getcwd() and friends.

[Brandon S. Allbery]

As quoted from <12625 at swan.ulowell.edu> by arosen at hawk.ulowell.edu (MFHorn):
+---------------
| From article <811 at mtxinu.UUCP>, by ed at mtxinu.COM (Ed Gould):
| >  >It is possible to be handed a file descriptor
| >  >which you would not be able to have opened because some component of
| >  >the search path denies access now, but at some point in the past did
| >  >allow access.
| > 
| > Worse than that, the permission required to open a directory is "r"
| > (since one may not open a directory for writing), whereas the
| > permission required to change to one is "x".  Hence, Unix protection
| > would be completely violated by the existance of fchdir().
| 
| Why can't the kernel check access permissions *at the time* of the
| fchdir() call just like it does for chdir() and open()?
+---------------

Because it can only check access to the specific inode -- *not* to the path
to the inode.  What happens if you have:

	chdir("/usr/spool/uucppublic");
	fd = open(".", 0);
	chdir("/tmp");
	...do something...
	...root makes /usr/spool mode 0700 owner bin...
	fchdir(fd);	<-- succeeds

BUT, under the present system:

	chdir("/usr/spool/uucppublic");
	chdir("/tmp");
	...do something...
	...root changes permissions as above...
	chdir("/usr/spool/uucppublic");	<-- FAILS!

There is a distinction.  The permissions on a file are dependent on the
permissions of each component of the path; which means that two (hard) links
to a file may have different *effective* permissions, if (say) one link is
in a directory which is mode 0700 owner root and the other is in /tmp (mode
0777, or maybe 2777 or 3777 on more recent systems), *regardless* of the
permissions on the file.  (Symbolic links are another matter; a symbolic
link contains a path, the permissions of whose components must be figured
into it.  Interestingly, a symlink itself has no relevant permissions; every
one I've seen lstat()'s as mode 0000....)

++Brandon
-- 
Brandon S. Allbery, moderator of comp.sources.misc	     allbery at ncoast.org
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