Re: [GIT PULL] overlayfs update for 4.9

From: Amir Goldstein
Date: Fri Oct 14 2016 - 06:11:33 EST

On Fri, Oct 14, 2016 at 9:48 AM, Amir Goldstein <amir73il@xxxxxxxxx> wrote:
> On Fri, Oct 14, 2016 at 7:03 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>> On Thu, Oct 13, 2016 at 7:37 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>> Please pull from:
>> No.
>> Or rather, I pulled and then immediately unpulled. When I look at the
>> diff, I saw an obvious bug in the very first chunk. I'm not going to
>> pull something that is this obviously buggy and untested.
>> Your change to fs/ioctl.c to add a -EXDEV error case very clearly
>> leaks a reference to 'src_file'.
> On the charge of writing obviously buggy code I plead guilty :-/
> On the charge of not testing my code I plead not guilty.
> I exercised the FICLONERANGE intensively with the xfstests clone test group
> and never experienced any problem and any warning running with all
> relevant kernel debugging enabled.
> So how come this leak went unnoticed?
> Because fdget (__fget_light) does not take a reference if the fd is not shared.
> So what can we do to catch silly mistakes like this one earlier and
> without relying
> on Linus's spidy senses?
> Writing xfstests to test all fd interfaces with cloned fds? Is this a
> scalable solution?

Well, I added an idle loop thread to xfs_io and sure enough it catches the leak
in test generic/157 (Try cross-device reflink).
I will post patches to fstests.


> Or maybe it's best to add a trivial CONFIG_DEBUG_FDGET that will always
> skip the no refcount optimization?
> To me the second option seems preferred from engineering pov
> I can post this simple patch if you guys agree to this solution.
>> It's too late in the merge window for this to be fixed up any more.
>> This has been a painful enough merge window for me that I'm not going
>> to play around with obviously buggy pull requests.
> It has been a rocky merge window and I apologize for adding this last straw
> and pooping the party for the entire overlay pull request.
> In the hope that this may help to sweeten your verdict -
> I just got my hands on a brand new testing machine, which is dedicated
> to stress testing file systems on the latest rc.
> Amir.
>> Linus
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