Re: [PATCH 24/32] vfs: syscall: Add fsopen() to prepare for superblock creation [ver #9]

On Thu, Jul 12, 2018 at 1:23 PM David Howells [off-list ref] wrote:

It's all very well to say "use file->f_creds".  The problem is this has to be
handed down all the way through the filesystem and down into the block layer
as appropriate to anywhere there's an LSM call, a CAP_* check or a pathwalk -
but there's not currently any way to do that.

.. and the reason is simple: you damn well shouldn't do that.

The unix semantics are that credentials are checked at open time.

If your interface involves checking credentials at write() time, your
interface is garbage shit.

Really.

This is the whole "write() is only for data". If you ever have
credentials mattering at write time, you're doing something wrong.

Really really.

Don't do it.

             Linus