当然原则上 interpreter 可以 ET_EXEC 和 ET_DYN 里二选一即可，不过选 ET_EXEC 的话，会加载到固定地址上，可能会和实际可执行文件的地址冲突，ET_DYN 之后则可以允许在其他地址加载，没有规定 interpreter 在 ET_DYN 模式下是否可以是一个可执行文件，只要求不能递归指定 interpreter ，一般来说约定有 ET_DYN 没有 interpreter 表示它自身就是 interp ，可以参考内核里的注释
/*
* This logic is run once for the first LOAD Program
* Header for ET_DYN binaries to calculate the
* randomization (load_bias) for all the LOAD
* Program Headers.
*
* There are effectively two types of ET_DYN
* binaries: programs (i.e. PIE: ET_DYN with INTERP)
* and loaders (ET_DYN without INTERP, since they
* _are_ the ELF interpreter). The loaders must
* be loaded away from programs since the program
* may otherwise collide with the loader (especially
* for ET_EXEC which does not have a randomized
* position). For example to handle invocations of
* "./ld.so someprog" to test out a new version of
* the loader, the subsequent program that the
* loader loads must avoid the loader itself, so
* they cannot share the same load range. Sufficient
* room for the brk must be allocated with the
* loader as well, since brk must be available with
* the loader.
*
* Therefore, programs are loaded offset from
* ELF_ET_DYN_BASE and loaders are loaded into the
* independently randomized mmap region (0 load_bias
* without MAP_FIXED nor MAP_FIXED_NOREPLACE).
*/