[PATCH bpf-next 4/8] bpf: add WARN_ONCE if bpf_seq_read show() return a positive number

[Yonghong Song]

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 <a...@kernel.org>
Signed-off-by: Yonghong Song <y...@fb.com>
---
 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);
                /* 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