[PATCH] exec: warn if process starts with executable stack

[Alexey Dobriyan <adobriyan@xxxxxxxxx>]

There were few episodes of silent downgrade to an executable stack:

1) linking innocent looking assembly file

	$ 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.

While without a double 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 much problems.

If the system is old or CPU is old, then there will be an early warning
against init and/or support personnel will write that "uh-oh, our Enterprise
Software absolutely requires executable stack" and close tickets and customers
will nod heads and life moves on.

Signed-off-by: Alexey Dobriyan <adobriyan@xxxxxxxxx>
---

 fs/exec.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -762,6 +762,11 @@ int setup_arg_pages(struct linux_binprm *bprm,
 		goto out_unlock;
 	BUG_ON(prev != vma);
 
+	if (vm_flags & VM_EXEC) {
+		pr_warn_once("process '%s'/%u started with executable stack\n",
+			     current->comm, current->pid);
+	}
+
 	/* Move stack pages down in memory. */
 	if (stack_shift) {
 		ret = shift_arg_pages(vma, stack_shift);