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Subject[PATCH v2] execve: warn if process starts with executable stack
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.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
---

v2: print pathname instead of comm/pid

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

--- 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);
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