fix(format): a v1 group with an empty link name fails its listing

libhdf5 refuses to list a symbol-table group that has an entry with an
empty name (H5G__ent_to_link: "invalid link name"), so h5py cannot list
cve-2021-46244's /BAG_root. We listed it, with an object at "/BAG_root/"
(the empty name, pointing at address 0). resolve_v1_group_entries — the
listing — now fails with the new FormatError::InvalidLinkName; path
lookups still find the group's other names, as libhdf5's by-name lookup
does.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

crates/clawhdf5-format/src/error.rs

@@ -223,6 +223,9 @@ pub enum FormatError {
         /// The file's actual length in bytes.
         actual_len: u64,
     },
+    /// A link libhdf5 refuses to list: a symbol-table entry with an empty
+    /// name ("invalid link name"). Listing the group fails, as in libhdf5.
+    InvalidLinkName,
 }
 
 impl fmt::Display for FormatError {
@@ -494,6 +497,9 @@ impl fmt::Display for FormatError {
                      but the file is {actual_len} bytes"
                 )
             }
+            FormatError::InvalidLinkName => {
+                write!(f, "invalid link name: a group entry has an empty name")
+            }
         }
     }
 }

crates/clawhdf5-format/src/group_v1.rs

@@ -21,12 +21,35 @@ pub struct GroupEntry {
     pub cache_type: u32,
 }
 
-/// Given a SymbolTableMessage, resolve all group children.
+/// Given a SymbolTableMessage, resolve all group children: the group's
+/// listing.
+///
+/// An entry with an empty name fails the listing with
+/// [`FormatError::InvalidLinkName`], as it fails libhdf5's link iteration
+/// (`H5G__ent_to_link`: "invalid link name"). Looking a name up
+/// ([`resolve_path`], and the path resolution in
+/// [`crate::group_v2::resolve_path_any`]) still works in such a group, as it
+/// does in libhdf5.
 pub fn resolve_v1_group_entries(
     file_data: &[u8],
     sym_table_msg: &SymbolTableMessage,
     offset_size: u8,
     length_size: u8,
+) -> Result<Vec<GroupEntry>, FormatError> {
+    let entries = v1_group_entries(file_data, sym_table_msg, offset_size, length_size)?;
+    if entries.iter().any(|e| e.name.is_empty()) {
+        return Err(FormatError::InvalidLinkName);
+    }
+    Ok(entries)
+}
+
+/// Every entry of a v1 group, empty names included — for looking a name up,
+/// which never matches an empty name.
+pub(crate) fn v1_group_entries(
+    file_data: &[u8],
+    sym_table_msg: &SymbolTableMessage,
+    offset_size: u8,
+    length_size: u8,
 ) -> Result<Vec<GroupEntry>, FormatError> {
     // Parse local heap
     let heap = LocalHeap::parse(
@@ -207,8 +230,7 @@ pub fn resolve_path(
     let mut current_sym_table = root_sym_table.clone();
 
     for (i, component) in components.iter().enumerate() {
-        let entries =
-            resolve_v1_group_entries(file_data, &current_sym_table, offset_size, length_size)?;
+        let entries = v1_group_entries(file_data, &current_sym_table, offset_size, length_size)?;
 
         let found = entries.iter().find(|e| e.name == *component);
         match found {
@@ -425,6 +447,22 @@ mod tests {
         assert_eq!(entries[1].object_header_address, 0x2000);
     }
 
+    /// cve-2021-46244 `/BAG_root`: a symbol-table entry with an empty name.
+    /// libhdf5 fails the group's listing ("invalid link name"); a lookup of
+    /// the other names still works.
+    #[test]
+    fn empty_entry_name_fails_the_listing_not_a_lookup() {
+        let (file, msg) = build_synthetic_group(&[("", 0x1000, 0), ("elevation", 0x2000, 0)], 8, 8);
+        assert_eq!(
+            resolve_v1_group_entries(&file, &msg, 8, 8).unwrap_err(),
+            FormatError::InvalidLinkName
+        );
+        assert_eq!(
+            resolve_path(&file, &msg, "elevation", 8, 8).unwrap(),
+            0x2000
+        );
+    }
+
     #[test]
     fn resolve_path_single_level() {
         let (file, msg) =

crates/clawhdf5-format/src/group_v2.rs

@@ -450,7 +450,9 @@ fn resolve_group_entries(
             .find(|m| m.msg_type == MessageType::SymbolTable)
             .ok_or_else(|| FormatError::PathNotFound(String::from("no symbol table message")))?;
         let stm = SymbolTableMessage::parse(&sym_msg.data, offset_size)?;
-        group_v1::resolve_v1_group_entries(file_data, &stm, offset_size, length_size)
+        // A lookup: an entry with an empty name (which fails a listing) is
+        // skipped by the name comparison, as in libhdf5.
+        group_v1::v1_group_entries(file_data, &stm, offset_size, length_size)
     } else if is_v2_group(object_header) {
         resolve_v2_group_entries(file_data, object_header, offset_size, length_size)
     } else {