Date: Sat, 29 Aug 1998 01:22:48 -0500 (CDT) From: Joel Ray Holveck <joelh@gnu.org> To: nbm@rucus.ru.ac.za Cc: luigi@labinfo.iet.unipi.it, chanders@timing.com, freebsd-hackers@FreeBSD.ORG, brhall@timing.com Subject: Re: Help with passing fd on FreeBSD Message-ID: <199808290622.BAA01509@detlev.UUCP> In-Reply-To: <19980828211404.A18096@rucus.ru.ac.za> (message from Neil Blakey-Milner on Fri, 28 Aug 1998 21:14:04 %2B0200) References: <199808281722.LAA10931@count.timing.com> <199808281628.SAA05374@labinfo.iet.unipi.it> <19980828211404.A18096@rucus.ru.ac.za>
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>>>> Can someone give me pointers on passing open file descriptors on
>>>> FreeBSD?
>> still, this looks to me one of the most obscure interfaces in the OS.
>> Does any real application use that ?
> I think this could be used to point children towards files to read
> for further input, or something like that.
You've got it backwards. To send a child process a fd at fork time,
you've just got to dup2 to a known (predetermined or passed at
runtime) fd before forking. Mike mentioned using it to pass fd's (in
his case, network sockets) to a preforked process, but that's a
slightly different case.
> I'm not terribly sure, but the children may then need not be run as
> root to read files not owned by their process id (one assumes the
^^^^^^^ user
> child uses set[gu]id).
Once a process has an fd, it stays, regardless of setuid's. The
uid/gid is checked when a file is opened, but not after. (This also
means that a process can receive a fd passed to it, either in its
creation (via fork) or by SCM_RIGHTS regardless of permissions.)
Emacs 20.4 will use the mechanism in just the way that Stevens'
example 1 mentioned. It can launch a suid root child which can pass
it back an fd that Emacs would not otherwise be able to handle. (It
does this to allow sysadmins to manipulate files as root while logged
in as a mortal, assuming authentication passes.)
> Of course, I'm sure there are better ways of doing these things. I'm not
> sure how apache logs to files owned by other users (apache set[gu]id's its
> children), this may be the way.
It could easily open the files as root (and with O_APPEND) before
forking.
Happy hacking,
joelh
--
Joel Ray Holveck - joelh@gnu.org - http://www.wp.com/piquan
Fourth law of programming:
Anything that can go wrong wi
sendmail: segmentation violation - core dumped
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