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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 4 Jan 2024 11:35:37 -0800
Message-ID: <CAHk-=wgxhmMcVGvyxTxvjeBaenOmG8t_Erahj16-68whbvh-Ug@mail.gmail.com>
Subject: Re: [PATCH] tracefs/eventfs: Use root and instance inodes as default ownership
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>, LKML <linux-kernel@vger.kernel.org>, 
	Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>, Masami Hiramatsu <mhiramat@kernel.org>, 
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>, Christian Brauner <brauner@kernel.org>, 
	linux-fsdevel@vger.kernel.org, 
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org
Content-Type: text/plain; charset="UTF-8"

On Thu, 4 Jan 2024 at 11:14, Steven Rostedt <rostedt@goodmis.org> wrote:
>
> "file descriptor" - is just what maps to a specific inode.

Nope. Technically and traditionally, file descriptor is just the
integer index that is used to look up a 'struct file *'.

Except in the kernel, we really just tend to use that term (well, I
do) for the 'struct file *' itself, since the integer 'fd' is usually
not really relevant except at the system call interface.

Which is *NOT* the inode, because the 'struct file' has other things
in it (the file position, the permissions that were used at open time
etc, close-on-exec state etc etc).

> "file description" - is how the file is accessed (position in the file and
>                         flags associated to how it was opened)

That's a horrible term that shouldn't be used at all. Apparently some
people use it for what is our 'struct file *", also known as a "file
table entry".  Avoid it.

If anything, just use "fd" for the integer representation, and "file"
for the pointer to a 'struct file".

But most of the time the two are conceptually interchangeable, in that
an 'fd' just translates directly to a 'struct file *'.

Note that while there's that conceptual direct translation, there's
also very much a "time of use" issue, in that a "fd -> file"
translation happens at one particular time and in one particular user
context, and then it's *done* (so closing and possibly re-using the fd
after it's been looked up does not actually affect an existing 'struct
file *').

And while 'fd -> file' lookup is quick and common, the other way
doesn't exist, because multiple 'fd's can map to one 'struct file *'
thanks to dup() (and 'fork()', since a 'fd -> file' translation always
happens within the context of a particular user space, an 'fd' in one
process is obviously not the same as an 'fd' in another one).

               Linus