Re: [PATCH 11/19] bpf: Add support to load multi func tracing program

[Yonghong Song <yhs@fb.com>]

On 6/5/21 4:10 AM, Jiri Olsa wrote:
> Adding support to load tracing program with new BPF_F_MULTI_FUNC flag,
> that allows the program to be loaded without specific function to be
> attached to.
> 
> The verifier assumes the program is using all (6) available arguments

Is this a verifier failure or it is due to the check in the
beginning of function arch_prepare_bpf_trampoline()?

         /* x86-64 supports up to 6 arguments. 7+ can be added in the 
future */
         if (nr_args > 6)
                 return -ENOTSUPP;

If it is indeed due to arch_prepare_bpf_trampoline() maybe we
can improve it instead of specially processing the first argument
"ip" in quite some places?

> as unsigned long values. We can't add extra ip argument at this time,
> because JIT on x86 would fail to process this function. Instead we
> allow to access extra first 'ip' argument in btf_ctx_access.
> 
> Such program will be allowed to be attached to multiple functions
> in following patches.
> 
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>   include/linux/bpf.h            |  1 +
>   include/uapi/linux/bpf.h       |  7 +++++++
>   kernel/bpf/btf.c               |  5 +++++
>   kernel/bpf/syscall.c           | 35 +++++++++++++++++++++++++++++-----
>   kernel/bpf/verifier.c          |  3 ++-
>   tools/include/uapi/linux/bpf.h |  7 +++++++
>   6 files changed, 52 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 6cbf3c81c650..23221e0e8d3c 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -845,6 +845,7 @@ struct bpf_prog_aux {
>   	bool sleepable;
>   	bool tail_call_reachable;
>   	struct hlist_node tramp_hlist;
> +	bool multi_func;

Move this field right after "tail_call_reachable"?

>   	/* BTF_KIND_FUNC_PROTO for valid attach_btf_id */
>   	const struct btf_type *attach_func_proto;
>   	/* function name for valid attach_btf_id */
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 2c1ba70abbf1..ad9340fb14d4 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -1109,6 +1109,13 @@ enum bpf_link_type {
>    */
>   #define BPF_F_SLEEPABLE		(1U << 4)
>   
> +/* If BPF_F_MULTI_FUNC is used in BPF_PROG_LOAD command, the verifier does
> + * not expect BTF ID for the program, instead it assumes it's function
> + * with 6 u64 arguments. No trampoline is created for the program. Such
> + * program can be attached to multiple functions.
> + */
> +#define BPF_F_MULTI_FUNC	(1U << 5)
> +
>   /* When BPF ldimm64's insn[0].src_reg != 0 then this can have
>    * the following extensions:
>    *
> diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
> index a6e39c5ea0bf..c233aaa6a709 100644
> --- a/kernel/bpf/btf.c
> +++ b/kernel/bpf/btf.c
> @@ -4679,6 +4679,11 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
>   		args++;
>   		nr_args--;
>   	}
> +	if (prog->aux->multi_func) {
> +		if (arg == 0)
> +			return true;
> +		arg--;

Some comments in the above to mention like "the first 'ip' argument
is omitted" will be good.

> +	}
>   
>   	if (arg > nr_args) {
>   		bpf_log(log, "func '%s' doesn't have %d-th argument\n",
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