xiaoxiang781216 commented on code in PR #3536:
URL: https://github.com/apache/nuttx-apps/pull/3536#discussion_r3429432759
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testing/sig_sp_test/sig_sp_test_main.c:
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@@ -0,0 +1,198 @@
+/****************************************************************************
+ * apps/testing/sig_sp_test/sig_sp_test_main.c
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * Test: Verify that modifying SP (REG_R13) in the saved register context
+ * is honored on context restore (exception return path).
+ *
+ * Scenario (simulates runtime unwinding past a trampoline):
+ * 1. Push values 1 and 2 onto the stack (simulating a trampoline push)
+ * 2. Busy-wait for a timer signal (SIGALRM)
+ * 3. In the handler, "emulate a pop" by advancing SP by 4
+ * (simulating a runtime deciding the top frame was a stub)
+ * 4. Redirect PC to resume_after_signal
+ * 5. After signal return, pop a value — it should be 1
+ *
+ * This exercises:
+ * - The SP relocation fix in arm_exception.S
+ * - The backward sliding copy (new SP > old SP, stack shrinks)
+ * - A realistic use case: unwinding past a trampoline frame
+ *
+ * Requirements:
+ * - Flat build only (accesses nxsched_self() for saved_regs)
+ * - ARM architecture (uses ARMv7-M register layout)
+ *
+ ****************************************************************************/
+
+#include <nuttx/config.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <signal.h>
+#include <string.h>
+#include <stdint.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <nuttx/arch.h>
+#include <nuttx/sched.h>
+#include <arch/irq.h>
+
+static volatile int g_result = -1;
+static volatile int g_ready = 0;
+
+/* Forward declarations */
+
+void verify_result(uint32_t value);
+
+/****************************************************************************
+ * Name: verify_result
+ *
+ * Description:
+ * Called with the popped value in r0. Verifies it equals 1.
+ ****************************************************************************/
+
+void __attribute__((noinline, used)) verify_result(uint32_t value)
+{
+ printf("sig_sp_test: popped value = %lu (expected 1)\n",
+ (unsigned long)value);
+
+ if (value == 1)
+ {
+ printf("sig_sp_test: PASS\n");
+ g_result = 0;
+ }
+ else
+ {
+ printf("sig_sp_test: FAIL - expected 1, got %lu\n",
+ (unsigned long)value);
+ g_result = 1;
+ }
+
+ exit(g_result);
+}
+
+/****************************************************************************
+ * Name: resume_after_signal
+ *
+ * Description:
+ * We land here after the signal handler adjusts SP and PC.
+ * Stack now has [SP] = 1 (the '2' was skipped by SP += 4).
+ * Pop one value and verify it equals 1.
+ ****************************************************************************/
+
+static void __attribute__((naked, used)) resume_after_signal(void)
+{
+ __asm__ __volatile__(
+ "pop {r0}\n\t"
+ "b verify_result\n\t"
+ );
+}
+
+/****************************************************************************
+ * Name: sigalrm_handler
+ *
+ * Description:
+ * Called from the timer interrupt path (async signal delivery).
+ * Accesses saved_regs via nxsched_self() to modify the context
+ * that will be restored after signal return.
+ ****************************************************************************/
+
+static void sigalrm_handler(int signo, siginfo_t *info, void *ucontext)
+{
+ struct tcb_s *rtcb = nxsched_self();
+ uint32_t *regs;
+
+ (void)signo;
+ (void)info;
+ (void)ucontext;
+
+ regs = rtcb->xcp.saved_regs;
Review Comment:
it's better to pass regs context through ucontext, and remove depends on
BUILD_FLAT
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