Re: [PATCH 1/5] WIP: Add syscall unlinkat_s (currently x86* only)

[Alexander Holler <holler@ahsoftware.de>]

Am 04.02.2015 um 00:33 schrieb Al Viro:
> On Tue, Feb 03, 2015 at 07:01:50PM +0100, Alexander Holler wrote:
>
>> Yeah, as I've already admitted in the bug, I never should have use
>> the word secure, because everyone nowadays seems to end up in panic
>> when reading that word.
>>
>> So, if I would be able to use sed on my mails, I would replace
>> unlinkat_s() with unlinkat_w() (for wipe) or would say that _s does
>> stand for 'shred' in the means of shred(1).
>
> TBH, I suspect that the saner API would be something like EXT2_IOC_[SG[ETFLAGS,
> allowing to set and query that along with other flags (append-only, etc.).
>
> Forget about unlink; first of all, whatever API you use should only _mark_
> the inode as "zero freed blocks" (or trim, for that matter).  You can't
> force freeing of an inode, so either you make sure that subsequent freeing
> of inode, whenever it happens, will do that work, or your API is hopelessly
> racy.  Moreover, when link has been removed it's too late to report that
> fs has no way to e.g. trim those blocks, so you really want to have it done
> _before_ the actual link removal.  And if the file contents is that sensitive,
> you'd better extend the same protection to all operations that free its
> blocks, including truncate(), fallocate() hole-punching, whatever.  What's
> more, if you divorce that from link removal, you probably don't want it as
> in-core-only flag - have it stored in inode, if fs supports that.
>
> Alternatively, you might want to represent it as xattr - as much as I hate
> those, it might turn out to be the best fit in this case, if we end up
> with several variants for freed blocks disposal.  Not sure...
>
> But whichever way we represent that state, IMO
> 	a) operation should be similar to chmod/chattr/setfattr - modifying
> inode metadata.
> 	b) it should affect _all_ operations freeing blocks of that file
> from that point on
> 	c) it should be able to fail, telling you that you can't do that for
> this backing store.

My intention to use unlinkat() or unlinkat_s() was the following:

- It can be supported by most filesystems (see my fat patch)

- It doesn't really make any promises it can't, like deleting leftovers 
of an already modified file. That's where a much more complicated 
solution like the 's' attribute would appropriate. It just should try to 
wipe the current contents of a file.

The second reason was also the reason why I've crafted the subject of 
the RFC very carefully: "Offer a way for userspace to request real 
deletion of files".

I did that to avoid the nitpickers. It doesn't say how the request is or 
has to be handled. I was aware of all the problems which arise if one 
tries to fullfill what the 's' flag promises. The final result of trying 
to get a 100 percent solution is just what we have now: nothing at all.

It wasn't the first time I've posted a patch to LKML, I know that 
maintainers like to request high towers from ordinary people and 
therefor very often nice dog houses were refused. There might be a 
legitimate reason to request a high tower from a big company, but that's 
something totally different.

And I refuse to try to understand why maintainers request high towers. ;)

And because hope never dies, I was again silly enough to post a simple 
patch. ;)

Regards,

Alexander Hpller