netdev - Re: [PATCH v2 bpf-next 3/6] libbpf: Modify bpf_printk to choose helper based on arg count

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Date:   Wed, 25 Aug 2021 18:01:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Dave Marchevsky <davemarchevsky@...com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Yonghong Song <yhs@...com>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 bpf-next 3/6] libbpf: Modify bpf_printk to choose
 helper based on arg count

On Wed, Aug 25, 2021 at 12:58 PM Dave Marchevsky <davemarchevsky@...com> wrote:
>
> Instead of being a thin wrapper which calls into bpf_trace_printk,
> libbpf's bpf_printk convenience macro now chooses between
> bpf_trace_printk and bpf_trace_vprintk. If the arg count (excluding
> format string) is >3, use bpf_trace_vprintk, otherwise use the older
> helper.
>
> The motivation behind this added complexity - instead of migrating
> entirely to bpf_trace_vprintk - is to maintain good developer experience
> for users compiling against new libbpf but running on older kernels.
> Users who are passing <=3 args to bpf_printk will see no change in their
> bytecode.
>
> __bpf_vprintk functions similarly to BPF_SEQ_PRINTF and BPF_SNPRINTF
> macros elsewhere in the file - it allows use of bpf_trace_vprintk
> without manual conversion of varargs to u64 array. Previous
> implementation of bpf_printk macro is moved to __bpf_printk for use by
> the new implementation.
>
> This does change behavior of bpf_printk calls with >3 args in the "new
> libbpf, old kernels" scenario. On my system, using a clang built from
> recent upstream sources (14.0.0 https://github.com/llvm/llvm-project.git
> 50b62731452cb83979bbf3c06e828d26a4698dca), attempting to use 4 args to
> __bpf_printk (old impl) results in a compile-time error:
>
>   progs/trace_printk.c:21:21: error: too many args to 0x6cdf4b8: i64 = Constant<6>
>         trace_printk_ret = __bpf_printk("testing,testing %d %d %d %d\n",

and with a new bpf_printk it will compile to use bpf_trace_vprintk
and gets rejected during load on old kernels, right?
That will be the case for any clang.
It's fine.
Would be good to clarify the commit log.

> I was able to replicate this behavior with an older clang as well. When
> the format string has >3 format specifiers, there is no output to the
> trace_pipe in either case.

I don't understand this paragraph. What are the cases?

> After this patch, using bpf_printk with 4 args would result in a
> trace_vprintk helper call being emitted and a load-time failure on older
> kernels.

right.

> +#define __bpf_printk(fmt, ...)                         \
> +({                                                     \
> +       char ____fmt[] = fmt;                           \

Andrii was suggesting to make it const while we're at it,
but that could be done in a follow up.