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

David Howells
2018-07-27T21:51:45Z

Andy Lutomirski <luto@amacapital.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.

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);

which indicates that you are actually providing a path.

David