src: fix slice of slice of file-backed Blob

The value for `new_end` was wrong: While the members `start_` and `end_`
refer to the entire length of the file, the parameters `start` and `end`
are relative to the current slice.

The new end would apparently have the current start_ subtracted from it,
and the length would possibly overflow when the FdEntry is asked for its
size or when get_reader is called, resulting in a subslice which extends
past the current slice, which shouldn't be possible. Add a CHECK if this
happens, rather than returning data outside the current slice.

There aren't any C++ tests for FdEntry, and on the javascript side there
isn't a way to ask the blob handle for its nominal size. That size could
be a large uint64, which gets converted to int64 to when FileHandle::new
is called, which interprets a negative length as unlimited.

Fixes: #53908
PR-URL: #53972
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>
Reviewed-By: James M Snell <[email protected]>

src/dataqueue/queue.cc

@@ -840,7 +840,9 @@ class FdEntry final : public EntryImpl {
         path_(std::move(path_)),
         stat_(stat),
         start_(start),
-        end_(end) {}
+        end_(end) {
+    CHECK_LE(start, end);
+  }
 
   std::shared_ptr<DataQueue::Reader> get_reader() override {
     return ReaderImpl::Create(this);
@@ -851,7 +853,7 @@ class FdEntry final : public EntryImpl {
     uint64_t new_start = start_ + start;
     uint64_t new_end = end_;
     if (end.has_value()) {
-      new_end = std::min(end.value(), end_);
+      new_end = std::min(end.value() + start_, end_);
     }
 
     CHECK(new_start >= start_);

test/parallel/test-blob-file-backed.js

@@ -86,6 +86,16 @@ writeFileSync(testfile5, '');
 
   const res1 = blob.slice(995, 1005);
   strictEqual(await res1.text(), data.slice(995, 1005));
+
+  // Refs: https://github.com/nodejs/node/issues/53908
+  for (const res2 of [
+    blob.slice(995, 1005).slice(),
+    blob.slice(995).slice(0, 10),
+    blob.slice(0, 1005).slice(995),
+  ]) {
+    strictEqual(await res2.text(), data.slice(995, 1005));
+  }
+
   await unlink(testfile2);
 })().then(common.mustCall());