sparse: Fix u32 overflow when unpacking files with large holes

CHUNK_TYPE_RAW is the only chunk type that's guaranteed to not overflow
a u32 when its number of blocks is multiplied by the block size.
CHUNK_TYPE_FILL and CHUNK_TYPE_DONT_CARE require a u64.

Issue: #472

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

avbroot/src/cli/sparse.rs

@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: 2024 Andrew Gunnerson
+// SPDX-FileCopyrightText: 2024-2025 Andrew Gunnerson
 // SPDX-License-Identifier: GPL-3.0-only
 
 use std::{
@@ -317,11 +317,11 @@ fn unpack_subcommand(
                 })?;
             }
             ChunkData::Hole => {
-                // This cannot overflow.
-                let to_skip = chunk.bounds.len() * metadata.header.block_size;
+                // Unlike ChunkData::Data, this can overflow a u32.
+                let to_skip = i64::from(chunk.bounds.len()) * i64::from(metadata.header.block_size);
 
                 writer
-                    .seek(SeekFrom::Current(to_skip.into()))
+                    .seek(SeekFrom::Current(to_skip))
                     .with_context(|| format!("Failed to seek file: {:?}", cli.output))?;
             }
             ChunkData::Crc32(_) => {}

avbroot/src/format/sparse.rs

@@ -1,4 +1,4 @@
-// SPDX-FileCopyrightText: 2024 Andrew Gunnerson
+// SPDX-FileCopyrightText: 2024-2025 Andrew Gunnerson
 // SPDX-License-Identifier: GPL-3.0-only
 
 use std::{
@@ -374,6 +374,9 @@ impl fmt::Debug for ChunkData {
 /// metadata they contain.
 #[derive(Clone, Copy, PartialEq, Eq)]
 pub struct Chunk {
+    /// When [`Self::data`] is [`ChunkData::Data`], this is guaranteed to not
+    /// exceed the bounds of [`u32`] when multiplied by [`Header::block_size`].
+    /// For other types of data, a 64-bit signed or unsigned integer is needed.
     pub bounds: ChunkBounds,
     pub data: ChunkData,
 }