final decision about *at syscalls

[Robert Watson]

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