linux-kernel - [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS

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Message-Id: <1524709611-29437-1-git-send-email-leo.yan@linaro.org>
Date:   Thu, 26 Apr 2018 10:26:51 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jonathan Corbet <corbet@....net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Cc:     Leo Yan <leo.yan@...aro.org>
Subject: [PATCH bpf-next] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

When CONFIG_BPF_JIT_ALWAYS_ON is enabled, kernel has limitation for
bpf_jit_enable, so it has fixed value 1 and we cannot set it to 2
for JIT opcode dumping; this patch is to update the doc for it.

Signed-off-by: Leo Yan <leo.yan@...aro.org>
---
 Documentation/networking/filter.txt | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
index fd55c7d..feddab9 100644
--- a/Documentation/networking/filter.txt
+++ b/Documentation/networking/filter.txt
@@ -483,6 +483,12 @@ Example output from dmesg:
 [ 3389.935851] JIT code: 00000030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 00 00
 [ 3389.935852] JIT code: 00000040: eb 02 31 c0 c9 c3
 
+When CONFIG_BPF_JIT_ALWAYS_ON is enabled, bpf_jit_enable is set to 1 by default
+and it returns failure if change to any other value from proc node; this is
+for security consideration to avoid leaking info to unprivileged users. In this
+case, we can't directly dump JIT opcode image from kernel log, alternatively we
+need to use bpf tool for the dumping.
+
 In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
 generating disassembly out of the kernel log's hexdump:
 
-- 
1.9.1