netdev - [PATCH net-next 4/9] bpf: reconcile bpf_tail_call and stack

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Date:   Tue, 30 May 2017 13:31:30 -0700
From:   Alexei Starovoitov <ast@...com>
To:     "David S . Miller" <davem@...emloft.net>
CC:     Daniel Borkmann <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: [PATCH net-next 4/9] bpf: reconcile bpf_tail_call and stack_depth

The next set of patches will take advantage of stack_depth tracking,
so make sure that the program that does bpf_tail_call() has
stack depth large enough for the callee.
We could have tracked the stack depth of the prog_array owner program
and only allow insertion of the programs with stack depth less
than the owner, but it will break existing applications.
Some of them have trivial root bpf program that only does
multiple bpf_tail_calls and at init time the prog array is empty.
In the future we may add a flag to do such tracking optionally,
but for now play simple and safe.

Signed-off-by: Alexei Starovoitov <ast@...nel.org>
Acked-by: Daniel Borkmann <daniel@...earbox.net>
---
 kernel/bpf/verifier.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index d96f27ff9f6f..14ccb0759fa4 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -3470,6 +3470,7 @@ static int fixup_bpf_calls(struct bpf_verifier_env *env)
 			 * the program array.
 			 */
 			prog->cb_access = 1;
+			env->prog->aux->stack_depth = MAX_BPF_STACK;

 			/* mark bpf_tail_call as different opcode to avoid
 			 * conditional branch in the interpeter for every normal
-- 
2.9.3