Re: [PATCHv4 RESEND 0/3] syscalls,x86: Add execveat() system call

["H. Peter Anvin" <hpa@zytor.com>]

On 10/19/2014 01:20 PM, Al Viro wrote:
> On Fri, Oct 17, 2014 at 02:45:03PM -0700, Andy Lutomirski wrote:
> 
>> For example, I want to be able to reliably do something like nsenter
>> --namespace-flags-here toybox sh.  Toybox's shell is unusual in that
>> it is more or less fully functional, so this should Just Work (tm),
>> except that the toybox binary might not exist in the namespace being
>> entered.  If execveat were available, I could rig nsenter or a similar
>> tool to open it with O_CLOEXEC, enter the namespace, and then call
>> execveat.
> 
> The question I hadn't seen really answered through all of that was how to
> deal with #!...  "Just use d_path()" isn't particulary appealing - if that
> file has a pathname reachable for you, you could bloody well use _that_
> from the very beginning.
> 
> Frankly, I wonder if it would make sense to provide something like
> dupfs.  We can't mount it by default on /dev/fd (more's the pity), but
> it might be a good thing to have.
> 
> What it is, for those who are not familiar with Plan 9: a filesystem with
> one directory and a bunch of files in it.  Directory contents depends on
> who's looking; for each opened descriptor in your descriptor table, you'll
> see two files there.  One series is 0, 1, ... - opening one of those gives
> dup().  IOW, it's *not* giving you a new struct file; it gives you a new
> reference to existing one, complete with sharing IO position, etc.  Another
> is 0ctl, 1ctl, ... - those are read-only and reading from them gives pretty
> much a combination of our /proc/self/fdinfo/n with readlink of /proc/self/fd/n.
> 
> It's actually a better match for what one would expect at /dev/fd than what
> we do.  Example:
> 

Yes, it is really unfortunate that /proc/self/fd/* had the wrong
semantics for the start, due to then-existing implementation issues.

	-hpa