-r broken on root_squash NFS?

zigdon has asked for the wisdom of the Perl Monks concerning the following question:

I'm a little bit confused here, maybe one of you guys can spot what I'm missing. I have some files on an NFS filesystem, that is set to root_squash. The file is owned by my network id (dboger), while I'm logged in locally as dan:

dan@dbdb$ ls -l somefile.png
-r-------- 1 dboger 3813 2008-06-05 16:46 somefile.png
[download]

I'm trying to figure out if a file is readable, so I do something like:

dan@dbdb$ perl -le 'print "lies" if -r shift' somefile.png
dan@dbdb$
[download]

That is correct - the file is owned by a different UID, and is set to 0400. Now, I try the same thing as root:

dan@dbdb$ sudo perl -le 'print "lies" if -r shift' somefile.png
lies
dan@dbdb$ sudo file somefile.png
couldn't open file
somefile.png: regular file, no read permission
[download]

Is perl just assuming that if I'm root I can always read the file, even though the permissions say that's wrong? The perldoc page for -r says:

Also note that, for the superuser on the local filesystems, the "-r", "-R", "-w", and "-W" tests always return 1, and "-x" and "-X" return 1 if any execute bit is set in the mode. Scripts run by the superuser may thus need to do a stat() to determine the actual mode of the file, or temporarily set their effective uid to something else.

But surely NFS doesn't count as a local filesystem? What am I missing?

-- zigdon

Comment on -r broken on root_squash NFS? Select or Download Code

Replies are listed 'Best First'.
Re: -r broken on root_squash NFS? by pc88mxer (Vicar) on Jun 06, 2008 at 21:22 UTC
~~The `root_squash` option means that `root` will be mapped to the user `nobody`. That's why the file is not readable by `root`.~~ Sorry, misunderstood the question. See a better answer below.	[reply] [d/l] [select]
Re^2: -r broken on root_squash NFS? by zigdon (Deacon) on Jun 06, 2008 at 21:23 UTC
Right... But why does '-r' still says it's readable? -- zigdon	[reply]
Re^3: -r broken on root_squash NFS? by pc88mxer (Vicar) on Jun 06, 2008 at 21:37 UTC
You're right. I bet perl is determining readability by calling `access(2)`. From the man page: access() may not work correctly on NFS file systems with UID mapping enabled, because UID mapping is done on the server and hidden from the client, which checks permissions. and `root_squash` is a server option.	[reply] [d/l] [select]
Re^4: -r broken on root_squash NFS? by zigdon (Deacon) on Jun 06, 2008 at 22:18 UTC
Re^5: -r broken on root_squash NFS? by sgifford (Prior) on Jun 06, 2008 at 22:31 UTC


No such thing as a small change
	PerlMonks