linux-kernel - Re: [RFC] proc: fix create timestamp of files in proc

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Date:   Thu, 21 Jul 2022 12:47:06 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Zhang Yuchen <zhangyuchen.lcr@...edance.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Deepa Dinamani <deepa.kernel@...il.com>,
        Christoph Hellwig <hch@....de>,
        Muchun Song <songmuchun@...edance.com>,
        linux-api@...r.kernel.org, keescook@...omium.org,
        yzaikin@...gle.com, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC] proc: fix create timestamp of files in proc

On Thu, Jul 21, 2022 at 12:45:42PM -0500, Eric W. Biederman wrote:
> Luis Chamberlain <mcgrof@...nel.org> writes:
> 
> > On Thu, Jul 21, 2022 at 04:16:17PM +0800, Zhang Yuchen wrote:
> >> A user has reported a problem that the /proc/{pid} directory
> >> creation timestamp is incorrect.
> >
> > The directory?
> 
> A bit of history that I don't think made it to the git log is that
> procps uses the /proc/<pid> directory, to discover the uid and gid of the
> process.

Oy vei.

In that sense, if *really* the directory for a PID all of a sudden
disappear and reapper with another time stamp, I wonder if its
possible to change the uid/gid.

> I have memories of Albert Cahalan reporting regressions because I
> had tweaked the attributes of proc in ways that I expected no
> one would care about and caused a regression in procps.

Thanks for bringing this little bit of history to light.

> So it is not unreasonable for people to have used proc in surprising
> ways.
> 
> I took a quick read through procps and it looks like procps reads
> /proc/<pid>/stat to get the start_time of the process.

OK so at least for that world of userspace *if* indeed the pid directory
is changing time somehow (the exact way in which this can happen as
reported is yet to be determined) the existing procps would *not* have
been affected.

If *procps* is not being used and someone is trying to re-implement it,
and if indeed it is using the time of the inode, and *if* this really
can change, then we have an explanation to the current situation.

> Which leads us to this quality of implementation issue that the time
> on the inode of a proc directory is the first time that someone read
> the directory and observed the file.  Which does not need to be anything
> at all related to the start time.

Best I think we can do, is document this, and if we *want* to accept
a *new mechanism*, add a kconfig entry so to distinguish this, so to
not break old reliance on prior behaviour.

> I think except for the symlinks and files under /proc/pid/fd and
> /proc/pid/fdinfo there is a very good case for making all of the files
> /proc/pid have a creation time of equal to the creation of the process
> in question.

A new kconfig entry could enable this. But that still would not
prevent userspace from modifying file's creation time and I'm not
sure why we'd want to change things.

> Although the files under /proc/pid/task/ need to have
> a time equal to the creation time of the thread in question.
> 
> Improving the quality of implementation requires caring enough to make
> that change, and right now I don't.

My biggest fear is breaking things.

If we *really* are somehow modifying the timestamp of the directory
inode though, I'd like to understand how that happened.

> At the same time I would say the suggested patch is a bad idea.
> Any application that breaks because we hard set the timestamp on a proc
> file or directory to the beginning of time is automatically counts as a
> regression.

It is why I was seriously confused, why would someone purposely try to
create a regression.

> Since the entire point of the patch is to break applications that are
> doing things wrong, aka cause regressions I don't think the patch
> make sense.

And hence my serious distaste for it.

> So I would vote for understanding what the problem user is doing.  Then
> either proc can be improved to better support users, or we can do
> nothing.
> 
> Except for explaining the history and how people have legitimately used
> implementation details of proc before, I am not really interested.  But
> I do think we can do better.

Thanks for the feedback.

  Luis