WIP: json-sysctl: fix offset handling

Author: geofft

tests/json-sysctl/src/lib.rs

@@ -1,6 +1,7 @@
 #![no_std]
 #![feature(const_str_as_bytes)]
 
+use core::cmp::min;
 use core::convert::TryInto;
 use core::sync::atomic::{AtomicBool, Ordering};
 
@@ -36,9 +37,11 @@ impl linux_kernel_module::chrdev::FileOperations for JsonChrdev {
             c: C.load(Ordering::Relaxed),
         };
         let mut s =
-            serde_json_core::to_string::<typenum::U32, _>(&o).map_err(|_| error::Error::ENOMEM)?;
-        s.push_str("\n").map_err(|_| error::Error::ENOMEM)?;
-        buf.write(&s.into_bytes()[offset.try_into()?..][..buf.len()])?;
+            serde_json_core::to_vec::<typenum::U32, _>(&o).map_err(|_| error::Error::ENOMEM)?;
+        s.push(b'\n').map_err(|_| error::Error::ENOMEM)?;
+        let start = min(offset.try_into()?, s.len());
+        let end = min(start + buf.len(), s.len());
+        buf.write(&s[start..end])?;
         Ok(())
     }
 }