Enable read and write operations of the attribute to handle offset

kernel/comps/systree/src/lib.rs

@@ -76,6 +76,8 @@ pub enum Error {
     PermissionDenied,
     /// Other internal error
     InternalError(&'static str),
+    /// Arithmetic overflow occurred
+    Overflow,
 }
 
 impl core::fmt::Display for Error {
@@ -88,6 +90,7 @@ impl core::fmt::Display for Error {
             Error::AttributeError => write!(f, "Attribute error"),
             Error::PermissionDenied => write!(f, "Permission denied for operation"),
             Error::InternalError(msg) => write!(f, "Internal error: {}", msg),
+            Error::Overflow => write!(f, "Numerical overflow occurred"),
         }
     }
 }

kernel/comps/systree/src/node.rs

@@ -118,6 +118,33 @@ pub trait SysNode: SysObj {
     /// Writes the value of an attribute.
     fn write_attr(&self, name: &str, reader: &mut VmReader) -> Result<usize>;
 
+    /// Reads the value of an attribute from the specified `offset`.
+    fn read_attr_at(&self, name: &str, offset: usize, writer: &mut VmWriter) -> Result<usize> {
+        let (attr_buffer, attr_len) = {
+            let attr_buffer_len = writer.avail().checked_add(offset).ok_or(Error::Overflow)?;
+            let mut buffer = vec![0; attr_buffer_len];
+            let len = self.read_attr(
+                name,
+                &mut VmWriter::from(buffer.as_mut_slice()).to_fallible(),
+            )?;
+            (buffer, len)
+        };
+
+        if attr_len <= offset {
+            return Ok(0);
+        }
+
+        writer
+            .write_fallible(VmReader::from(attr_buffer.as_slice()).skip(offset))
+            .map_err(|_| Error::AttributeError)
+    }
+
+    /// Writes the value of an attribute at the specified `offset`.
+    fn write_attr_at(&self, name: &str, _offset: usize, reader: &mut VmReader) -> Result<usize> {
+        // In general, the `offset` for attribute write operations is ignored directly.
+        self.write_attr(name, reader)
+    }
+
     /// Shows the string value of an attribute.
     ///
     /// Most attributes are textual, rather binary (see `SysAttrFlags::IS_BINARY`).

kernel/src/error.rs

@@ -339,6 +339,7 @@ impl From<aster_systree::Error> for Error {
             AttributeError => Error::new(Errno::EIO),
             PermissionDenied => Error::new(Errno::EACCES),
             InternalError(msg) => Error::with_message(Errno::EIO, msg),
+            Overflow => Error::new(Errno::EOVERFLOW),
         }
     }
 }

kernel/src/fs/sysfs/inode.rs

@@ -376,29 +376,38 @@ impl Inode for SysFsInode {
         self.read_direct_at(offset, buf)
     }
 
-    fn read_direct_at(&self, _offset: usize, buf: &mut VmWriter) -> Result<usize> {
-        // TODO: it is unclear whether we should simply ignore the offset
-        // or report errors if it is non-zero.
-
+    fn read_direct_at(&self, offset: usize, buf: &mut VmWriter) -> Result<usize> {
         let InnerNode::Attr(attr, leaf) = &self.inner_node else {
             return Err(Error::new(Errno::EINVAL));
         };
 
-        // TODO: check read permission
-        Ok(leaf.read_attr(attr.name(), buf)?)
+        let len = if offset == 0 {
+            leaf.read_attr(attr.name(), buf)?
+        } else {
+            // The `read_attr_at` method is more general than `read_attr`,
+            // but it could be less efficient. So we only use the more general form when necessary.
+            leaf.read_attr_at(attr.name(), offset, buf)?
+        };
+
+        Ok(len)
     }
 
     fn write_at(&self, offset: usize, buf: &mut VmReader) -> Result<usize> {
         self.write_direct_at(offset, buf)
     }
 
-    fn write_direct_at(&self, _offset: usize, buf: &mut VmReader) -> Result<usize> {
+    fn write_direct_at(&self, offset: usize, buf: &mut VmReader) -> Result<usize> {
         let InnerNode::Attr(attr, leaf) = &self.inner_node else {
             return Err(Error::new(Errno::EINVAL));
         };
 
-        // TODO: check write permission
-        Ok(leaf.write_attr(attr.name(), buf)?)
+        let len = if offset == 0 {
+            leaf.write_attr(attr.name(), buf)?
+        } else {
+            leaf.write_attr_at(attr.name(), offset, buf)?
+        };
+
+        Ok(len)
     }
 
     fn create(&self, _name: &str, _type_: InodeType, _mode: InodeMode) -> Result<Arc<dyn Inode>> {