Re: [PATCH bpf-next v3 2/7] bpf: add two kfunc for TRACE_SESSION

[Menglong Dong <menglong.dong@linux.dev>]

On 2025/10/26 11:01, Menglong Dong wrote:
> If TRACE_SESSION exists, we will use extra 8-bytes in the stack of the
> trampoline to store the flags that we needed, and the 8-bytes lie after
> the return value, which means ctx[nr_args + 1]. And we will store the
> flag "is_exit" to the first bit of it.
> 
> Introduce the kfunc bpf_tracing_is_exit(), which is used to tell if it
> is fexit currently. Meanwhile, inline it in the verifier.
> 
> Add the kfunc bpf_fsession_cookie(), which is similar to
> bpf_session_cookie() and return the address of the session cookie. The
> address of the session cookie is stored after session flags, which means
> ctx[nr_args + 2]. Inline this kfunc in the verifier too.
> 
> Signed-off-by: Menglong Dong <dongml2@chinatelecom.cn>
> Co-developed-by: Leon Hwang <leon.hwang@linux.dev>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
> ---
> v3:
> - merge the bpf_tracing_is_exit and bpf_fsession_cookie into a single
>   patch
> 
> v2:
> - store the session flags after return value, instead of before nr_args
> - inline the bpf_tracing_is_exit, as Jiri suggested
> ---
>  include/linux/bpf.h      |  1 +
>  kernel/bpf/verifier.c    | 33 ++++++++++++++++++++--
>  kernel/trace/bpf_trace.c | 59 ++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 88 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 6b5855c80fa6..ce55d3881c0d 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -1736,6 +1736,7 @@ struct bpf_prog {
>  				enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */
>  				call_get_stack:1, /* Do we call bpf_get_stack() or bpf_get_stackid() */
>  				call_get_func_ip:1, /* Do we call get_func_ip() */
> +				call_session_cookie:1, /* Do we call bpf_fsession_cookie() */
>  				tstamp_type_access:1, /* Accessed __sk_buff->tstamp_type */
>  				sleepable:1;	/* BPF program is sleepable */
>  	enum bpf_prog_type	type;		/* Type of BPF program */
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 818deb6a06e4..6f8aa4718d6f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12293,6 +12293,8 @@ enum special_kfunc_type {
>  	KF___bpf_trap,
>  	KF_bpf_task_work_schedule_signal,
>  	KF_bpf_task_work_schedule_resume,
> +	KF_bpf_tracing_is_exit,
> +	KF_bpf_fsession_cookie,
>  };
>  
>  BTF_ID_LIST(special_kfunc_list)
> @@ -12365,6 +12367,8 @@ BTF_ID(func, bpf_res_spin_unlock_irqrestore)
>  BTF_ID(func, __bpf_trap)
>  BTF_ID(func, bpf_task_work_schedule_signal)
>  BTF_ID(func, bpf_task_work_schedule_resume)
> +BTF_ID(func, bpf_tracing_is_exit)
> +BTF_ID(func, bpf_fsession_cookie)
>  
>  static bool is_task_work_add_kfunc(u32 func_id)
>  {
> @@ -12419,7 +12423,9 @@ get_kfunc_ptr_arg_type(struct bpf_verifier_env *env,
>  	struct bpf_reg_state *reg = &regs[regno];
>  	bool arg_mem_size = false;
>  
> -	if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx])
> +	if (meta->func_id == special_kfunc_list[KF_bpf_cast_to_kern_ctx] ||
> +	    meta->func_id == special_kfunc_list[KF_bpf_tracing_is_exit] ||
> +	    meta->func_id == special_kfunc_list[KF_bpf_fsession_cookie])
>  		return KF_ARG_PTR_TO_CTX;
>  
>  	/* In this function, we verify the kfunc's BTF as per the argument type,
> @@ -13912,7 +13918,8 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  		}
>  	}
>  
> -	if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie]) {
> +	if (meta.func_id == special_kfunc_list[KF_bpf_session_cookie] ||
> +	    meta.func_id == special_kfunc_list[KF_bpf_fsession_cookie]) {
>  		meta.r0_size = sizeof(u64);
>  		meta.r0_rdonly = false;
>  	}
> @@ -14193,6 +14200,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  			return err;
>  	}
>  
> +	if (meta.func_id == special_kfunc_list[KF_bpf_fsession_cookie])
> +		env->prog->call_session_cookie = true;
> +
>  	return 0;
>  }
>  
> @@ -22012,6 +22022,25 @@ static int fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>  		   desc->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {
>  		insn_buf[0] = BPF_MOV64_REG(BPF_REG_0, BPF_REG_1);
>  		*cnt = 1;
> +	} else if (desc->func_id == special_kfunc_list[KF_bpf_tracing_is_exit]) {
> +		/* Load nr_args from ctx - 8 */
> +		insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +		/* add rax, 1 */
> +		insn_buf[1] = BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 1);
> +		insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
> +		insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
> +		insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
> +		insn_buf[5] = BPF_ALU64_IMM(BPF_AND, BPF_REG_0, 1);
> +		*cnt = 6;
> +	} else if (desc->func_id == special_kfunc_list[KF_bpf_fsession_cookie]) {
> +		/* Load nr_args from ctx - 8 */
> +		insn_buf[0] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_1, -8);
> +		/* add rax, 2 */
> +		insn_buf[1] = BPF_ALU64_IMM(BPF_ADD, BPF_REG_0, 2);
> +		insn_buf[2] = BPF_ALU64_IMM(BPF_LSH, BPF_REG_0, 3);
> +		insn_buf[3] = BPF_ALU64_REG(BPF_ADD, BPF_REG_0, BPF_REG_1);
> +		insn_buf[4] = BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0);
> +		*cnt = 5;
>  	}
>  
>  	if (env->insn_aux_data[insn_idx].arg_prog) {
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 4f87c16d915a..4a8568bd654d 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -3356,12 +3356,65 @@ static const struct btf_kfunc_id_set bpf_kprobe_multi_kfunc_set = {
>  	.filter = bpf_kprobe_multi_filter,
>  };
>  
> -static int __init bpf_kprobe_multi_kfuncs_init(void)
> +__bpf_kfunc_start_defs();
> +
> +__bpf_kfunc bool bpf_tracing_is_exit(void *ctx)
>  {
> -	return register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_kprobe_multi_kfunc_set);
> +	/* This helper call is inlined by verifier. */
> +	u64 nr_args = ((u64 *)ctx)[-1];
> +
> +	/*
> +	 * ctx[nr_args + 1] is the session flags, and the last bit is
> +	 * is_exit.
> +	 */
> +	return ((u64 *)ctx)[nr_args + 1] & 1;
> +}
> +
> +__bpf_kfunc u64 *bpf_fsession_cookie(void *ctx)
> +{
> +	/* This helper call is inlined by verifier. */
> +	u64 nr_args = ((u64 *)ctx)[-1];
> +
> +	/* ctx[nr_args + 2] is the session cookie address */
> +	return (u64 *)((u64 *)ctx)[nr_args + 2];

