> On Oct 13, 2021, at 12:33 AM, Kumar Kartikeya Dwivedi <memxor@xxxxxxxxx> wrote:
>
> Add a simple wrapper for passing an fd and getting a new one >= 3 if it
> is one of 0, 1, or 2. There are two primary reasons to make this change:
> First, libbpf relies on the assumption a certain BPF fd is never 0 (e.g.
> most recently noticed in [0]). Second, Alexei pointed out in [1] that
> some environments reset stdin, stdout, and stderr if they notice an
> invalid fd at these numbers. To protect against both these cases, switch
> all internal BPF syscall wrappers in libbpf to always return an fd >= 3.
> We only need to modify the syscall wrappers and not other code that
> assumes a valid fd by doing >= 0, to avoid pointless churn, and because
> it is still a valid assumption. The cost paid is two additional syscalls
> if fd is in range [0, 2].
Acked-by: Song Liu <songliubraving@xxxxxx>
With on nitpick below.
[...]
> +static inline int skel_reserve_bad_fds(struct bpf_load_and_run_opts *opts, int *fds)
> +{
> + int fd, err, i;
> +
> + for (i = 0; i < 3; i++) {
> + fd = open("/dev/null", O_RDONLY | O_CLOEXEC);
> + if (fd < 0) {
> + opts->errstr = "failed to reserve fd 0, 1, and 2";
> + err = -errno;
> + return err;
> + }
> + if (fd >= 3)
> + close(fd);
nit: Maybe likely(fd >= 3) and break here?
> + else
> + fds[i] = fd;
> + }
> + return 0;
> +}
> +
> static inline int bpf_load_and_run(struct bpf_load_and_run_opts *opts)
> {
> - int map_fd = -1, prog_fd = -1, key = 0, err;
> + int map_fd = -1, prog_fd = -1, key = 0, err, i;
> + int res_fds[3] = { -1, -1, -1 };
> union bpf_attr attr;
>
> + /* ensures that we don't open fd 0, 1, or 2 from here on out */
> + err = skel_reserve_bad_fds(opts, res_fds);
> + if (err < 0) {
> + errno = -err;
> + goto out;
> + }
> +
> map_fd = bpf_create_map_name(BPF_MAP_TYPE_ARRAY, "__loader.map", 4,
> opts->data_sz, 1, 0);
> if (map_fd < 0) {
> @@ -115,6 +143,10 @@ static inline int bpf_load_and_run(struct bpf_load_and_run_opts *opts)
> }
> err = 0;
> out:
> + for (i = 0; i < 3; i++) {
> + if (res_fds[i] >= 0)
> + close(res_fds[i]);
> + }
> if (map_fd >= 0)
> close(map_fd);
> if (prog_fd >= 0)
> --
> 2.33.0
>
