Hi Jun, Thank you for the patch! Yet something to improve: [auto build test ERROR on kselftest/next] [also build test ERROR on v5.14-rc7 next-20210823] [If your patch is applied to the wrong git tree, kindly drop us a note. And when submitting patch, we suggest to use '--base' as documented in https://git-scm.com/docs/git-format-patch] url: https://github.com/0day-ci/linux/commits/Jun-Miao/selftests-x86-Fix-error... base: https://git.kernel.org/pub/scm/linux/kernel/git/shuah/linux-kselftest.git next config: x86_64-rhel-8.3-kselftests (attached as .config) compiler: gcc-9 (Debian 9.3.0-22) 9.3.0 reproduce (this is a W=1 build): # https://github.com/0day-ci/linux/commit/3752736483bd2f09d350c5d3a42460d74... git remote add linux-review https://github.com/0day-ci/linux git fetch --no-tags linux-review Jun-Miao/selftests-x86-Fix-error-variably-modified-altstack_data-at-file-scope/20210823-215819 git checkout 3752736483bd2f09d350c5d3a42460d74ea9c53a # save the attached .config to linux build tree mkdir build_dir make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash -C tools/testing/selftests install If you fix the issue, kindly add following tag as appropriate Reported-by: kernel test robot <lkp(a)intel.com&gt; All errors (new ones prefixed by >>): single_step_syscall.c: In function 'main': 230 | free(stack.ss_sp); | ^~~~~ | stack_t single_step_syscall.c:230:7: note: each undeclared identifier is reported only once for each function it appears in -- mov_ss_trap.c: In function 'main': 284 | free(stack.ss_sp); | ^~~~~ | obstack mov_ss_trap.c:284:7: note: each undeclared identifier is reported only once for each function it appears in --- 0-DAY CI Kernel Test Service, Intel Corporation https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org