Re: git 2.2.x: Unexpected, overstrict file permissions after "git update-server-info"

From: Jeff King <peff@peff.net>
Date: Jan 6, 2015, 19:37 UTC
Message-ID: <20150106193742.GA28440@peff.net>
In-Reply-To: <xmqqd26sql0v.fsf@gitster.dls.corp.google.com>

On Tue, Jan 06, 2015 at 02:08:16AM -0800, Junio C Hamano wrote:

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> Jeff King <peff@peff.net> writes:
> 
> > Yeah, I didn't consider the mode impact of using mkstemp. That is
> > definitely a regression that should be fixed. Though of course if you
> > really do want 0644, you should set your umask to 0022. :)
> > ...
> > If you haven't set core.sharedrepository, then adjust_shared_perm is a
> > noop. But you shouldn't have to do that. Git should just respect your
> > umask in this case.
> 
> Thanks for a nicely done patch series, but I am not sure if I agree
> with the analysis and its conclusion.
> 
> If adjust_shared_perm is a no-op, how do we ensure that other files
> that need to be served by a dumb HTTP server are readable by it?  Is
> it because we just happen not to use mkstemp() to create them (and
> also is it because the pushers do not have umask 007 or stronger to
> prevent files from being read by the HTTP server user)?

I think there are two things at play here.

One is that we accidentally tightened the permissions on the info/* files, and that is a regression that should be fixed regardless. So the patch series is doing the right thing, even if the commit message is up for debate. And I think you agree with that, from what you've written.

As for "should it work", I would tend to say yes. As long as "work" is "respect your umask". Git has no reason to do anything other than "0666 & umask"[1] when creating new files. The umask is the traditional way to configure the permissions on files you create, and git should follow it, unless it happens to know a particular file is sensitive (and I cannot really think of any that are, aside from a few related to credential storage).

So if you do not have "0004" in your umask, everything git creates should be world-readable, and other users should be able to access it.

Grepping around, there are a few other calls to mkstemp (and not mkstemp_mode). But they are all for true temporary files (which will be read by subprocesses of the current process), and not files which we expect to live on in the repo. So I think we are mostly following that rule already.

There are a couple spots where we use 0600 explicitly, for no good reason (e.g., some BISECT_* files, which I guess might be left in the filesystem for later processes to read).

[1] We actually use 0444 for object and packfile creation, but I think
    that still follows the same line of reasoning.

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> Is our goal here to give the users this statement?
> 
>     For shared repository served by dumb HTTP and written by users
>     who are different from the user that runs the HTTP server, you
>     need to do nothing special.

No, I don't think so. We should follow the umask, and in most cases that will just work for serving by another user (and if it _doesn't_, then perhaps it is because the user with the draconian umask _wanted_ to prevent other people, including the http user, from reading it).

And as you noted, if you want to override that umask, we already have core.sharedrepository.

So maybe my commit message overstated things. And it should just say "we should be respecting the umask, because setting a permissive umask is enough to make dumb http work, and we broke that".

> It feels to me that the old set-up were "working" by accident, not
> by design (I may be mistaken--so correct me if that were the case).

I do not think it was consciously designed as part of git, but rather that general good taste and fitting in with Unix traditions made it work. We follow the umask, and the umask is typically enough to make it work.

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> And if that is the case, I do not think it is a good idea to try to
> hide the broken configuration under the rug longer.  "As long as
> everybody writes world-readable files, you do not have to do
> anything" will break when the next person with 0xx7 umask setting
> pushes, no?

Yes, but it will be the fault of the person with the 0xx7 umask. ;)

-Peff