arm64_task/pthread_start: Convert the C / inline ASM code to assembly

The aforementioned functions can/will fail if the C compiler decides
to use the stack for the incoming entrypt/etc. parameters.

Fix this issue by converting the jump to user part into pure assembly,
ensuring the stack is NOT used for the parameters.

arch/arm64/src/common/arm64_internal.h

@@ -260,6 +260,8 @@ EXTERN uint8_t g_idle_topstack[];   /* End+1 of heap */
  ****************************************************************************/
 
 void arm64_new_task(struct tcb_s *tak_new);
+void arm64_jump_to_user(uint64_t entry, uint64_t x0, uint64_t x1,
+                        uint64_t *regs) noreturn_function;
 
 /* Low level initialization provided by chip logic */
 

arch/arm64/src/common/arm64_pthread_start.c

@@ -68,12 +68,6 @@
 void up_pthread_start(pthread_trampoline_t startup,
                       pthread_startroutine_t entrypt, pthread_addr_t arg)
 {
-  uint64_t *regs = this_task()->xcp.initregs;
-
-  /* This must be performed atomically, the C-section ends upon user entry */
-
-  enter_critical_section();
-
   /* Set up to enter the user-space pthread start-up function in
    * unprivileged mode. We need:
    *
@@ -83,24 +77,8 @@ void up_pthread_start(pthread_trampoline_t startup,
    *   SPSR = user mode
    */
 
-  regs[REG_ELR]  = (uint64_t)startup;
-  regs[REG_X0]   = (uint64_t)entrypt;
-  regs[REG_X1]   = (uint64_t)arg;
-  regs[REG_SPSR] = (regs[REG_SPSR] & ~SPSR_MODE_MASK) | SPSR_MODE_EL0T;
-
-  /* Fully unwind the kernel stack and drop to user space */
-
-  asm
-  (
-    "mov x0, %0\n" /* Get context registers */
-    "mov sp, x0\n" /* Stack pointer = context */
-    "b arm64_exit_exception\n"
-    :
-    : "r" (regs)
-    : "x0", "memory"
-  );
-
-  PANIC();
+  arm64_jump_to_user((uint64_t)startup, (uint64_t)entrypt, (uint64_t)arg,
+                     this_task()->xcp.initregs);
 }
 
 #endif /* !CONFIG_BUILD_FLAT && __KERNEL__ && !CONFIG_DISABLE_PTHREAD */

arch/arm64/src/common/arm64_task_start.c

@@ -65,12 +65,6 @@
 
 void up_task_start(main_t taskentry, int argc, char *argv[])
 {
-  uint64_t *regs = this_task()->xcp.initregs;
-
-  /* This must be performed atomically, the C-section ends upon user entry */
-
-  enter_critical_section();
-
   /* Set up to return to the user-space _start function in
    * unprivileged mode.  We need:
    *
@@ -80,24 +74,8 @@ void up_task_start(main_t taskentry, int argc, char *argv[])
    *   SPSR = user mode
    */
 
-  regs[REG_ELR]  = (uint64_t)taskentry;
-  regs[REG_X0]   = (uint64_t)argc;
-  regs[REG_X1]   = (uint64_t)argv;
-  regs[REG_SPSR] = (regs[REG_SPSR] & ~SPSR_MODE_MASK) | SPSR_MODE_EL0T;
-
-  /* Fully unwind the kernel stack and drop to user space */
-
-  asm
-  (
-    "mov x0, %0\n" /* Get context registers */
-    "mov sp, x0\n" /* Stack pointer = context */
-    "b arm64_exit_exception\n"
-    :
-    : "r" (regs)
-    : "x0", "memory"
-  );
-
-  PANIC();
+  arm64_jump_to_user((uint64_t)taskentry, (uint64_t)argc, (uint64_t)argv,
+                     this_task()->xcp.initregs);
 }
 
 #endif /* !CONFIG_BUILD_FLAT */

arch/arm64/src/common/arm64_vectors.S

@@ -174,6 +174,34 @@ restore_new:
 
     ret
 
+/****************************************************************************
+ * Function: arm64_jump_to_user
+ *
+ * Description:
+ *  Routine to jump to user space, called when a user process is started and
+ *  the kernel is ready to give control to the user task in user space.
+ *
+ * arm64_jump_to_user(entry, x0, x1, regs)
+ *     entry: process entry point
+ *     x0:    parameter 0 for process
+ *     x1:    parameter 1 for process
+ *     regs:  integer register save area to use
+ *
+ ****************************************************************************/
+
+GTEXT(arm64_jump_to_user)
+SECTION_FUNC(text, arm64_jump_to_user)
+    msr daifset, #IRQ_DAIF_MASK
+    mov sp,  x3
+    str x0,  [sp, #8 * REG_ELR]
+    str x1,  [sp, #8 * REG_X0]
+    str x2,  [sp, #8 * REG_X1]
+    mrs x19, spsr_el1
+    and x19, x19, #~SPSR_MODE_MASK
+    #orr x19, x19, #SPSR_MODE_EL0T # EL0T=0x00, out of range for orr
+    str x19, [sp, #8 * REG_SPSR]
+    b arm64_exit_exception
+
 /****************************************************************************
  * Function: arm64_sync_exc
  *