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Subject: Re: [PATCH v6 09/11] fs/pipe.c: export create_pipe_files() and
 replace_fd()
From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Christoph Hellwig <hch@infradead.org>
Cc: platform-driver-x86@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Alexander Viro <viro@zeniv.linux.org.uk>,
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Date: Thu, 30 Nov 2017 10:38:30 -0800
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On Thu, 2017-11-30 at 18:43 +0200, Jarkko Sakkinen wrote:
> On Wed, Nov 29, 2017 at 03:13:57PM -0800, Christoph Hellwig wrote:
> > 
> > On Tue, Nov 28, 2017 at 11:57:53PM +0200, Jarkko Sakkinen wrote:
> > > 
> > > > 
> > > > Yes.  You still shall not play nasty games with file
> > > > descriptors.
> > > 
> > > I need to put something to file descriptors in order to have a IO
> > > channels for the launch enclave hosting process.
> > 
> > Just do it like any other program - open it from your userspace
> > program using open() and related syscalls.
> 
> In this case it would not work as the launch enclave is still part of
> the kernel and it would create a dependency how the user space
> defines paths. If using pipe specifically is an issue, I could easily
> use shmem file as a mean for communiation.

Can't you simply use 

sys_pipe2()
sys_close()
sys_dup2()

To achieve the same effect as replace_fd()/create_pipe_files()?

The point Christoph is making is that you can call sys_ interfaces from
within the kernel (carefully) and have them operate like direct
invocations.  Look at main.c:kernel_init_freeable() it's doing
something similar to what you want, except with the console, not a pipe
and it begins with the file table empty.

James