Fix metadata encoding and decoding to use the right length

Author: CraftSpider

compiler/rustc_codegen_ssa/src/back/metadata.rs

@@ -305,8 +305,13 @@ pub fn create_compressed_metadata_file(
     symbol_name: &str,
 ) -> Vec<u8> {
     let mut compressed = rustc_metadata::METADATA_HEADER.to_vec();
-    compressed.write_all(&(metadata.raw_data().len() as u32).to_be_bytes()).unwrap();
+    // Our length will be backfilled once we're done writing
+    compressed.write_all(&[0; 4]).unwrap();
     FrameEncoder::new(&mut compressed).write_all(metadata.raw_data()).unwrap();
+    let meta_len = rustc_metadata::METADATA_HEADER.len();
+    let data_len = (compressed.len() - meta_len - 4) as u32;
+    compressed[meta_len..meta_len + 4].copy_from_slice(&data_len.to_be_bytes());
+
     let Some(mut file) = create_object_file(sess) else {
         return compressed.to_vec();
     };

compiler/rustc_metadata/src/locator.rs

@@ -799,14 +799,14 @@ fn get_metadata_section<'p>(
             }
 
             // Length of the compressed stream - this allows linkers to pad the section if they want
-            let usize_len = core::mem::size_of::<u32>();
-            let Ok(len_bytes) = <[u8; 4]>::try_from(&buf[header_len..cmp::min(header_len + usize_len, buf.len())]) else {
+            let u32_len = core::mem::size_of::<u32>();
+            let Ok(len_bytes) = <[u8; 4]>::try_from(&buf[header_len..cmp::min(header_len + u32_len, buf.len())]) else {
                 return Err(MetadataError::LoadFailure("invalid metadata length found".to_string()));
             };
             let compressed_len = u32::from_be_bytes(len_bytes) as usize;
 
             // Header is okay -> inflate the actual metadata
-            let compressed_bytes = &buf[header_len..compressed_len + header_len];
+            let compressed_bytes = &buf[(header_len + u32_len)..(compressed_len + header_len + u32_len)];
             debug!("inflating {} bytes of compressed metadata", compressed_bytes.len());
             // Assume the decompressed data will be at least the size of the compressed data, so we
             // don't have to grow the buffer as much.