Fix fd_seek underflow

This commit adds a checked subtraction to `fd_seek` so that passing in a
large negative offset does not underflow.

fixes #4589

lib/wasix/src/syscalls/wasi/fd_seek.rs

@@ -75,8 +75,12 @@ pub(crate) fn fd_seek_internal(
                 fd_entry.offset.fetch_add(offset, Ordering::AcqRel) + offset
             } else if offset < 0 {
                 let offset = offset.unsigned_abs();
-                // FIXME: need to handle underflow!
-                fd_entry.offset.fetch_sub(offset, Ordering::AcqRel) - offset
+
+                wasi_try_ok_ok!(fd_entry
+                    .offset
+                    .fetch_sub(offset, Ordering::AcqRel)
+                    .checked_sub(offset)
+                    .ok_or(Errno::Inval))
             } else {
                 fd_entry.offset.load(Ordering::Acquire)
             }

tests/wasi-fyi/fd_seek_cur_negative_underflow.rs

@@ -0,0 +1,32 @@
+use std::os::fd::AsRawFd;
+
+#[link(wasm_import_module = "wasi_snapshot_preview1")]
+extern "C" {
+    pub fn fd_seek(fd: i32, offset: i64, whence: i32, filesize: i32) -> i32;
+}
+
+const ERRNO_INVAL: i32 = 28;
+
+const WHENCE_CUR: i32 = 1;
+
+fn main() {
+    unsafe {
+        let large_negative_offset = -6551085931117533355;
+        let mut filesize = 0u64;
+
+        let f = std::fs::OpenOptions::new()
+            .read(true)
+            .write(true)
+            .create(true)
+            .open("test.sh")
+            .unwrap();
+        let errno = fd_seek(
+            f.as_raw_fd(),
+            large_negative_offset,
+            WHENCE_CUR,
+            &mut filesize as *mut u64 as usize as i32,
+        );
+
+        assert_eq!(errno, ERRNO_INVAL);
+    }
+}