On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>
> Since the code assumes in various places that BTF fd for modules is
> never 0, if we end up getting fd as 0, obtain a new fd > 0. Even though
> fd 0 being free for allocation is usually an application error, it is
> still possible that we end up getting fd 0 if the application explicitly
> closes its stdin. Deal with this by getting a new fd using dup and
> closing fd 0.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d286dec73b5f..3e5e460fe63e 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4975,6 +4975,20 @@ static int load_module_btfs(struct bpf_object *obj)
> pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
> return err;
> }
> + /* Make sure module BTF fd is never 0, as kernel depends on it
> + * being > 0 to distinguish between vmlinux and module BTFs,
> + * e.g. for BPF_PSEUDO_BTF_ID ld_imm64 insns (ksyms).
> + */
> + if (!fd) {
> + fd = dup(0);
This is not the only place where we make assumptions that fd > 0 but
technically can get fd == 0. Instead of doing such a check in every
such place, would it be possible to open (cheaply) some FD (/dev/null
or whatever, don't know what's the best file to open), if we detect
that FD == 0 is not allocated? Can we detect that fd 0 is not
allocated?
Doing something like that in bpf_object__open() or bpf_object__load()
would make everything much simpler and we'll have a guarantee that fd
== 0 is not going to be allocated (unless someone accidentally or not
accidentally does close(0), but that's entirely different story).
> + if (fd < 0) {
> + err = -errno;
> + pr_warn("failed to dup BTF object #%d FD 0 to FD > 0: %d\n", id, err);
> + close(0);
> + return err;
> + }
> + close(0);
> + }
>
> len = sizeof(info);
> memset(&info, 0, sizeof(info));
> --
> 2.33.0
>
