Date: Mon, 9 Dec 1996 12:09:56 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: ccsanady@friley216.res.iastate.edu (Chris Csanady) Cc: joerg_wunsch@uriah.heep.sax.de, freebsd-current@freebsd.org Subject: Re: mfs module -- seems to work.. Message-ID: <199612091909.MAA01078@phaeton.artisoft.com> In-Reply-To: <199612091403.IAA00305@friley216.res.iastate.edu> from "Chris Csanady" at Dec 9, 96 08:03:26 am
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> Ok. I did some looking into this, and now I am really stumped. There seems
> to be something quite strange afoot here. :\ I have traced the problem to
> vfsload(). (/usr/src/lib/libc/gen/getvfsent.c:233) It would seem that the
> chdir() call is failing on a bad file descriptor!? Does anyone have any
> ideas why this might happen? How a chdir("/var/tmp") ever fail?
chdir works by doing a lookup, getting a locked vp to a dir back (and
referenced as if it were opened -- see fchdir), and then putting the
vp into the vp area for "current directory" in the proc struct. Then
it closes the vp that was previously there.
A currently directory is, in a very real sense, an open file. This is
why you must gett all processes current directories out of an FS
before it will unmount.
A lookup picks a start point:
For a non-chroot'ed process, this is the vp for /.
For a chroot'ed process, this is the chroot vp from the proc struct
(the current implementation of "namei/lookup" is inconsistent; the
root vp should be set as the chroot vp, and the root vp should not
be special cased for non-chroot'ed processes).
The lookup proceeds a component at a time:
lock start_point vnode
dereference vfs_ops for lookup from the start_point
my_locked_vnode = lookup(start_point, "var")
unlock start_point
start_point = my_locked_vnode
dereference vfs_ops for lookup from the start_point
my_locked_vnode = lookup(start_point, "tmp")
temp_old_dir = current_dir
current_dir = my_locked_vnode
unlock temp_dir
So to answer your question:
It can fail if "var" does not exist in the root vnode namespace when
lookup in the root FS type is called
It can fail if "tmp" does not exist in the "var" vnode namespace when
lookup in the "var" FS type is called
In general, if the FS types change (ie: lookup() traverses a mount point),
you should unmount the mount and look to make sure that the permissions
on the underlying vnode(s) will let you cd into it, or ls it, or whatever.
If not, you will need to chmod them.
Terry Lambert
terry@lambert.org
---
Any opinions in this posting are my own and not those of my present
or previous employers.
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