Re: [PATCH v2 bpf-next 06/14] bpf: add bpf_get_user_ctx() BPF helper to access user_ctx value

[Yonghong Song <yhs@fb.com>]

On 7/26/21 9:12 AM, Andrii Nakryiko wrote:
> Add new BPF helper, bpf_get_user_ctx(), which can be used by BPF programs to
> get access to the user_ctx value, specified during BPF program attachment (BPF
> link creation) time.
> 
> Currently all perf_event-backed BPF program types support bpf_get_user_ctx()
> helper. Follow-up patches will add support for fentry/fexit programs as well.
> 
> While at it, mark bpf_tracing_func_proto() as static to make it obvious that
> it's only used from within the kernel/trace/bpf_trace.c.
> 
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>   include/linux/bpf.h            |  3 ---
>   include/uapi/linux/bpf.h       | 16 ++++++++++++++++
>   kernel/trace/bpf_trace.c       | 35 +++++++++++++++++++++++++++++++++-
>   tools/include/uapi/linux/bpf.h | 16 ++++++++++++++++
>   4 files changed, 66 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 74b35faf0b73..94ebedc1e13a 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -2110,9 +2110,6 @@ extern const struct bpf_func_proto bpf_btf_find_by_name_kind_proto;
>   extern const struct bpf_func_proto bpf_sk_setsockopt_proto;
>   extern const struct bpf_func_proto bpf_sk_getsockopt_proto;
>   
> -const struct bpf_func_proto *bpf_tracing_func_proto(
> -	enum bpf_func_id func_id, const struct bpf_prog *prog);
> -
>   const struct bpf_func_proto *tracing_prog_func_proto(
>     enum bpf_func_id func_id, const struct bpf_prog *prog);
>   
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index bc1fd54a8f58..96afeced3467 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -4856,6 +4856,21 @@ union bpf_attr {
>    * 		Get address of the traced function (for tracing and kprobe programs).
>    * 	Return
>    * 		Address of the traced function.
> + *
> + * u64 bpf_get_user_ctx(void *ctx)
> + * 	Description
> + * 		Get user_ctx value provided (optionally) during the program
> + * 		attachment. It might be different for each individual
> + * 		attachment, even if BPF program itself is the same.
> + * 		Expects BPF program context *ctx* as a first argument.
> + *
> + * 		Supported for the following program types:
> + *			- kprobe/uprobe;
> + *			- tracepoint;
> + *			- perf_event.

I think it is possible in the future we may need to support more
program types with user_ctx, not just u64 but more than 64bit value. 
Should we may make this helper extensible like
     long bpf_get_user_ctx(void *ctx, void *user_ctx, u32 user_ctx_len)

The return value will 0 to be good and a negative indicating an error.
What do you think?

> + * 	Return
> + *		Value specified by user at BPF link creation/attachment time
> + *		or 0, if it was not specified.
>    */
>   #define __BPF_FUNC_MAPPER(FN)		\
>   	FN(unspec),			\
> @@ -5032,6 +5047,7 @@ union bpf_attr {
>   	FN(timer_start),		\
>   	FN(timer_cancel),		\
>   	FN(get_func_ip),		\
> +	FN(get_user_ctx),		\
>   	/* */
>   
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index c9cf6a0d0fb3..b14978b3f6fb 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -975,7 +975,34 @@ static const struct bpf_func_proto bpf_get_func_ip_proto_kprobe = {
>   	.arg1_type	= ARG_PTR_TO_CTX,
>   };
>   
> -const struct bpf_func_proto *
> +BPF_CALL_1(bpf_get_user_ctx_trace, void *, ctx)
> +{
> +	struct bpf_trace_run_ctx *run_ctx;
> +
> +	run_ctx = container_of(current->bpf_ctx, struct bpf_trace_run_ctx, run_ctx);
> +	return run_ctx->user_ctx;
> +}
> +
> +static const struct bpf_func_proto bpf_get_user_ctx_proto_trace = {
> +	.func		= bpf_get_user_ctx_trace,
> +	.gpl_only	= false,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_PTR_TO_CTX,
> +};
> +
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