linux-kernel - Re: [PATCH bpf-next 1/4] bpf: crypto: Use the correct destructor kfunc type

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Message-ID: <6bab8404-f830-4855-be83-df57705491f9@linux.dev>
Date: Fri, 25 Jul 2025 09:13:00 -0700
From: Yonghong Song <yonghong.song@...ux.dev>
To: Sami Tolvanen <samitolvanen@...gle.com>, bpf@...r.kernel.org
Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
 Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>,
 Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next 1/4] bpf: crypto: Use the correct destructor
 kfunc type



On 7/24/25 3:32 PM, Sami Tolvanen wrote:
> With CONFIG_CFI_CLANG enabled, the kernel strictly enforces that
> indirect function calls use a function pointer type that matches the
> target function. I ran into the following type mismatch when running
> BPF self-tests:
>
>    CFI failure at bpf_obj_free_fields+0x190/0x238 (target:
>      bpf_crypto_ctx_release+0x0/0x94; expected type: 0xa488ebfc)
>    Internal error: Oops - CFI: 00000000f2008228 [#1]  SMP
>    ...
>
> As bpf_crypto_ctx_release() is also used in BPF programs and using
> a void pointer as the argument would make the verifier unhappy, add
> a simple stub function with the correct type and register it as the
> destructor kfunc instead.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> ---
>   kernel/bpf/crypto.c | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/crypto.c b/kernel/bpf/crypto.c
> index 94854cd9c4cc..b703b1d1c282 100644
> --- a/kernel/bpf/crypto.c
> +++ b/kernel/bpf/crypto.c
> @@ -261,6 +261,11 @@ __bpf_kfunc void bpf_crypto_ctx_release(struct bpf_crypto_ctx *ctx)
>   		call_rcu(&ctx->rcu, crypto_free_cb);
>   }
>   
> +__bpf_kfunc void __bpf_crypto_ctx_release(void *ctx)

We are not really creating a kfunc here. The function is merely
to be used for destructor. So you can replace '__bpf_kfunc' with
'__used __retain'.

> +{
> +	bpf_crypto_ctx_release(ctx);
> +}
> +
>   static int bpf_crypto_crypt(const struct bpf_crypto_ctx *ctx,
>   			    const struct bpf_dynptr_kern *src,
>   			    const struct bpf_dynptr_kern *dst,
> @@ -368,7 +373,7 @@ static const struct btf_kfunc_id_set crypt_kfunc_set = {
>   
>   BTF_ID_LIST(bpf_crypto_dtor_ids)
>   BTF_ID(struct, bpf_crypto_ctx)
> -BTF_ID(func, bpf_crypto_ctx_release)
> +BTF_ID(func, __bpf_crypto_ctx_release)
>   
>   static int __init crypto_kfunc_init(void)
>   {