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

[Daniel Borkmann]

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 
> Signed-off-by: Leo Yan 

Applied to bpf-next, thanks Leo!
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[PATCH bpf-next v2] bpf, doc: Update bpf_jit_enable limitation for CONFIG_BPF_JIT_ALWAYS_ON

[Leo Yan]

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 
Signed-off-by: Leo Yan 
---
 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: 0030: 00 e8 28 94 ff e0 83 f8 01 75 07 b8 ff ff 
00 00
 [ 3389.935852] JIT code: 0040: 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

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