shred:add checks for block sizes

This commit adds checks for block size constants. Misconfigured sizes
could lead to overflows or performance issues. While such cases are
expected to be rare or unlikely, it's better to be notified of them
early.

Author: alexs-sh

src/uu/shred/src/shred.rs

@@ -514,12 +514,17 @@ fn wipe_file(
 }
 
 fn split_on_blocks(file_size: u64, exact: bool) -> (u64, u64) {
+    // OPTIMAL_IO_BLOCK_SIZE must not exceed BLOCK_SIZE. Violating this may cause overflows due
+    // to alignment or performance issues.This kind of misconfiguration is
+    // highly unlikely but would indicate a serious error.
+    const _: () = assert!(OPTIMAL_IO_BLOCK_SIZE <= BLOCK_SIZE);
+
     let file_size = if exact {
         file_size
     } else {
-        // The main idea here is to align the file size to the OPTIMAL_IO_BLOCK_SIZE, and then split it into
-        // BLOCK_SIZE + remaining bytes. Since the input data is already aligned to N * OPTIMAL_IO_BLOCK_SIZE,
-        // the output file size will also be aligned and correct.
+        // The main idea here is to align the file size to the OPTIMAL_IO_BLOCK_SIZE, and then
+        // split it into BLOCK_SIZE + remaining bytes. Since the input data is already aligned to N
+        // * OPTIMAL_IO_BLOCK_SIZE, the output file size will also be aligned and correct.
         file_size.div_ceil(OPTIMAL_IO_BLOCK_SIZE as u64) * OPTIMAL_IO_BLOCK_SIZE as u64
     };
     (file_size / BLOCK_SIZE as u64, file_size % BLOCK_SIZE as u64)