Re: [PATCH 2/5] Replace invocation of weak PRNG in kernel/bpf/core.c

[Yonghong Song <yhs@meta.com>]

On 12/11/22 2:16 PM, david.keisarschm@mail.huji.ac.il wrote:
> From: David <david.keisarschm@mail.huji.ac.il>
> 
> We changed the invocation of
>   prandom_u32_state to get_random_u32.
>   We deleted the maintained state,
>   which was a CPU-variable,
>   since get_random_u32 maintains its own CPU-variable.
>   We also deleted the state initializer,
>   since it is not needed anymore.
> 
> Signed-off-by: David <david.keisarschm@mail.huji.ac.il>
> ---
>   include/linux/bpf.h   |  1 -
>   kernel/bpf/core.c     | 13 +------------
>   kernel/bpf/verifier.c |  2 --
>   net/core/filter.c     |  1 -
>   4 files changed, 1 insertion(+), 16 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index c1bd1bd10..0689520b9 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2572,7 +2572,6 @@ const struct bpf_func_proto *tracing_prog_func_proto(
>     enum bpf_func_id func_id, const struct bpf_prog *prog);
>   
>   /* Shared helpers among cBPF and eBPF. */
> -void bpf_user_rnd_init_once(void);
>   u64 bpf_user_rnd_u32(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
>   u64 bpf_get_raw_cpu_id(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
>   
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 4cb5421d9..a6f06894e 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -2579,13 +2579,6 @@ void bpf_prog_free(struct bpf_prog *fp)
>   }
>   EXPORT_SYMBOL_GPL(bpf_prog_free);
>   
> -/* RNG for unpriviledged user space with separated state from prandom_u32(). */
> -static DEFINE_PER_CPU(struct rnd_state, bpf_user_rnd_state);
> -
> -void bpf_user_rnd_init_once(void)
> -{
> -	prandom_init_once(&bpf_user_rnd_state);
> -}
>   
>   BPF_CALL_0(bpf_user_rnd_u32)
>   {
> @@ -2595,12 +2588,8 @@ BPF_CALL_0(bpf_user_rnd_u32)
>   	 * transformations. Register assignments from both sides are
>   	 * different, f.e. classic always sets fn(ctx, A, X) here.
>   	 */
> -	struct rnd_state *state;
>   	u32 res;
> -
> -	state = &get_cpu_var(bpf_user_rnd_state);
> -	res = predictable_rng_prandom_u32_state(state);
> -	put_cpu_var(bpf_user_rnd_state);
> +	res = get_random_u32();
>   
>   	return res;
>   }

Please see the discussion here.
https://lore.kernel.org/bpf/87edtctz8t.fsf@toke.dk/
There is a performance concern with the above change.

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 264b3dc71..9f22fb3fa 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14049,8 +14049,6 @@ static int do_misc_fixups(struct bpf_verifier_env *env)
>   
>   		if (insn->imm == BPF_FUNC_get_route_realm)
>   			prog->dst_needed = 1;
> -		if (insn->imm == BPF_FUNC_get_prandom_u32)
> -			bpf_user_rnd_init_once();
>   		if (insn->imm == BPF_FUNC_override_return)
>   			prog->kprobe_override = 1;
>   		if (insn->imm == BPF_FUNC_tail_call) {
> diff --git a/net/core/filter.c b/net/core/filter.c
> index bb0136e7a..7a595ac00 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -443,7 +443,6 @@ static bool convert_bpf_extensions(struct sock_filter *fp,
>   			break;
>   		case SKF_AD_OFF + SKF_AD_RANDOM:
>   			*insn = BPF_EMIT_CALL(bpf_user_rnd_u32);
> -			bpf_user_rnd_init_once();
>   			break;
>   		}
>   		break;