Now... go back to my original email:
%pavel () toy:/tmp/my_priv$ chmod 700 .
%# relax file permissions, directory is private, so this is safe
%# check link count on unwritable_file. We would not want someone
%# to have a hard link to work around our permissions, would we?
%pavel () toy:/tmp/my_priv$ chmod 666 unwritable_file
Yes, you are right, open file descriptor acts as a kind of hardlink
here. Except that
a) this kind of hardlink does not exist when /proc is mounted (and on
b) unlike other hardlinks, you can't see it on the link count
(and c) writing to file descriptor opened read-only is bad).
Plus, you may run traditional unix/POSIX application, expecting
directory access controls to prevent the write. (Or can you see a way
to write to that file when /proc is unmounted?)
Directory permissions control an access just to the directory
itself, not to the files in it, so your pretensions are in fact
Demonstrate how to get access to the file with /proc unmounted and you
have a point. Demonstrate how to get access on anything else then
Linux and you have a point. Otherwise there's a security hole.