Fix corruption recovery miscalculating when a frame runs past EOF

Author: akrieger

src/zzip.cpp

@@ -854,36 +854,29 @@ bool zzip::rewrite_footer()
         size_t zzip_remaining = zzip_len - scan_offset;
 
         // For each entry we expect at least a filename and checksum header.
-        void *file_ptr = nullptr; ;
-        size_t file_len = 0;
-        std::tie( file_ptr, file_len ) = read_and_skip_entry_metadata(
-                                             file_base_plus( scan_offset ),
-                                             zzip_remaining,
-                                             &filename_opt,
-                                             &checksum_opt );
-        if( file_ptr == nullptr || file_len == 0 || file_len > zzip_remaining ) {
+        void *file_ptr = nullptr;
+        std::tie( file_ptr, zzip_remaining ) = read_and_skip_entry_metadata(
+                file_base_plus( scan_offset ),
+                zzip_remaining,
+                &filename_opt,
+                &checksum_opt );
+        if( file_ptr == nullptr || zzip_remaining == 0 ) {
             break;
         }
-        // zzip_remaining = length of zzip from start of entry
-        // file_len = length of zzip from start of compressed file frame
-        // -> offset of file from start of entry = zzip_remaining - file_len
-        file_offset = scan_offset + ( zzip_remaining - file_len );
+
+        file_offset = zzip_len - zzip_remaining;
 
         if( !filename_opt.has_value() || !checksum_opt.has_value() ) {
             // Missing required metadata.
             break;
         }
-        if( file_offset >= file_len ) {
-            // We've run off the end of the file and read past the original footer.
-            break;
-        }
 
-        void *file_frame_begin = file_base_plus( file_offset );
-        size_t file_frame_size = ZSTD_findFrameCompressedSize( file_frame_begin, file_len );
+        size_t file_frame_size = ZSTD_findFrameCompressedSize( file_ptr, zzip_remaining );
         if( ZSTD_isError( file_frame_size ) ) {
             break;
         }
-        uint64_t checksum = XXH64( file_frame_begin, file_frame_size, kCheckumSeed );
+
+        uint64_t checksum = XXH64( file_ptr, file_frame_size, kCheckumSeed );
         if( checksum != checksum_opt ) {
             // Corruption in the compressed frame. Don't try to recover it, assume
             // everything after is lost.