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

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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$

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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

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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

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.

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      Right... But why does '-r' still says it's readable?

      -- zigdon

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        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.
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