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Message-ID: <CALCETrWDGa+ut=xsoki+XFKzdD7KruJJicfH1tKdBtywLuk7iQ@mail.gmail.com>
Date: Fri, 27 Jul 2018 14:57:51 -0700
From: Andy Lutomirski <luto@...capital.net>
To: David Howells <dhowells@...hat.com>
Cc: 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]
On Fri, Jul 27, 2018 at 2:51 PM, David Howells <dhowells@...hat.com> wrote:
> Andy Lutomirski <luto@...capital.net> wrote:
>
>> Unless I'm rather confused, you have two or possibly three ways to
>> pass in an open fd. Can you clarify what the difference is and/or
>> remove all but one of them?
>
> No, they're not equivalent.
>
>> > (*) fsconfig_set_path: A non-empty path is specified. The parameter must
>> > be expecting a path object. value points to a NUL-terminated string
>> > that is the path and aux is a file descriptor at which to start a
>> > relative lookup or AT_FDCWD.
>
> So, an example:
>
> fsconfig(fd, fsconfig_set_path, "source", "/dev/sda1", AT_FDCWD);
>
> I don't want to require that the caller open /dev/sda1 and pass in an fd as
> that might prevent the filesystem from "holding" it exclusively.
>
>> > (*) fsconfig_set_path_empty: As fsconfig_set_path, but with AT_EMPTY_PATH
>> > implied.
>
> You can't do:
>
> fsconfig(fd, fsconfig_set_path, "source", "", dir_fd);
>
> because AT_EMPTY_PATH cannot be specified directly[*]. What you do instead is:
>
> fsconfig(fd, fsconfig_set_path_empty, "source", "", dir_fd);
>
> [*] Not without a 6-arg syscall or some other way of passing it.
Are there still architectures that have problems with 6-arg syscalls?
>
> I *could* require that the caller must call open(O_PATH) or openat(O_PATH)
> before calling fsconfig() - so you don't pass a string, but only a path-fd.
>
>> > (*) fsconfig_set_fd: An open file descriptor is specified. value must
>> > be NULL and aux indicates the file descriptor.
>
> See fd=%u on fuse. I think it's cleaner to do:
>
> fsconfig(fd, fsconfig_set_fd, "source", NULL, control_fd);
>
> saying explicitly that there's an open file to be passed rather than:
>
> fsconfig(fd, fsconfig_set_path, "source", NULL, control_fd);
Hmm. That should probably be clearly documented. 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.
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