On 04/26/2018 04:26 AM, 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. > > Signed-off-by: Leo Yan <leo....@linaro.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. > +
Could you change this doc text a bit, I think it's slightly misleading. From the first sentence one could also interpret that value 0 would leaking info to unprivileged users whereas here we're only talking about the case of value 2. Maybe something roughly like this to make it more clear: 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. Thanks, Daniel
