linux-kernel - Re: [PATCH][RFC] open_tree(2) (was Re: [PATCH 30/32] vfs: Allow cloning of a mount tree with open(O_PATH|O_CLONE

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Message-ID: <CAELBmZARJVUMhPC2w-RFic=6r7tcrJYGV4Yt03xxZXsKr+FR1w@mail.gmail.com>
Date:   Mon, 4 Jun 2018 21:38:00 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Matthew Wilcox <willy@...radead.org>,
        David Howells <dhowells@...hat.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-afs@...ts.infradead.org, LKML <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH][RFC] open_tree(2) (was Re: [PATCH 30/32] vfs: Allow
 cloning of a mount tree with open(O_PATH|O_CLONE_MOUNT) [ver #8])

On Mon, Jun 4, 2018 at 7:35 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Mon, Jun 04, 2018 at 10:16:30AM -0700, Matthew Wilcox wrote:
>> On Sun, Jun 03, 2018 at 01:55:37AM +0100, Al Viro wrote:
>> > +SYSCALL_DEFINE3(open_tree, int, dfd, const char *, filename, unsigned, flags)
>> > +{
>> > +   struct file *file;
>> > +   struct path path;
>> > +   int lookup_flags = LOOKUP_AUTOMOUNT | LOOKUP_FOLLOW;
>> > +   bool detached = flags & OPEN_TREE_CLONE;
>> > +   int error;
>> > +   int fd;
>> > +
>> > +   BUILD_BUG_ON(OPEN_TREE_CLOEXEC != O_CLOEXEC);
>>
>> Why do we need OPEN_TREE_CLOEXEC?  Wouldn't we be better off just making
>> the fd returned by open_tree implicitly close-on-exec?  I can think of
>> no good reason for these file descriptors to be inherited across exec()
>
> How are they different from any file descriptor?  It's not as if it was
> something usable only for mounting stuff - again, you can use them
> with any ...at() syscalls.

Defaulting to close on exec helps keep out clutter from the API.

Is there a disadvantage to needing an explicit fcntl(F_SETFD) call to
disable close on exec?

Thanks,
Miklos