Re: [PATCH bpf-next] bpf: Allow bpf_jit_enable = 2 with BPF_JIT_ALWAYS_ON config

[Leo Yan <leo.yan@linaro.org>]

Hi Daniel,

On Wed, Apr 25, 2018 at 11:12:21AM +0200, Daniel Borkmann wrote:
> On 04/25/2018 10:18 AM, Leo Yan wrote:
> > After enabled BPF_JIT_ALWAYS_ON config, bpf_jit_enable always equals to
> > 1; it is impossible to set 'bpf_jit_enable = 2' and the kernel has no
> > chance to call bpf_jit_dump().
> > 
> > This patch relaxes bpf_jit_enable range to [1..2] when kernel config
> > BPF_JIT_ALWAYS_ON is enabled so can invoke jit dump.
> > 
> > Signed-off-by: Leo Yan <leo.yan@linaro.org>
> 
> Is there a specific reason why you need this here instead of retrieving the
> dump from the newer interface available from bpftool (tools/bpf/bpftool/)?
> The bpf_jit_enable = 2 is not recommended these days since it dumps into the
> kernel log which is often readable from unpriv as well. bpftool makes use
> of the BPF_OBJ_GET_INFO_BY_FD interface via bpf syscall to get the JIT dump
> instead when bpf_jit_enable is set.

Thanks for reviewing.

When I read the doc Documentation/networking/filter.txt and the
section "JIT compiler" it suggests as below.  So I tried to set
'bpf_jit_enable = 2' to dump JIT code, but it failed.

If we have concern for security issue, should we remove support for
'bpf_jit_enable = 2' and modify the doc to reflect this change?

---8<---

For JIT developers, doing audits etc, each compile run can output the generated
opcode image into the kernel log via: 

  echo 2 > /proc/sys/net/core/bpf_jit_enable

Example output from dmesg:

[ 3389.935842] flen=6 proglen=70 pass=3 image=ffffffffa0069c8f
[ 3389.935847] JIT code: 00000000: 55 48 89 e5 48 83 ec 60 48 89 5d f8 44 8b 4f 68
[ 3389.935849] JIT code: 00000010: 44 2b 4f 6c 4c 8b 87 d8 00 00 00 be 0c 00 00 00
[ 3389.935850] JIT code: 00000020: e8 1d 94 ff e0 3d 00 08 00 00 75 16 be 17 00 00
[ 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

> > ---
> >  net/core/sysctl_net_core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/core/sysctl_net_core.c b/net/core/sysctl_net_core.c
> > index b1a2c5e..6a39b22 100644
> > --- a/net/core/sysctl_net_core.c
> > +++ b/net/core/sysctl_net_core.c
> > @@ -371,7 +371,7 @@ static int proc_dointvec_minmax_bpf_enable(struct ctl_table *table, int write,
> >  		.proc_handler	= proc_dointvec_minmax_bpf_enable,
> >  # ifdef CONFIG_BPF_JIT_ALWAYS_ON
> >  		.extra1		= &one,
> > -		.extra2		= &one,
> > +		.extra2		= &two,
> >  # else
> >  		.extra1		= &zero,
> >  		.extra2		= &two,
> > 
>