On 10.03.25 18:03, Oleg Nesterov wrote:
On 03/04, David Hildenbrand wrote:
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
