On Wed, Aug 17, 2022 at 03:57:24PM -0500, Eric Blake wrote:
> On Wed, Aug 17, 2022 at 04:38:11PM +0100, Richard W.M. Jones wrote:
> > This allows you to pass in a file descriptor to the plugin, eg:
> >
> > $ exec 6<>/var/tmp/fedora-36.img
> > $ ./nbdkit file fd=6 --run 'nbdinfo $uri'
> >
> > Note this was previously possible on most platforms using /dev/fd/<N>,
> > but not all platforms that have file descriptors support this
> > (although it is POSIX) and it seems better to make this explicit.
>
> The parenthetical feels odd here. /dev/fd/<N> is not POSIX; what IS a
> POSIX feature is the ability to pass an open file descriptor by
> inheritance.
Will fix.
> > @@ -202,6 +208,17 @@ file_config (const char *key, const char *value)
> > if (!directory)
> > return -1;
> > }
> > + else if (strcmp (key, "fd") == 0) {
> > + if (mode != mode_none) goto wrong_mode;
> > + mode = mode_filedesc;
> > + assert (filedesc == -1);
> > + if (nbdkit_parse_int ("fd", value, &filedesc) == -1)
> > + return -1;
> > + if (filedesc <= 2) {
> > + nbdkit_error ("file descriptor must be >= 3");
>
> Is it worth using STDERR_FILENO instead of open-coding 2? I'll be okay
> if your answer is that this is one place where <unistd.h> has too much
> indirection ;)
Will fix.
> > @@ -408,59 +436,69 @@ file_open (int readonly)
> ...
> > + case mode_filedesc:
> > + h->fd = dup (filedesc);
> > + if (h->fd == -1) {
> > + nbdkit_error ("dup fd=%d: %m", filedesc);
> > + free (h);
> > + return NULL;
> > + }
> > + file = "<file descriptor>"; /* This is needed for error messages. */
> > + break;
>
> Is it worth trying to figure out if the FD has O_RDWR privileges and
> set h->can_write accordingly, to match...
Is that actually possible? “fcntl (fd, F_GETFL) & O_WRONLY”
should do it?
> > default: abort ();
> > }
> >
> > - h = malloc (sizeof *h);
> > - if (h == NULL) {
> > - nbdkit_error ("malloc: %m");
> > - if (dfd != -1)
> > - close (dfd);
> > - return NULL;
> > - }
> > -
> > - flags = O_CLOEXEC|O_NOCTTY;
> > - if (readonly) {
> > - flags |= O_RDONLY;
> > - h->can_write = false;
> > - }
> > - else {
> > - flags |= O_RDWR;
> > - h->can_write = true;
> > - }
> > -
> > - h->fd = openat (dfd, file, flags);
> > - if (h->fd == -1 && !readonly) {
> > - nbdkit_debug ("open O_RDWR failed, falling back to read-only: %s: %m",
> > - file);
> > - flags = (flags & ~O_ACCMODE) | O_RDONLY;
> > - h->fd = openat (dfd, file, flags);
> > - h->can_write = false;
> > - }
>
> ... what we do for normal files?
>
> Overall looks like a nice addition.
>
> ACK series
I also want to add dirfd= similarly for directory FDs (seems better to
be explicit rather than overloading fd= and distinguishing through stat?)
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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