From 77d00a4f54ba65a2ab04a25ba501cf704e25fe60 Mon Sep 17 00:00:00 2001
From: Lucas Fernandes Nogueira <lucas@tauri.app>
Date: Fri, 9 Oct 2026 13:04:44 -0300
Subject: [PATCH] fix(fs): reject negative offsets when seeking from the start
 (#3633)

In plugins/fs/src/commands.rs seek, `SeekFrom::Start(offset as u64)`
turned -1 into u64::MAX. The offset is now converted with u64::try_from
and a negative one is an error. E2E spec added.
---
 .changes/fix-fs-seek-negative.md       |  6 ++++++
 packages/api-e2e/test/specs/fs.spec.ts | 13 +++++++++++++
 plugins/fs/src/commands.rs             | 19 ++++++++++---------
 3 files changed, 29 insertions(+), 9 deletions(-)
 create mode 100644 .changes/fix-fs-seek-negative.md

diff --git a/.changes/fix-fs-seek-negative.md b/.changes/fix-fs-seek-negative.md
new file mode 100644
index 000000000..0e46ce23a
--- /dev/null
+++ b/.changes/fix-fs-seek-negative.md
@@ -0,0 +1,6 @@
+---
+fs: patch
+fs-js: patch
+---
+
+`FileHandle.seek` now rejects a negative offset with `SeekMode.Start` instead of seeking to a huge position.
diff --git a/packages/api-e2e/test/specs/fs.spec.ts b/packages/api-e2e/test/specs/fs.spec.ts
index 790ee9105..68e9f22a3 100644
--- a/packages/api-e2e/test/specs/fs.spec.ts
+++ b/packages/api-e2e/test/specs/fs.spec.ts
@@ -371,6 +371,19 @@ describePlugin('fs', () => {
     })
   })
 
+  it('FileHandle.seek rejects a negative offset from the start', async () => {
+    const message = await tauriError(async (api, path) => {
+      const baseDir = api.fs.BaseDirectory.AppData
+      const file = await api.fs.create(path, { baseDir })
+      try {
+        await file.seek(-1, api.fs.SeekMode.Start)
+      } finally {
+        await file.close()
+      }
+    }, `${dir}/seek.txt`)
+    expect(message).toMatch(/must not be negative/)
+  })
+
   it('a closed FileHandle cannot be used again', async () => {
     const message = await tauriError(async (api, path) => {
       const baseDir = api.fs.BaseDirectory.AppData
diff --git a/plugins/fs/src/commands.rs b/plugins/fs/src/commands.rs
index 958e01de3..c66bb0d5a 100644
--- a/plugins/fs/src/commands.rs
+++ b/plugins/fs/src/commands.rs
@@ -888,16 +888,17 @@ pub async fn seek<R: Runtime>(
     whence: SeekMode,
 ) -> CommandResult<u64> {
     use std::io::{Seek, SeekFrom};
+    let position = match whence {
+        SeekMode::Start => SeekFrom::Start(u64::try_from(offset).map_err(|_| {
+            format!("invalid seek offset {offset}: it must not be negative with SeekMode.Start")
+        })?),
+        SeekMode::Current => SeekFrom::Current(offset),
+        SeekMode::End => SeekFrom::End(offset),
+    };
     let file: std::sync::Arc<StdFileResource<R>> = webview.resources_table().get(rid)?;
-    StdFileResource::with_lock(&file, |file| {
-        file.seek(match whence {
-            SeekMode::Start => SeekFrom::Start(offset as u64),
-            SeekMode::Current => SeekFrom::Current(offset),
-            SeekMode::End => SeekFrom::End(offset),
-        })
-    })
-    .map_err(|e| format!("failed to seek file with error: {e}"))
-    .map_err(Into::into)
+    StdFileResource::with_lock(&file, |file| file.seek(position))
+        .map_err(|e| format!("failed to seek file with error: {e}"))
+        .map_err(Into::into)
 }
 
 #[cfg(target_os = "android")]