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

From: Yonghong Song
Date: Mon Dec 12 2022 - 13:04:13 EST




On 12/11/22 2:16 PM, david.keisarschm@xxxxxxxxxxxxxxx wrote:
From: David <david.keisarschm@xxxxxxxxxxxxxxx>

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@xxxxxxxxxxxxxxx>
---
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@xxxxxxx/
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;