I'm pretty sure that the mode check done at the beginning of
spec_write() will ensure that the file is opened with write access.
:)
On Wed, 6 Jan 2016, Terry Moore wrote:
Isn't there a security risk with the fd_getfile() approach? This sounds (on
the face of it) similar to the kinds of problems that led tmpnam(3) to be
deprecated? For example, what if the monitoring program deliberately points
the fd at a file that it opened as read-only; will filemon then write to it?
--Terry
-----Original Message-----
From: tech-kern-ow...@netbsd.org [mailto:tech-kern-ow...@netbsd.org] On
Behalf Of Paul Goyette
Sent: Wednesday, January 6, 2016 16:55
To: Taylor R Campbell <campbell+netbsd-tech-k...@mumble.net>
Cc: tech-kern@netbsd.org
Subject: Re: In-kernel process exit hooks?
Another possibility would be to change filemon(4) to do fd_getfile
each it needs to use the file descriptor. This makes it a little
more brittle (fails if you close the descriptor), but would sidestep
the problem.
Hmmm, perhaps. Failure would not be a problem, since we would just
revert to the initial "output file unspecified" conditions.
I think I like this approach. :) I'll give it a try.
This actually works quite well. Please see the attached diffs for your
review.
One possible problem is what happens if the monitoring program closes
the file descriptor, and then re-uses that fd? I've included a check to
compare the original 'struct file *' pointer with the current one, which
will catch "some" instances, but not guaranteed to catch them all. It
could be a bit of a surprise if filemon output shows up in unexpected
places. :)
Because of this potential for surprising the user, I think I'm still
leaning to my earlier proposal of extending exithook processing. But
given the limited number of use-cases for filemon, I could live with
making the fd_getfile()-only-when-you-need-it change instead.
+------------------+--------------------------+------------------------+
| Paul Goyette | PGP Key fingerprint: | E-mail addresses: |
| (Retired) | FA29 0E3B 35AF E8AE 6651 | paul at whooppee.com |
| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd.org |
+------------------+--------------------------+------------------------+
+------------------+--------------------------+------------------------+
| Paul Goyette | PGP Key fingerprint: | E-mail addresses: |
| (Retired) | FA29 0E3B 35AF E8AE 6651 | paul at whooppee.com |
| Kernel Developer | 0786 F758 55DE 53BA 7731 | pgoyette at netbsd.org |
+------------------+--------------------------+------------------------+
