linux-kernel - Re: [PATCH] update ctime and mtime for mmaped write

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 22 Feb 2007 12:36:24 -0500
From:	Peter Staubach <staubach@...hat.com>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	akpm@...ux-foundation.org, hugh@...itas.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] update ctime and mtime for mmaped write

Miklos Szeredi wrote:
>>>>> Inspired by Peter Staubach's patch and the resulting comments.
>>>>>
>>>>>   
>>>>>       
>>>>>           
>>>> An updated version of the original patch was submitted to LKML
>>>> yesterday...  :-)
>>>>     
>>>>         
>>> Strange coincidence :)
>>>
>>>   
>>>       
>>>>>  		file = vma->vm_file;
>>>>>  		start = vma->vm_end;
>>>>> +		mapping_update_time(file);
>>>>>  		if ((flags & MS_SYNC) && file &&
>>>>>  				(vma->vm_flags & VM_SHARED)) {
>>>>>  			get_file(file);
>>>>>   
>>>>>       
>>>>>           
>>>> It seems to me that this might lead to file times being updated for
>>>> non-MAP_SHARED mappings.
>>>>     
>>>>         
>>> In theory no, because the COW-ed pages become anonymous and are not
>>> part of the original mapping any more.
>>>
>>>   
>>>       
>> I must profess to having a incomplete understanding of all of this
>> support, but then why would it be necessary to test VM_SHARED at
>> this point in msync()?
>>     
>
> That's basically just an optimization.  If it wasn't there, then data
> from a another (shared) mapping could be written back, which is not
> wrong, but not required either.
>
>   
>> I ran into problems early on with file times being updated incorrectly
>> so I am a little sensitive this aspect.
>>
>>     
>>>>> +int set_page_dirty_mapping(struct page *page);
>>>>>   
>>>>>       
>>>>>           
>>>> This aspect of the design seems intrusive to me.  I didn't see a strong
>>>> reason to introduce new versions of many of the routines just to handle
>>>> these semantics.  What motivated this part of your design?  Why the new
>>>> _mapping versions of routines?
>>>>     
>>>>         
>>> Because there's no way to know inside the set_page_dirty() functions
>>> if the dirtying comes from a memory mapping or from a modification
>>> through a normal write().  And they have different semantics, for
>>> write() the modification times are updated immediately.
>>>       
>> Perhaps I didn't understand what page_mapped() does, but it does seem to
>> have the right semantics as far as I could see.
>>     
>
> The problems will start, when you have a file that is both mapped and
> modified with write().  Then the dirying from the write() will set the
> flag, and that will have undesirable consequences.

I don't think that I quite follow the logic.  The dirtying from write()
will set the flag, but then the mtime will get updated and the flag will
be cleared by the hook in file_update_time().  Right?

    Thanx...

       ps
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/