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

[Song Liu <songliubraving@fb.com>]

Hi Oleg, 

>> 
>> 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?

I decided to follow your suggestion of "unmap old_page; fault in orig_page". 
Please see v9 of the set. 

Thanks,
Song