netdev - Re: [PATCH v1 bpf 1/4] bpf: Fix data-races around bpf_jit

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Date:   Thu, 18 Aug 2022 17:13:25 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kuniyuki Iwashima <kuni1840@...il.com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v1 bpf 1/4] bpf: Fix data-races around bpf_jit_enable.

On Thu, Aug 18, 2022 at 5:07 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
> Date:   Thu, 18 Aug 2022 15:49:46 -0700
> > On Wed, Aug 17, 2022 at 9:24 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > >
> > > A sysctl variable bpf_jit_enable is accessed concurrently, and there is
> > > always a chance of data-race.  So, all readers and a writer need some
> > > basic protection to avoid load/store-tearing.
> > >
> > > Fixes: 0a14842f5a3c ("net: filter: Just In Time compiler for x86-64")
> > > Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > ---
> > >  arch/arm/net/bpf_jit_32.c        | 2 +-
> > >  arch/arm64/net/bpf_jit_comp.c    | 2 +-
> > >  arch/mips/net/bpf_jit_comp.c     | 2 +-
> > >  arch/powerpc/net/bpf_jit_comp.c  | 5 +++--
> > >  arch/riscv/net/bpf_jit_core.c    | 2 +-
> > >  arch/s390/net/bpf_jit_comp.c     | 2 +-
> > >  arch/sparc/net/bpf_jit_comp_32.c | 5 +++--
> > >  arch/sparc/net/bpf_jit_comp_64.c | 5 +++--
> > >  arch/x86/net/bpf_jit_comp.c      | 2 +-
> > >  arch/x86/net/bpf_jit_comp32.c    | 2 +-
> > >  include/linux/filter.h           | 2 +-
> > >  net/core/sysctl_net_core.c       | 4 ++--
> > >  12 files changed, 19 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/arch/arm/net/bpf_jit_32.c b/arch/arm/net/bpf_jit_32.c
> > > index 6a1c9fca5260..4b6b62a6fdd4 100644
> > > --- a/arch/arm/net/bpf_jit_32.c
> > > +++ b/arch/arm/net/bpf_jit_32.c
> > > @@ -1999,7 +1999,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > >         }
> > >         flush_icache_range((u32)header, (u32)(ctx.target + ctx.idx));
> > >
> > > -       if (bpf_jit_enable > 1)
> > > +       if (READ_ONCE(bpf_jit_enable) > 1)
> >
> > Nack.
> > Even if the compiler decides to use single byte loads for some
> > odd reason there is no issue here.
>
> I see, and same for 2nd/3rd patches, right?
>
> Then how about this part?
> It's not data-race nor problematic in practice, but should the value be
> consistent in the same function?
> The 2nd/3rd patches also have this kind of part.

The bof_jit_enable > 1 is unsupported and buggy.
It will be removed eventually.

Why are you doing these changes if they're not fixing any bugs ?
Just to shut up some race sanitizer?

> ---8<---
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index 43e634126514..c71d1e94ee7e 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -122,6 +122,7 @@ bool bpf_jit_needs_zext(void)
>
>  struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>  {
> +       int jit_enable = READ_ONCE(bpf_jit_enable);
>         u32 proglen;
>         u32 alloclen;
>         u8 *image = NULL;
> @@ -263,13 +264,13 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>                 }
>                 bpf_jit_build_epilogue(code_base, &cgctx);
>
> -               if (bpf_jit_enable > 1)
> +               if (jit_enable > 1)
>                         pr_info("Pass %d: shrink = %d, seen = 0x%x\n", pass,
>                                 proglen - (cgctx.idx * 4), cgctx.seen);
>         }
>
>  skip_codegen_passes:
> -       if (bpf_jit_enable > 1)
> +       if (jit_enable > 1)
>                 /*
>                  * Note that we output the base address of the code_base
>                  * rather than image, since opcodes are in code_base.
> ---8<---