Messages in this thread | | | Date | Thu, 21 Jul 2022 12:47:06 -0700 | From | Luis Chamberlain <> | Subject | Re: [RFC] proc: fix create timestamp of files in proc |
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On Thu, Jul 21, 2022 at 12:45:42PM -0500, Eric W. Biederman wrote: > Luis Chamberlain <mcgrof@kernel.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
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