tape/changer/sg_pt_changer: read whole descriptor size for each entry

Some changer seem to append more data than we expect, but correctly
annotates that size in the subheader.

For each descriptor entry, read as much as the size given in the
subheader (or until the end of the reader), else our position in
the reader is wrong for the next entry, and we will parse
incorrect data.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

src/tape/changer/sg_pt_changer.rs

@@ -593,6 +593,8 @@ fn decode_element_status_page(
                     break;
                 }
 
+                let len_before = reader.len();
+
                 match subhead.element_type_code {
                     1 => {
                         let desc: TrasnsportDescriptor = unsafe { reader.read_be_value()? };
@@ -693,6 +695,19 @@ fn decode_element_status_page(
                     }
                     code => bail!("got unknown element type code {}", code),
                 }
+
+                // we have to consume the whole descriptor size, else
+                // our position in the reader is not correct
+                let len_after = reader.len();
+                let have_read = len_before - len_after;
+                let desc_len = subhead.descriptor_length as usize;
+                if desc_len > have_read {
+                    let mut left_to_read = desc_len - have_read;
+                    if left_to_read > len_after {
+                        left_to_read = len_after; // reader has not enough data?
+                    }
+                    let _ = reader.read_exact_allocated(left_to_read)?;
+                }
             }
         }