On Tue, Oct 08, 2024 at 07:42:51AM GMT, Jeff Layton wrote:
> On Tue, 2024-10-08 at 13:30 +0200, Christian Brauner wrote:
> > Currently when passing a closed file descriptor to
> > fcntl(fd, F_DUPFD_QUERY, fd_dup) the order matters:
> >
> > fd = open("/dev/null");
> > fd_dup = dup(fd);
> >
> > When we now close one of the file descriptors we get:
> >
> > (1) fcntl(fd, fd_dup) // -EBADF
> > (2) fcntl(fd_dup, fd) // 0 aka not equal
> >
> > depending on which file descriptor is passed first. That's not a huge
> > deal but it gives the api I slightly weird feel. Make it so that the
> > order doesn't matter by requiring that both file descriptors are valid:
> >
> > (1') fcntl(fd, fd_dup) // -EBADF
> > (2') fcntl(fd_dup, fd) // -EBADF
> >
> > Fixes: c62b758bae6a ("fcntl: add F_DUPFD_QUERY fcntl()")
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Reported-by: Lennart Poettering <lennart@xxxxxxxxxxxxxx>
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> > fs/fcntl.c | 3 +++
> > 1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/fcntl.c b/fs/fcntl.c
> > index 22dd9dcce7ec..3d89de31066a 100644
> > --- a/fs/fcntl.c
> > +++ b/fs/fcntl.c
> > @@ -397,6 +397,9 @@ static long f_dupfd_query(int fd, struct file *filp)
> > {
> > CLASS(fd_raw, f)(fd);
> >
> > + if (fd_empty(f))
> > + return -EBADF;
> > +
> > /*
> > * We can do the 'fdput()' immediately, as the only thing that
> > * matters is the pointer value which isn't changed by the fdput.
>
> Consistency is good, so:
>
> Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
>
> ...that said, we should document that -EBADF means that at least one of
> the fd's is bogus, but this API doesn't tell you which ones those are.
> To figure that out, I guess you'd need to do something like issue
> F_GETFD against each and see which ones return -EBADF?
It's actually worse because fcntl() can also give you EBADF if you have
an O_PATH file descriptor and you request an option that won't work on
an O_PATH file descriptor. It's complete nonsense.
So the most reliable way to figure out whether the fd is valid, is to
use a really really old fcntl() like idk F_GETFD and call it. Because
that should always work (ignoring really stupid things such as using
seccomp or an LSM to block F_GETFD) and if you get EBADF it must be
because the file descriptor isn't valid. Obviously that's racy if the
fdtable is shared but I don't think it's a big problem.
So if you get EBADF from F_DUPFD_QUERY and you really really need to
know whether the kernel supports it or any of the two fds was invalid
then yes, you need to follow this up with a F_GETFD. Again, racy but
won't matter most of the time.
Really, we should have returned something like EOPNOTSUPP from fcntl()
for the O_PATH case that would've meant that it's easy to detect new
flags.
