Re: [PATCH][RFC] %pd - for printing dentry name

[Nick Piggin]

On Tue, Feb 02, 2010 at 05:00:37AM +0000, Al Viro wrote:
> On Mon, Feb 01, 2010 at 08:22:24PM -0800, Linus Torvalds wrote:
> 
> > > We probably can get away with that, but we'll have to be a lot more careful
> > > with the order of updating these suckers in d_move_locked et.al.
> > 
> > I wouldn't worry about it too much. So what if we get a screwed up name? 
> > If we use "%.*s" to print the name, we know that we won't overstep the old 
> > name even if the NUL termination somehow went away (because we're busy 
> > copying a new, longer, name over it or whatever).
> 
> Actually, I'm not sure.  We can get hit by a switch to inline name with
> length still being that from a long earlier name.  And inline name is
> in the end of struct dentry, so we could end up stepping off the end
> of page.  Note that existing d_alloc() does put NUL in d_iname for a short
> name, but it won't clean the end of array, so overwrite during memcpy()
> can open up a whole lot of PITA.
> 
> And yes, it's theoretical and ought to be hard to hit - the sky isn't falling.
> OTOH, something like rename() vs. close() race as in ocfs2 might make it
> not all that theoretical.
> 
> We probably can get away with being careful with barriers and order of
> ->len vs. ->name updates (and being a bit more careful about cleaning the
> crap in d_alloc()), but it'll take an accurate analysis.  I'd really like
> to hear something from RCU folks, BTW; I still hope that it's one of the
> more or less standard problems and "memory barriers" and "reinventing
> the wheel" in the same sentence is something I'd rather avoid.

I ended up having to use a seqlock to do name comparison without locks
(and without holding references for that matter, just RCU).  However
name comparison is obviously a lot more critical because you can't
ignore races, so you might be able to do something simpler.

But I can't see a good way to do it completely just with RCU and memory
ordering. You could get multiple renames in there, so no matter the
ordering, I think you can get a mismatched len,name ptr tuple.

Could do something really awful by checking ksize or DNAME_INLINE_LEN
of your name pointer. This ensures you don't hit random memory. Would
require more work to avoid leaking kernel memory. Seems really nasty
though.

Seqlock should work nicely, but it requires some additions to core code
so I don't think it is justified unless it comes with other core
improvements (like scalability work).

 
> FWIW, speaking of fun printf extensions, there might be a completely
> different way to deal with all that crap.  %s modification doing kfree().
> I.e. "get char * from argument list, print it as %s would, then kfree() it".
> With something along the lines of
> 	printk("... %<something>...", build_some_string(...));
> as intended use, build_some_string() allocating a string and filling it.
> Might or might not be a good idea, but it's interesting to consider.  And
> yes, of course it's a deadlock if you do that under any kind of a spinlock,
> but that's the damn caller's responsibility - after all, they explicitly
> call a function that does allocation.  The real danger with that is that
> somebody will use it with %s and get a leak from hell...

Might not be a bad idea to have a kstrdup type helper that does the
right locking. But does it warrant a fancy new printk conversion?

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