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

[Song Liu <songliubraving@fb.com>]

> On Jul 25, 2019, at 1:14 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 
> 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.

I see. Let me see how we can cover this. 

> 
>>> 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.

I guess I know the case now. We can probably avoid this with an simp\x10le 
check for old_page == new_page?

Thanks,
Song