Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are removed

[Oleg Nesterov <oleg@redhat.com>]

On 07/24, Song Liu wrote:
>
>
> > On Jul 24, 2019, at 4:37 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > On 07/24, Song Liu wrote:
> >>
> >> 	lock_page(old_page);
> >> @@ -177,15 +180,24 @@ static int __replace_page(struct vm_area_struct *vma, unsigned long addr,
> >> 	mmu_notifier_invalidate_range_start(&range);
> >> 	err = -EAGAIN;
> >> 	if (!page_vma_mapped_walk(&pvmw)) {
> >> -		mem_cgroup_cancel_charge(new_page, memcg, false);
> >> +		if (!orig)
> >> +			mem_cgroup_cancel_charge(new_page, memcg, false);
> >> 		goto unlock;
> >> 	}
> >> 	VM_BUG_ON_PAGE(addr != pvmw.address, old_page);
> >>
> >> 	get_page(new_page);
> >> -	page_add_new_anon_rmap(new_page, vma, addr, false);
> >> -	mem_cgroup_commit_charge(new_page, memcg, false, false);
> >> -	lru_cache_add_active_or_unevictable(new_page, vma);
> >> +	if (orig) {
> >> +		lock_page(new_page);  /* for page_add_file_rmap() */
> >> +		page_add_file_rmap(new_page, false);
> >
> >
> > Shouldn't we re-check new_page->mapping after lock_page() ? Or we can't
> > race with truncate?
>
> We can't race with truncate, because the file is open as binary and
> protected with DENYWRITE (ETXTBSY).

No. Yes, deny_write_access() protects mm->exe_file, but not the dynamic
libraries or other files which can be mmaped.

> > and I am worried this code can try to lock the same page twice...
> > Say, the probed application does MADV_DONTNEED and then writes "int3"
> > into vma->vm_file at the same address to fool verify_opcode().
> >
>
> Do you mean the case where old_page == new_page?

Yes,

> I think this won't
> happen, because in uprobe_write_opcode() we only do orig_page for
> !is_register case.

See above.

!is_register doesn't necessarily mean the original page was previously cow'ed.
And even if it was cow'ed, MADV_DONTNEED can restore the original mapping.

Oleg.