multiboot2: Follow C behavior for string parsing

The various strings in the Multiboot2 spec are specified as C-style
zero-terminated strings.

Previously, if the multiboot2 crate is handed a string with non-NUL characters
present between the terminating NUL (within the tag), they were returned as
well.

This revision instead terminates the strings on the first NUL character,
as would be typical in C handling. Although unterminated strings in Multiboot2
are technically an error, current unterminated string behavior is maintained.

multiboot2/src/tag.rs

@@ -43,24 +43,17 @@ impl Tag {
     /// UTF-8 Rust string slice without a terminating null byte or an error is
     /// returned.
     pub fn get_dst_str_slice(bytes: &[u8]) -> Result<&str, Utf8Error> {
-        if bytes.is_empty() {
-            // Very unlikely. A sane bootloader would omit the tag in this case.
-            // But better be safe.
-            return Ok("");
-        }
+        // Determine length of string before first NUL character
+
+        let length = match bytes.iter().position(|ch| (*ch) == 0x00) {
+            // Unterminated string case:
+            None => bytes.len(),
 
-        // Return without a trailing null byte. By spec, the null byte should
-        // always terminate the string. However, for safety, we do make an extra
-        // check.
-        let str_slice = if bytes.ends_with(&[b'\0']) {
-            let str_len = bytes.len() - 1;
-            &bytes[0..str_len]
-        } else {
-            // Unlikely that a bootloader doesn't follow the spec and does not
-            // add a terminating null byte.
-            bytes
+            // Terminated case:
+            Some(idx) => idx,
         };
-        core::str::from_utf8(str_slice)
+        // Convert to Rust string:
+        core::str::from_utf8(&bytes[..length])
     }
 }
 
@@ -141,9 +134,11 @@ mod tests {
         // also unlikely case
         assert_eq!(Ok(""), Tag::get_dst_str_slice(&[b'\0']));
         // unlikely case: missing null byte. but the lib can cope with that
-        assert_eq!(Ok("foobar"), Tag::get_dst_str_slice("foobar".as_bytes()));
+        assert_eq!(Ok("foobar"), Tag::get_dst_str_slice(b"foobar"));
         // test that the null bytes is not included in the string slice
-        assert_eq!(Ok("foobar"), Tag::get_dst_str_slice("foobar\0".as_bytes()));
+        assert_eq!(Ok("foobar"), Tag::get_dst_str_slice(b"foobar\0"));
+        // test that C-style null string termination works as expected
+        assert_eq!(Ok("foo"), Tag::get_dst_str_slice(b"foo\0bar"));
         // test invalid utf8
         assert!(matches!(
             Tag::get_dst_str_slice(&[0xff, 0xff]),