netdev - [PATCH bpf-next v2 3/7] bpf: add comments to interpret bpf

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Message-ID: <20200513180218.2949517-1-yhs@fb.com>
Date:   Wed, 13 May 2020 11:02:18 -0700
From:   Yonghong Song <yhs@...com>
To:     Andrii Nakryiko <andriin@...com>, <bpf@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>, <netdev@...r.kernel.org>
CC:     Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>, <kernel-team@...com>
Subject: [PATCH bpf-next v2 3/7] bpf: add comments to interpret bpf_prog return values

Add a short comment in bpf_iter_run_prog() function to
explain how bpf_prog return value is converted to
seq_ops->show() return value:
  bpf_prog return           seq_ops()->show() return
     0                         0
     1                         -EAGAIN

When show() return value is -EAGAIN, the current
bpf_seq_read() will end. If the current seq_file buffer
is empty, -EAGAIN will return to user space. Otherwise,
the buffer will be copied to user space.
In both cases, the next bpf_seq_read() call will
try to show the same object which returned -EAGAIN
previously.

Acked-by: Andrii Nakryiko <andriin@...com>
Signed-off-by: Yonghong Song <yhs@...com>
---
 kernel/bpf/bpf_iter.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/kernel/bpf/bpf_iter.c b/kernel/bpf/bpf_iter.c
index 30efd15cd4a0..0a45a6cdfabd 100644
--- a/kernel/bpf/bpf_iter.c
+++ b/kernel/bpf/bpf_iter.c
@@ -526,5 +526,11 @@ int bpf_iter_run_prog(struct bpf_prog *prog, void *ctx)
 	migrate_enable();
 	rcu_read_unlock();
 
+	/* bpf program can only return 0 or 1:
+	 *  0 : okay
+	 *  1 : retry the same object
+	 * The bpf_iter_run_prog() return value
+	 * will be seq_ops->show() return value.
+	 */
 	return ret == 0 ? 0 : -EAGAIN;
 }
-- 
2.24.1