_frame_size,
+	 * where hw_frame_size depends on whether the frame includes FPU
+	 * state (determined by EXC_RETURN bit 4 in r14).
+	 * If they differ, we must relocate the HW frame so that the final
+	 * SP after hardware pop equals the desired value (r2).
+	 */
+
+	push		{r1, r3}				/* Save psplim + basepri on MSP */
+
+#ifdef CONFIG_ARCH_FPU
+	/* Determine actual HW frame size from EXC_RETURN:
+	 *   bit 4 set (STD_CONTEXT): 8 words = 32 bytes (no FPU)
+	 *   bit 4 clear:            26 words = 104 bytes (with FPU)
+	 */
+
+	tst		r14, #EXC_RETURN_STD_CONTEXT
+	ite		ne
+	movne		r1, #(4*HW_INT_REGS)			/* Standard frame: 32 bytes */
+	moveq		r1, #HW_XCPT_SIZE			/* Extended frame: 104 bytes */
+#else
+	mov		r1, #HW_XCPT_SIZE			/* Always 32 bytes without FPU */
+#endif
+	/* r1 = actual HW frame size in bytes */
+
+	add		r1, r0, r1				/* r1 = implied SP = r0 + frame_size */
+	cmp		r2, r1					/* desired SP == implied SP? */
+	beq		.Lno_sp_relocate			/* Yes, skip relocation */
+
+	/* Relocate HW exception frame from r0 to (r2 - frame_size).
+	 * frame_size = r1 - r0. We have r0 (source), r2 (desired SP).
+	 * r4-r11 are already restored. Available scratch: r1, r2, r3.
+	 *
+	 * Copy direction matters for overlapping regions:
+	 *   dest < source: forward copy (low to high)
+	 *   dest > source: backward copy (high to low)
+	 */
+
+	sub		r3, r1, r0				/* r3 = frame_size = implied_SP - src */
+	sub		r2, r2, r3				/* r2 = dest = desired_SP - frame_size */
+	cmp		r2, r0					/* dest vs source? */
+	bhs		.Lsp_copy_backward			/* dest >= source: copy backward */
+
+	/* Forward copy (dest < source) */
+
+	push		{r2}					/* Save dest start for later */
+.Lsp_copy_fwd:
+	ldr		r1, [r0], #4
+	str		r1, [r2], #4
+	subs		r3, r3, #4
+	bne		.Lsp_copy_fwd
+	pop		{r0}					/* r0 = dest start = relocated frame */
+	b		.Lno_sp_relocate
+
+.Lsp_copy_backward:
+	/* Backward copy (dest >= source) */
+
+	push		{r2}					/* Save dest start for later */
+	add		r0, r0, r3				/* r0 = source end (one past) */
+	add		r2, r2, r3				/* r2 = dest end (one past) */
+.Lsp_copy_bwd:
+	ldr		r1, [r0, #-4]!				/* Pre-decrement, load */
+	str		r1, [r2, #-4]!				/* Pre-decrement, store */
+	subs		r3, r3, #4
+	bne		.Lsp_copy_bwd
+	pop		{r0}					/* r0 = dest start = relocated frame */
+
+.Lno_sp_relocate:
+	pop		{r1, r3}				/* Restore psplim + basepri */
+#endif /* CONFIG_ARMV8M_SP_CONTEXT_RESTORE */
+
 	/* The EXC_RETURN value tells us whether we are returning on the MSP or PSP
 	 */