fix clear bss when load user program. 'sh' works!

kernel/src/process/context.rs

@@ -80,12 +80,11 @@ impl ContextImpl {
                     let virt_addr = ph.virtual_addr() as usize;
                     let offset = ph.offset() as usize;
                     let file_size = ph.file_size() as usize;
-                    if file_size == 0 {
-                        return;
-                    }
-                    use core::slice;
-                    let target = unsafe { slice::from_raw_parts_mut(virt_addr as *mut u8, file_size) };
-                    target.copy_from_slice(&data[offset..offset + file_size]);
+                    let mem_size = ph.mem_size() as usize;
+
+                    let target = unsafe { ::core::slice::from_raw_parts_mut(virt_addr as *mut u8, mem_size) };
+                    target[..file_size].copy_from_slice(&data[offset..offset + file_size]);
+                    target[file_size..].iter_mut().for_each(|x| *x = 0);
                 }
                 ustack_top = push_args_at_stack(args, ustack_top);
             });