Re: Potential regression after fsnotify_nameremove() rework in 5.3

[Thorsten Leemhuis <regressions@leemhuis.info>]

[TLDR: I'm adding this regression to regzbot, the Linux kernel
regression tracking bot; most text you find below is compiled from a few
templates paragraphs some of you might have seen already.]

Hi, this is your Linux kernel regression tracker speaking.

Adding the regression mailing list to the list of recipients, as it
should be in the loop for all regressions, as explained here:
https://www.kernel.org/doc/html/latest/admin-guide/reporting-issues.html

On 15.01.22 04:11, Ivan Delalande wrote:
> Hi,
> 
> Sorry to bring this up so late but we might have found a regression
> introduced by your "Sort out fsnotify_nameremove() mess" patch series
> merged in 5.3 (116b9731ad76..7377f5bec133), and that can still be
> reproduced on v5.16.
> 
> Some of our processes use inotify to watch for IN_DELETE events (for
> files on tmpfs mostly), and relied on the fact that once such events are
> received, the files they refer to have actually been unlinked and can't
> be open/read. So if and once open() succeeds then it is a new version of
> the file that has been recreated with new content.
> 
> This was true and working reliably before 5.3, but changed after
> 49246466a989 ("fsnotify: move fsnotify_nameremove() hook out of
> d_delete()") specifically. There is now a time window where a process
> receiving one of those IN_DELETE events may still be able to open the
> file and read its old content before it's really unlinked from the FS.
> 
> I'm not very familiar with the VFS and fsnotify internals, would you
> consider this a regression, or was there never any intentional guarantee
> for that behavior and it's best we work around this change in userspace?

Thanks for the report.

To be sure this issue doesn't fall through the cracks unnoticed, I'm
adding it to regzbot, my Linux kernel regression tracking bot:

#regzbot ^introduced 116b9731ad76..7377f5bec133
#regzbot title fsnotify: regression due to the fsnotify_nameremove()
rework in 5.3
#regzbot ignore-activity

Reminder: when fixing the issue, please add a 'Link:' tag with the URL
to the report (the parent of this mail) using the kernel.org redirector,
as explained in 'Documentation/process/submitting-patches.rst'. Regzbot
then will automatically mark the regression as resolved once the fix
lands in the appropriate tree. For more details about regzbot see footer.

Sending this to everyone that got the initial report, to make all aware
of the tracking. I also hope that messages like this motivate people to
directly get at least the regression mailing list and ideally even
regzbot involved when dealing with regressions, as messages like this
wouldn't be needed then.

Don't worry, I'll send further messages wrt to this regression just to
the lists (with a tag in the subject so people can filter them away), as
long as they are intended just for regzbot. With a bit of luck no such
messages will be needed anyway.

Ciao, Thorsten (wearing his 'Linux kernel regression tracker' hat)

P.S.: As a Linux kernel regression tracker I'm getting a lot of reports
on my table. I can only look briefly into most of them. Unfortunately
therefore I sometimes will get things wrong or miss something important.
I hope that's not the case here; if you think it is, don't hesitate to
tell me about it in a public reply, that's in everyone's interest.

BTW, I have no personal interest in this issue, which is tracked using
regzbot, my Linux kernel regression tracking bot
(https://linux-regtracking.leemhuis.info/regzbot/). I'm only posting
this mail to get things rolling again and hence don't need to be CC on
all further activities wrt to this regression.

---
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https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md
https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md

The last two documents will explain how you can interact with regzbot
yourself if your want to.

Hint for reporters: when reporting a regression it's in your interest to
tell #regzbot about it in the report, as that will ensure the regression
gets on the radar of regzbot and the regression tracker. That's in your
interest, as they will make sure the report won't fall through the
cracks unnoticed.

Hint for developers: you normally don't need to care about regzbot once
it's involved. Fix the issue as you normally would, just remember to
include a 'Link:' tag to the report in the commit message, as explained
in Documentation/process/submitting-patches.rst
That aspect was recently was made more explicit in commit 1f57bd42b77c:
https://git.kernel.org/linus/1f57bd42b77c