abi: proper handling for empty slice arguments

kernel/src/syscall/arg.rs

@@ -17,6 +17,11 @@ pub fn ref_mut<'a, T: Sized>(addr: usize) -> Result<&'a mut T, Error> {
 
 /// Validates a &str passed as base address and length
 pub fn str_ref<'a>(base: usize, len: usize) -> Result<&'a str, Error> {
+    if len == 0 {
+        // Ignore the base
+        return Ok("");
+    }
+
     let slice = slice_ref(base, len)?;
     if slice.contains(&0) {
         log::warn!("User-supplied string contains NUL characters");
@@ -27,12 +32,22 @@ pub fn str_ref<'a>(base: usize, len: usize) -> Result<&'a str, Error> {
 
 /// Validates a &[T] passed as base address and element count
 pub fn slice_ref<'a, T: Sized>(base: usize, count: usize) -> Result<&'a [T], Error> {
+    if count == 0 {
+        // Ignore the base
+        return Ok(&[]);
+    }
+
     let proc = Thread::current();
     let ptr = base as *const T;
     unsafe { ptr.validate_user_slice(count, proc.address_space()) }
 }
 /// Validates a &mut [T] passed as base address and element count
 pub fn slice_mut<'a, T: Sized>(base: usize, count: usize) -> Result<&'a mut [T], Error> {
+    if count == 0 {
+        // Ignore the base
+        return Ok(&mut []);
+    }
+
     let proc = Thread::current();
     let ptr = base as *mut T;
     unsafe { ptr.validate_user_slice_mut(count, proc.address_space()) }