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

[Andy Lutomirski]

On Fri, Jul 27, 2018 at 2:51 PM, David Howells <dhowells@xxxxxxxxxx> wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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.