This casting cause build warning in 32-bits arch. I'll make it
like this in the next version after more human comments
on this series:

    return (u64 *)(long)((u64 *)ctx)[nr_args + 2];

> +}
> +
> +__bpf_kfunc_end_defs();
> +
> +BTF_KFUNCS_START(tracing_kfunc_set_ids)
> +BTF_ID_FLAGS(func, bpf_tracing_is_exit, KF_FASTCALL)
> +BTF_ID_FLAGS(func, bpf_fsession_cookie, KF_FASTCALL)
> +BTF_KFUNCS_END(tracing_kfunc_set_ids)
> +
> +static int bpf_tracing_filter(const struct bpf_prog *prog, u32 kfunc_id)
> +{
> +	if (!btf_id_set8_contains(&tracing_kfunc_set_ids, kfunc_id))
> +		return 0;
> +
> +	if (prog->type != BPF_PROG_TYPE_TRACING ||
> +	    prog->expected_attach_type != BPF_TRACE_SESSION)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
> +static const struct btf_kfunc_id_set bpf_tracing_kfunc_set = {
> +	.owner = THIS_MODULE,
> +	.set = &tracing_kfunc_set_ids,
> +	.filter = bpf_tracing_filter,
> +};
> +
> +static int __init bpf_trace_kfuncs_init(void)
> +{
> +	int err = 0;
> +
> +	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_KPROBE, &bpf_kprobe_multi_kfunc_set);
> +	err = err ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_tracing_kfunc_set);
> +
> +	return err;
>  }
>  
> -late_initcall(bpf_kprobe_multi_kfuncs_init);
> +late_initcall(bpf_trace_kfuncs_init);
>  
>  typedef int (*copy_fn_t)(void *dst, const void *src, u32 size, struct task_struct *tsk);
>  
>