aarch64: userspace: Introduce arch_user_string_nlen

Introduce the arch_user_string_nlen() assembly routine and the necessary
C code bits.

Signed-off-by: Carlo Caione <[email protected]>
Signed-off-by: Nicolas Pitre <[email protected]>

Author: carlocaione

arch/arm/core/aarch64/fatal.c

@@ -15,9 +15,18 @@
 
 #include <kernel.h>
 #include <logging/log.h>
+#include <exc_handle.h>
 
 LOG_MODULE_DECLARE(os, CONFIG_KERNEL_LOG_LEVEL);
 
+#ifdef CONFIG_USERSPACE
+Z_EXC_DECLARE(z_arm64_user_string_nlen);
+
+static const struct z_exc_handle exceptions[] = {
+	Z_EXC_HANDLE(z_arm64_user_string_nlen),
+};
+#endif /* CONFIG_USERSPACE */
+
 #ifdef CONFIG_EXCEPTION_DEBUG
 static void dump_esr(uint64_t esr, bool *dump_far)
 {
@@ -168,7 +177,18 @@ static bool is_recoverable(z_arch_esf_t *esf, uint64_t esr, uint64_t far,
 	if (!esf)
 		return false;
 
-	/* Empty */
+#ifdef CONFIG_USERSPACE
+	for (int i = 0; i < ARRAY_SIZE(exceptions); i++) {
+		/* Mask out instruction mode */
+		uint64_t start = (uint64_t)exceptions[i].start;
+		uint64_t end = (uint64_t)exceptions[i].end;
+
+		if (esf->elr >= start && esf->elr < end) {
+			esf->elr = (uint64_t)(exceptions[i].fixup);
+			return true;
+		}
+	}
+#endif
 
 	return false;
 }

arch/arm/core/aarch64/userspace.S

@@ -13,6 +13,42 @@
 
 _ASM_FILE_PROLOGUE
 
+/*
+ * size_t arch_user_string_nlen(const char *s, size_t maxsize, int *err_arg)
+ */
+
+GTEXT(z_arm64_user_string_nlen_fault_start)
+GTEXT(z_arm64_user_string_nlen_fault_end)
+GTEXT(z_arm64_user_string_nlen_fixup)
+
+GTEXT(arch_user_string_nlen)
+SECTION_FUNC(TEXT, arch_user_string_nlen)
+
+	mov	x3, x0
+	mov	x0, #0
+	mov	x4, #0
+
+strlen_loop:
+
+	cmp	x0, x1
+	beq	strlen_done
+
+z_arm64_user_string_nlen_fault_start:
+	ldrb	w5, [x3, x0]
+z_arm64_user_string_nlen_fault_end:
+	cbz	x5, strlen_done
+
+	add	x0, x0, #1
+	b	strlen_loop
+
+z_arm64_user_string_nlen_fixup:
+	mov	x4, #-1
+	mov	x0, #0
+
+strlen_done:
+	str	w4, [x2]
+	ret
+
 /*
  * Routine to jump into userspace
  *