On 10.03.25 18:03, Oleg Nesterov wrote:

On 03/04, David Hildenbrand wrote:

quoted

uprobe_write_opcode() does some pretty low-level things that really, it
shouldn't be doing:

Agreed. Thanks again for doing this.

David, as I said, I can't review. I don't understand this mm/folio magic
with or without your changes.

No worries! Thanks for taking a look!

However. With your changes the code looks "better" and more understandable
to me. So I'd vote for your patches even if I can't ack them.

But I'd like to ask some stupid (no, really) questions.
__uprobe_write_opcode() does:

	/* We're done if we don't find an anonymous folio when unregistering. */
	if (!folio_test_anon(folio))
		return is_register ? -EFAULT : 0;

Yes, but we do not expect !folio_test_anon() if register == true, right?

 > See also below.>

	/* Verify that the page content is still as expected. */
	if (verify_opcode(fw->page, opcode_vaddr, &opcode) <= 0) {
		set_pte_at(vma->vm_mm, vaddr, fw->ptep, fw->pte);
		return -EAGAIN;
	}

The caller, uprobe_write_opcode(), has already called verify_opcode(),
why do we need to re-check?

Regarding both questions, the code is fairly racy. Nothing would stop 
user space from (a) modifying that memory (b) zapping the anon page 
using MADV_DONTNEED (if we don't hold the mmap lock in write mode).

Regarding the latter, uprobe_write_opcode() is documented to: "Called 
with mm->mmap_lock held for read or write.".

Note that both checks are fairly cheap.

But whatever reason we have. Can we change uprobe_write_opcode() to
"delay" put_page() and instead of

I was debating with myself whether we should do that and went back and 
forth a couple of times.

	/* Walk the page tables again, to perform the actual update. */
	folio = folio_walk_start(&fw, vma, vaddr, 0);
	if (folio) {
		ret = __uprobe_write_opcode(vma, &fw, folio, opcode_vaddr,
					    opcode);
		folio_walk_end(&fw, vma);
	} else {
		ret = -EAGAIN;
	}

do something like

	/* Walk the page tables again, to perform the actual update. */
	ret = -EAGAIN;
	folio = folio_walk_start(&fw, vma, vaddr, 0);
	if (folio) {
		if (fw.page == page) {
			WARN_ON(is_register && !folio_test_anon(folio));

Yes, that would work (we could leave the WARN_ON in 
__uprobe_write_opcode), but I am not sure if the end result is better 
better. No strong opinion on the details though.

			ret = __uprobe_write_opcode(vma, &fw, folio, opcode_vaddr,
					            opcode);
		}
		folio_walk_end(&fw, vma);
	}

?

Once again, I am not trying to review. I am trying to understand the
basics of your code.

Any feedback is welcome, thanks!

-- 
Cheers,

David / dhildenb