netdev - [PATCH bpf-next 1/2] bpf: remove incorrect 'verifier bug' warning

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Date:   Wed, 20 Mar 2019 13:58:27 +0100
From:   Paul Chaignon <paul.chaignon@...nge.com>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Cc:     xiao.han@...nge.com, paul.chaignon@...il.com,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>
Subject: [PATCH bpf-next 1/2] bpf: remove incorrect 'verifier bug' warning

The BPF verifier checks the maximum number of call stack frames twice,
first in the main CFG traversal (do_check) and then in a subsequent
traversal (check_max_stack_depth).  If the second check fails, it logs a
'verifier bug' warning and errors out, as the number of call stack frames
should have been verified already.

However, the second check may fail without indicating a verifier bug: if
the excessive function calls reside in dead code, the main CFG traversal
may not visit them; the subsequent traversal visits all instructions,
including dead code.

This case raises the question of how invalid dead code should be treated.
This patch implements the conservative option and rejects such code.

Signed-off-by: Paul Chaignon <paul.chaignon@...nge.com>
Tested-by: Xiao Han <xiao.han@...nge.com>
---
 kernel/bpf/verifier.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 86f9cd5d1c4e..6719bb9b332e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -1888,8 +1888,9 @@ static int check_max_stack_depth(struct bpf_verifier_env *env)
 		}
 		frame++;
 		if (frame >= MAX_CALL_FRAMES) {
-			WARN_ONCE(1, "verifier bug. Call stack is too deep\n");
-			return -EFAULT;
+			verbose(env, "the call stack of %d frames is too deep !\n",
+				frame);
+			return -E2BIG;
 		}
 		goto process_func;
 	}
-- 
2.17.1