On Tue, May 12, 2020 at 8:54 AM Yonghong Song <yhs@xxxxxx> wrote:
>
> In seq_read() implementation, a positive integer return value
> of seq_ops->show() indicates that the current object seq_file
> buffer is discarded and next object should be checked.
> bpf_seq_read() implemented in a similar way if show()
> returns a positive integer value.
>
> But for bpf_seq_read(), show() didn't return positive integer for
> all currently supported targets. Let us add a WARN_ONCE for
> such cases so we can get an alert when things are changed.
>
> Suggested-by: Alexei Starovoitov <ast@xxxxxxxxxx>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
> kernel/bpf/bpf_iter.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
> index 0a45a6cdfabd..b0c8b3bdf3b0 100644
> --- a/kernel/bpf/bpf_iter.c
> +++ b/kernel/bpf/bpf_iter.c
> @@ -120,6 +120,7 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
>
> err = seq->op->show(seq, p);
> if (err > 0) {
> + WARN_ONCE(1, "seq_ops->show() returns %d\n", err);
This makes it look like it's a bug or non-safe, honestly. I'd drop the
warning altogether, but if not, probably leaving a comment explaining
why we added WARN_ONCE here and that it's ok to remove it would be
good.
> /* object is skipped, decrease seq_num, so next
> * valid object can reuse the same seq_num.
> */
> @@ -156,6 +157,7 @@ static ssize_t bpf_seq_read(struct file *file, char __user *buf, size_t size,
>
> err = seq->op->show(seq, p);
> if (err > 0) {
> + WARN_ONCE(1, "seq_ops->show() returns %d\n", err);
> bpf_iter_dec_seq_num(seq);
> seq->count = offs;
> } else if (err < 0 || seq_has_overflowed(seq)) {
> --
> 2.24.1
>
