Re: [Libguestfs] [PATCH nbdkit 2/2] file: Allow a file descriptor to be passed to the plugin
On Wed, Aug 17, 2022 at 10:56:58PM +0100, Richard W.M. Jones wrote:
On Wed, Aug 17, 2022 at 10:37:00PM +0100, Richard W.M. Jones wrote:
> Is that actually possible? “fcntl (fd, F_GETFL) & O_WRONLY”
> should do it?
So the answer is no as it's a kind of tri-state.
More like 5-state (by POSIX): O_RDONLY, O_WRONLY, O_RDWR, O_SEARCH,
O_EXEC (although O_SEARCH and O_EXEC can be the same bit, because
their uses are mutually exclusive). And in reality, since Linux burns
3 bits on it, there can be other states as well (O_PATH, for example).
Some systems have O_RDONLY==1, O_WRONLY==2, O_RDWR==3 (then an obvious
extension for O_PATH would be 0); but most systems have O_RDONLY==0,
O_WRONLY==1, O_RDWR==2, which makes bit-wise testing impossible.
I think this should work (untested)?
r = fcntl (fd, F_GETFL);
if (r == -1) ...
r &= O_ACCMODE;
if (r == O_RDONLY)
h->can_write = false;
Yeah, that's one approach. Another might be as Lazslo suggested:
switch (r & O_ACCMODE) {
case O_RDWR: break;
case O_RDONLY: h->can_write = false; break;
default: //error about unsupported mode
}
There's also the case where r == O_WRONLY which the plugin (and NBD)
cannot deal with. Not sure what to do about that - error?
Or allow it, but with the caveat that every NBD_CMD_READ will fail.
The only reason to special case h->can_write=false for O_RDONLY is
because then we don't advertise it to the client; to save them from
getting failures on NBD_CMD_WRITE - but that's because it is an easy
thing to advertise. Advertising that NBD_CMD_READ will fail is not
easy (and less likely to happen in practice), so failing to serve the
file is just as viable as serving it and letting every NBD_CMD_READ
fail.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org