Remove the shim kernel crate

kernel/src/syscall/close.rs

@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: MPL-2.0
+
+use super::SyscallReturn;
+use crate::{fs::file_table::FileDesc, prelude::*};
+
+pub fn sys_close(fd: FileDesc, ctx: &Context) -> Result<SyscallReturn> {
+    debug!("fd = {}", fd);
+
+    let file = {
+        let mut file_table = ctx.process.file_table().lock();
+        let _ = file_table.get_file(fd)?;
+        file_table.close_file(fd).unwrap()
+    };
+
+    // Cleanup work needs to be done in the `Drop` impl.
+    //
+    // We don't mind the races between closing the file descriptor and using the file descriptor,
+    // because such races are explicitly allowed in the man pages. See the "Multithreaded processes
+    // and close()" section in <https://man7.org/linux/man-pages/man2/close.2.html>.
+    drop(file);
+
+    // Linux has error codes for the close() system call for diagnostic and remedial purposes, but
+    // only for a small subset of file systems such as NFS. We currently have no support for such
+    // file systems, so it's fine to just return zero.
+    //
+    // For details, see the discussion at <https://github.com/asterinas/asterinas/issues/506> and
+    // the "Dealing with error returns from close()" section at
+    // <https://man7.org/linux/man-pages/man2/close.2.html>.
+    Ok(SyscallReturn::Return(0))
+}