[PATCH v6 proxmox-backup 07/18] datastore: verify that chunk_size is a power of two

[Robert Obkircher <r.obkircher@proxmox.com>]

Guarantee that the bitmasks used to compute the modulo actually yield
the correct value and implicitly guard against divisions by zero.

Signed-off-by: Robert Obkircher <r.obkircher@proxmox.com>
---
 pbs-datastore/src/fixed_index.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/pbs-datastore/src/fixed_index.rs b/pbs-datastore/src/fixed_index.rs
index 7f0ef91c..953f4a20 100644
--- a/pbs-datastore/src/fixed_index.rs
+++ b/pbs-datastore/src/fixed_index.rs
@@ -86,6 +86,10 @@ impl FixedIndexReader {
         let ctime = i64::from_le(header.ctime);
         let chunk_size = u64::from_le(header.chunk_size);
 
+        if !chunk_size.is_power_of_two() {
+            bail!("got non-power-of-two chunk size: {chunk_size}");
+        }
+
         let index_length = size.div_ceil(chunk_size) as usize;
         let index_size = index_length * 32;
 
@@ -98,6 +102,8 @@ impl FixedIndexReader {
             );
         }
 
+        let chunk_size = usize::try_from(chunk_size)?;
+
         let data = unsafe {
             nix::sys::mman::mmap(
                 None,
@@ -114,7 +120,7 @@ impl FixedIndexReader {
 
         Ok(Self {
             _file: file,
-            chunk_size: chunk_size as usize,
+            chunk_size,
             size,
             index_length,
             index: data,
@@ -263,6 +269,9 @@ impl FixedIndexWriter {
         }
 
         let chunk_size = u64::from(chunk_size);
+        if !chunk_size.is_power_of_two() {
+            bail!("got non-power-of-two chunk size: {chunk_size}");
+        }
 
         let ctime = proxmox_time::epoch_i64();
 
-- 
2.47.3