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

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Message-ID: <A6BFA766-141D-445E-8F64-BE8E50C4AC0E@fb.com>
Date:   Fri, 26 Jul 2019 06:07:21 +0000
From:   Song Liu <songliubraving@...com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     lkml <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "Kernel Team" <Kernel-team@...com>,
        "william.kucharski@...cle.com" <william.kucharski@...cle.com>
Subject: Re: [PATCH v8 2/4] uprobe: use original page when all uprobes are
 removed


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.le 
> 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