linux-kernel - Re: [PATCH 29/38] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #10]

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Date:   Fri, 27 Jul 2018 23:27:34 +0100
From:   David Howells <dhowells@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 29/38] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #10]

Andy Lutomirski <luto@...capital.net> wrote:

> > [*] Not without a 6-arg syscall or some other way of passing it.
> 
> Are there still architectures that have problems with 6-arg syscalls?

As I understand it, 6-arg syscalls are frowned upon.

> I suppose that, as long as there is never a case where fsconfig_set_path and
> fsconfig_set_fd both succeed, then it's not a big deal.

fsconfig_set_path/path_empty requires the 'value' argument to point to a
string, possibly "", and fsconfig_set_fd requires it to be NULL.

I can't stop you from doing:

	fd = open("/some/path", O_PATH);
	fsconfig(fsfd, fsconfig_set_fd, "fd", NULL, fd);

or:

	fd = open("/dev/sda6", O_RDWR);
	fsconfig(fsfd, fsconfig_set_path_empty, "foo", "", fd);

The first should fail because I'm using fget() not fget_raw() and the
second will pass the string and fd number to the filesystem, which will
presumably then call fs_lookup_param() to invoke pathwalk upon it - which will
likely also fail.

David