Re: [PATCH v1 bpf 1/4] bpf: Fix data-races around bpf_jit_enable.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Thu, 18 Aug 2022 15:49:46 -0700
> On Wed, Aug 17, 2022 at 9:24 PM Kuniyuki Iwashima <kuniyu@amazon.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@amazon.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.

---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<---