fix(format): overflow-checked sizes and fallible allocation on chunked reads

Dataspace and chunk dimensions are untrusted 64-bit fields, but the chunked
read paths computed `num_elements() as usize * elem_size` and
`chunk_dims.product() * elem_size` with plain arithmetic and fed the result to
`vec![0u8; n]`. A crafted file could wrap the product (under-sizing the output
buffer that chunks are then copied into) or request an allocation large enough
to abort the process.

- Dataspace::checked_num_elements, checked_byte_len, checked_chunk_byte_len
  and alloc_output (try_reserve_exact) replace the plain products and
  vec![0; n] at every chunked read site, plus the VDS and hyperslab paths.
  Overflow and allocation failure are FormatError::Overflow.
- Dataspace::num_elements saturates instead of wrapping.
- A zero-element dataset returns early, which also keeps the stride products
  in range when another dimension is huge.
- parallel_read.rs: the three `c_addr + size > len` bounds checks used a raw
  add; they now use checked_add like the rest of the crate.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/clawhdf5-format/src/dataspace.rs

@@ -1,5 +1,7 @@
 //! HDF5 Dataspace message parsing (message type 0x0001).
 
+#[cfg(not(feature = "std"))]
+use alloc::format;
 #[cfg(not(feature = "std"))]
 use alloc::vec::Vec;
 
@@ -167,6 +169,27 @@ impl Dataspace {
         }
     }
 
+    /// [`Dataspace::num_elements`] with the product overflow-checked. The
+    /// dimensions are untrusted 64-bit fields; read paths that size a buffer
+    /// from them must use this one.
+    pub fn checked_num_elements(&self) -> Result<u64, FormatError> {
+        match self.space_type {
+            DataspaceType::Null => Ok(0),
+            DataspaceType::Scalar => Ok(1),
+            DataspaceType::Simple if self.dimensions.is_empty() => Ok(0),
+            DataspaceType::Simple => self
+                .dimensions
+                .iter()
+                .try_fold(1u64, |acc, &d| acc.checked_mul(d))
+                .ok_or_else(|| {
+                    FormatError::Overflow(format!(
+                        "dataspace dimensions {:?} overflow the element count",
+                        self.dimensions
+                    ))
+                }),
+        }
+    }
+
     /// Total number of elements. Scalar = 1, Null = 0.
     pub fn num_elements(&self) -> u64 {
         match self.space_type {
@@ -176,7 +199,12 @@ impl Dataspace {
                 if self.dimensions.is_empty() {
                     0
                 } else {
-                    self.dimensions.iter().product()
+                    // Saturate rather than wrap: a wrapped product could
+                    // under-size a buffer. Size-critical callers use
+                    // `checked_num_elements`.
+                    self.dimensions
+                        .iter()
+                        .fold(1u64, |acc, &d| acc.saturating_mul(d))
                 }
             }
         }