Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes: > On Thu, 4 Jun 2020 10:33:41 -0700 > Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote: > >> On Thu, Jun 04, 2020 at 10:40:06AM -0600, David Ahern wrote: >> > On 6/4/20 9:48 AM, Jesper Dangaard Brouer wrote: >> > > I will NOT send a patch that expose this in uapi/bpf.h. As I explained >> > > before, this caused the issues for my userspace application, that >> > > automatically picked-up struct bpf_devmap_val, and started to fail >> > > (with no code changes), because it needed minus-1 as input. I fear >> > > that this will cause more work for me later, when I have to helpout and >> > > support end-users on e.g. xdp-newbies list, as it will not be obvious >> > > to end-users why their programs map-insert start to fail. I have given >> > > up, so I will not NACK anyone sending such a patch. >> >> Jesper, >> >> you gave wrong direction to David during development of the patches and >> now the devmap uapi is suffering the consequences. >> >> > > >> > > Why is it we need to support file-descriptor zero as a valid >> > > file-descriptor for a bpf-prog? >> > >> > That was a nice property of using the id instead of fd. And the init to >> > -1 is not unique to this; adopters of the bpf_set_link_xdp_fd_opts for >> > example have to do the same. >> >> I think it's better to adopt "fd==0 -> invalid" approach. >> It won't be unique here. We're already using it in other places in bpf syscall. >> I agree with Jesper that requiring -1 init of 2nd field is quite ugly >> and inconvenient. > > Great. If we can remove this requirement of -1 init (and let zero mean > feature isn't used), then I'm all for exposing expose in uapi/bpf.h. If we're going to officially deprecate fd 0 as a valid BPF fd, we should at least make sure users don't end up with such an fd after opening a BPF object. Not sure how the fd number assignment works, but could we make sure that the kernel never returns fd 0 for a BPF program/map? Alternatively, we could add a check in libbpf and either reject the call, or just call dup() before passing the fd to the kernel. Right now it's quite trivial to get a BPF program ref with fd0 - all you have to do is open a BPF program is the first thing you do after closing stdin (like a daemon might). I'd really rather not have to help anyone debug that... -Toke
