Re: [PATCH bpf-next v3 2/2] bpf: Make BPF_JIT_DEFAULT_ON selectable in Kconfig

[Tiezhu Yang]

On 03/01/2022 07:53 AM, Daniel Borkmann wrote:
> Hi Tiezhu,
>
> (patch 1/2 applied so far, thanks!)
>
> On 2/22/22 10:57 AM, Tiezhu Yang wrote:
> > Currently, only x86, arm64 and s390 select ARCH_WANT_DEFAULT_BPF_JIT,
> > the other archs do not select ARCH_WANT_DEFAULT_BPF_JIT. On the archs
> > without ARCH_WANT_DEFAULT_BPF_JIT, if we want to set bpf_jit_enable to
> > 1 by default, the only way is to enable CONFIG_BPF_JIT_ALWAYS_ON, then
> > the users can not change it to 0 or 2, it seems bad for some users. We
>
> Can you elaborate on the "it seems bad for some users" part? What's the
> concrete

Hi Daniel,

Sorry for the late reply.

I saw the following two similar patches, some users want to set
bpf_jit_enable to 1 by default, at the same time, they want to
change it to 0 or 2 to debug, only enable CONFIG_BPF_JIT_DEFAULT_ON
is a proper way in such a case.

[PATCH bpf-next] bpf: trace jit code when enable BPF_JIT_ALWAYS_ON
https://lore.kernel.org/bpf/20210326124030.1138964-1-Jianlin.Lv@xxxxxxx/

[PATCH bpf-next] bpf: support bpf_jit_enable=2 for CONFIG_BPF_JIT_ALWAYS_ON
https://lore.kernel.org/bpf/20211231153550.3807430-1-houtao1@xxxxxxxxxx/


> use case? Also, why not add (e.g. mips) JIT to ARCH_WANT_DEFAULT_BPF_JIT
> if the
> CI suite passes with high degree/confidence?

Yes, we can let the specific arch select ARCH_WANT_DEFAULT_BPF_JIT in 
Kconfig, this commit only gives another chance to enable or disable 
CONFIG_BPF_JIT_DEFAULT_ON manually when make menuconfig, this is useful
to debug when develop JIT.

> > can select ARCH_WANT_DEFAULT_BPF_JIT for those archs if it is proper,
> > but at least for now, make BPF_JIT_DEFAULT_ON selectable can give them
> > a chance.
> >
> > Additionally, with this patch, under !BPF_JIT_ALWAYS_ON, we can disable
> > BPF_JIT_DEFAULT_ON on the archs with ARCH_WANT_DEFAULT_BPF_JIT when make
> > menuconfig, it seems flexible for some developers.
> >
> > Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> > ---
> >   kernel/bpf/Kconfig | 13 +++++++++++--
> >   1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/bpf/Kconfig b/kernel/bpf/Kconfig
> > index f3db15a..8521874 100644
> > --- a/kernel/bpf/Kconfig
> > +++ b/kernel/bpf/Kconfig
> > @@ -54,6 +54,7 @@ config BPF_JIT
> >   config BPF_JIT_ALWAYS_ON
> >       bool "Permanently enable BPF JIT and remove BPF interpreter"
> >       depends on BPF_SYSCALL && HAVE_EBPF_JIT && BPF_JIT
> > +    select BPF_JIT_DEFAULT_ON
>
> Is the above needed if ...
>
> >       help
> >         Enables BPF JIT and removes BPF interpreter to avoid speculative
> >         execution of BPF instructions by the interpreter.
> > @@ -63,8 +64,16 @@ config BPF_JIT_ALWAYS_ON
> >         failure.
> >     config BPF_JIT_DEFAULT_ON
> > -    def_bool ARCH_WANT_DEFAULT_BPF_JIT || BPF_JIT_ALWAYS_ON
> > -    depends on HAVE_EBPF_JIT && BPF_JIT
> > +    bool "Enable BPF JIT by default"
> > +    default y if ARCH_WANT_DEFAULT_BPF_JIT
>
> ... we retain the prior `default y if ARCH_WANT_DEFAULT_BPF_JIT ||
> BPF_JIT_ALWAYS_ON` ?

After add

  bool "Enable BPF JIT by default"

if use

  default y if ARCH_WANT_DEFAULT_BPF_JIT || BPF_JIT_ALWAYS_ON

under !ARCH_WANT_DEFAULT_BPF_JIT, when enable CONFIG_BPF_JIT_ALWAYS_ON
manually through make menuconfig, CONFIG_BPF_JIT_DEFAULT_ON is not set
automatically, it seems not reasonable, but I do not know the reason,
maybe this is because CONFIG_BPF_JIT_ALWAYS_ON is user selectable rather
than selected via Kconfig only (like ARCH_WANT_DEFAULT_BPF_JIT), so here
let BPF_JIT_ALWAYS_ON select BPF_JIT_DEFAULT_ON.

> > +    depends on BPF_SYSCALL && HAVE_EBPF_JIT && BPF_JIT
>
> Why is the extra BPF_SYSCALL dependency needed? You could still have
> this for cBPF->eBPF
> translations when BPF syscall is compiled out (e.g. seccomp, sock/packet
> filters, etc).

Sorry, just copy-paste from "config BPF_JIT_ALWAYS_ON".

If BPF_SYSCALL dependency is not needed by BPF_JIT_DEFAULT_ON,
should we remove BPF_SYSCALL dependency in "config BPF_JIT_ALWAYS_ON"?

Thanks,
Tiezhu

> > +    help
> > +      Enables BPF JIT by default to avoid speculative execution of BPF
> > +      instructions by the interpreter.
> > +
> > +      When CONFIG_BPF_JIT_DEFAULT_ON is enabled but
> > CONFIG_BPF_JIT_ALWAYS_ON
> > +      is disabled, /proc/sys/net/core/bpf_jit_enable is set to 1 by
> > default
> > +      and can be changed to 0 or 2.
> >     config BPF_UNPRIV_DEFAULT_OFF
> >       bool "Disable unprivileged BPF by default"