2018-04-27 10:04:39

by Leo Yan

Subject: [PATCH bpf-next v2] 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.

Suggested-by: Daniel Borkmann <[email protected]>
Signed-off-by: Leo Yan <[email protected]>
---
Documentation/networking/filter.txt | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/Documentation/networking/filter.txt b/Documentation/networking/filter.txt
index fd55c7d..5032e12 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 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:

--
1.9.1

2018-04-27 10:11:52

by Daniel Borkmann

Subject: Re: [PATCH bpf-next v2] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

On 04/27/2018 12:02 PM, Leo Yan wrote:
> 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 <[email protected]>
> Signed-off-by: Leo Yan <[email protected]>

Applied to bpf-next, thanks Leo!