Re: [PATCH -V18 04/13] vfs: Allow handle based open on symlinks

[Al Viro]

On Fri, Aug 20, 2010 at 07:53:03PM +1000, Neil Brown wrote:
> On Fri, 20 Aug 2010 04:30:57 -0400
> Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> 
> > Suddenly getting an file pointer for a symlink which could never happen
> > before is a really bad idea.  Just add a proper readlink_by_handle
> > system call, similar to what's done in the XFS interface.
> 
> Why is that?
> With futexes we suddenly get a file descriptor for something we could never
> get a file descriptor on before and that doesn't seem to be a problem.
> 
> Why should symlinks be special as the only thing that you cannot have a file
> descriptor for?  Uniformity of interface is a very valuable property.

You are welcome to review the codepaths around pathname resolution for
assumptions of presense of ->follow_link() and friends; there _are_
subtle cases and dumping your "opened symlinks" in there is far from
a trivial change.  Note that it affects more than just the starting
points of lookups; /proc/*/fd/* stuff is also involved.

BTW, speaking of NULL pathname, linkat() variant that allows creating a link
to an opened file is also a very dubious thing; at the very least, you get
non-trivial security implications, since now a process that got an opened
descriptor passed to it by somebody else may create hardlinks to the sucker.
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