Date: Wed, 16 Apr 2008 18:52:12 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Alfred Perlstein <alfred@freebsd.org> Cc: kib@freebsd.org, Roman Divacky <rdivacky@freebsd.org>, Pawel Jakub Dawidek <pjd@FreeBSD.org>, freebsd-arch@freebsd.org Subject: Re: final decision about *at syscalls Message-ID: <20080416184522.F1046@fledge.watson.org> In-Reply-To: <20080416170341.GN95731@elvis.mu.org> References: <20071218092222.GA9695@freebsd.org> <200712201138.56423.jhb@freebsd.org> <20080412112019.GI45299@garage.freebsd.pl> <200804161014.41025.jhb@freebsd.org> <20080416165612.GA31094@garage.freebsd.pl> <20080416170341.GN95731@elvis.mu.org>
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On Wed, 16 Apr 2008, Alfred Perlstein wrote:
>> File descriptor proposal works like this:
>>
>> userland
>> openat(fd, path)
>> kernel
>> NDINIT_AT(&vp, path, fd);
>> /* operate on vp */
>>
>> Vnode proposal works this way:
>>
>> userland
>> openat(fd, path)
>> kernel
>> dvp = file_descriptor_to_vnode(fd);
>> NDINIT_AT(&vp, path, dvp);
>> /* operate on vp */
>
> My first impression is that passing fp to vp code is a layering
> violation and bad news. I need to think about it more.
NDINIT() is already aware of the file descriptor array because it uses that to
get the current working and root directories. And what the *at() system calls
are effectively doing is substituting another directory for the current
working directory. The exact expression of all this doesn't matter all that
much to me, but I think evaluating the file descriptor array for directory
stuff all in one place, rather than spread over the caller and NDINIT(), is
cleaner and avoids a lot of code everywhere. Nothing says you can't have:
void
NDINIT(struct nameidata *ndp, u_long op, u_long flags,
enum uio_seg segflg, const char *namep, struct thread *td);
void
NDINIT_AT(struct nameidata *ndp, u_long op, u_long flags,
enum uio_seg segflg, const char *namep, int fd, struct thread *td);
NDINIT_DVP(struct nameidata *ndp, u_long op, u_long flags,
enum uio_seg segflg, const char *namep, struct vnode *vp,
struct thread *td);
However, I think I wouldn't want NDINIT_AT() to be a wrapper for NDINIT_DVP(),
because I'd like all that fdp following to occur together.
Robert N M Watson
Computer Laboratory
University of Cambridge
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