On Sun, 8 Dec 2019 20:19:18 +0300 Alexey Dobriyan <adobriyan@xxxxxxxxx> wrote:
> There were few episodes of silent downgrade to an executable stack over
> years:
>
> 1) linking innocent looking assembly file will silently add executable
> stack if proper linker options is not given as well:
>
> $ cat f.S
> .intel_syntax noprefix
> .text
> .globl f
> f:
> ret
>
> $ cat main.c
> void f(void);
> int main(void)
> {
> f();
> return 0;
> }
>
> $ gcc main.c f.S
> $ readelf -l ./a.out
> GNU_STACK 0x0000000000000000 0x0000000000000000 0x0000000000000000
> 0x0000000000000000 0x0000000000000000 RWE 0x10
> ^^^
>
> 2) converting C99 nested function into a closure
> https://nullprogram.com/blog/2019/11/15/
>
> void intsort2(int *base, size_t nmemb, _Bool invert)
> {
> int cmp(const void *a, const void *b)
> {
> int r = *(int *)a - *(int *)b;
> return invert ? -r : r;
> }
> qsort(base, nmemb, sizeof(*base), cmp);
> }
>
> will silently require stack trampolines while non-closure version will not.
>
> Without doubt this behaviour is documented somewhere, add a warning so that
> developers and users can at least notice. After so many years of x86_64 having
> proper executable stack support it should not cause too many problems.
hm, OK, let's give it a trial run.
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -761,6 +761,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
> goto out_unlock;
> BUG_ON(prev != vma);
>
> + if (unlikely(vm_flags & VM_EXEC)) {
> + pr_warn_once("process '%pD4' started with executable stack\n",
> + bprm->file);
> + }
> +
> /* Move stack pages down in memory. */
> if (stack_shift) {
> ret = shift_arg_pages(vma, stack_shift);
What are poor users supposed to do if this message comes out?
Hopefully google the message and end up at this thread. What do you
want to tell them?
