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bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

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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.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
This commit is contained in:
Leo Yan 2018-04-27 18:02:54 +08:00 committed by Daniel Borkmann
parent 79741a38b4
commit 2c25fc9a50
1 changed files with 6 additions and 0 deletions
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6
Documentation/networking/filter.txt
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@ -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 permanently set to 1 and
setting any other value than that will return in failure. This is even the case for
setting bpf_jit_enable to 2, since dumping the final JIT image into the kernel log
is discouraged and introspection through bpftool (under tools/bpf/bpftool/) is the
generally recommended approach instead.
In the kernel source tree under tools/bpf/, there's bpf_jit_disasm for
generating disassembly out of the kernel log's hexdump: