Re: [PATCH v2 7/11] Uprobes Implementation

[Oleg Nesterov <oleg@redhat.com>]

On 04/22, Srikar Dronamraju wrote:
>
> * Oleg Nesterov <oleg@redhat.com> [2010-04-21 18:05:15]:
>
> > 3. mprotect(). write_opcode() checks !VM_WRITE. This is correct,
> >    otherwise we can race with the user-space writing to the same
> >    page.
> >
> >    But suppose that the application does mprotect(PROT_WRITE) after
> >    register_uprobe() installs the bp, now unregister_uprobe/etc can't
> >    restore the original insn?
> >
>
> I still need to verify this. I shall get back to you on this.
> However are there applications that mprotect(PROT_WRITE) text pages?

Well, I think the kernel should assume that the user-space can do
anything.

Hmm. And if this vma is VM_SHARED, then this bp could be actually
written to vm_file after mprotect().

But I think this doesn't really matter. When I actually look at
patches 3 and 4, I am starting to think this all is very wrong.

> I am copying Mel Gorman and Andrea Arcangeli so that they can provide
> their inputs on VM and KSM related issues.

Yes. We need vm experts here, I am not. Still, I'd like to share my
concerns. I also added Rik and Hugh.


So, 3/11 does

	@@ -2617,7 +2617,10 @@ int replace_page(struct vm_area_struct *vma, struct page *page,
		}
	 
		get_page(kpage);
	-	page_add_anon_rmap(kpage, vma, addr);
	+	if (PageAnon(kpage))
	+		page_add_anon_rmap(kpage, vma, addr);
	+	else
	+		page_add_file_rmap(kpage);
	 
		flush_cache_page(vma, addr, pte_pfn(*ptep));
		ptep_clear_flush(vma, addr, ptep);

I see no point in this patch, please see below.

The next 4/11 patch introduces write_opcode() which roughly does:

	int write_opcode(unsigned long vaddr, user_bkpt_opcode_t opcode)
	{
		get_user_pages(write => false, &old_page);

		new_page = alloc_page_vma(...);

		... insert the bp into the new_page ...

		new_page->mapping = old_page->mapping;
		new_page->index = old_page->index;

		replace_page(old_page, new_page);
	}

This doesn't look right at all to me.

	IF PageAnon(old_page):

		in this case replace_page() calls page_add_anon_rmap() which
		needs the locked page.

	ELSE:

		I don't think the new page should evere preserve the mapping,
		this looks just wrong. It should be always anonymous.


And in fact, I do not understand why write_opcode() needs replace_page().
It could just use get_user_pages(FOLL_WRITE | FOLL_FORCE), no? It should
create the anonymous page correctly.

Either way, I think register_uprobe() should disallow the probes in
VM_SHARED/VM_MAYWRITE vmas.

Oleg.