linux-kernel - Re: [PATCH v2] Add BPF Kernel Function bpf_ptrace

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Message-ID: <CAADnVQJ5xCsBg057gKOQOYA1+9pD-X86bjYJVrTbpRNstvW=DQ@mail.gmail.com>
Date: Sun, 29 Sep 2024 10:10:10 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Eric Yan <eric.yan@...o.com>
Cc: kbuild test robot <lkp@...el.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Hao Luo <haoluo@...gle.com>, 
	John Fastabend <john.fastabend@...il.com>, Jiri Olsa <jolsa@...nel.org>, 
	KP Singh <kpsingh@...nel.org>, LKML <linux-kernel@...r.kernel.org>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, oe-kbuild-all@...ts.linux.dev, 
	Stanislav Fomichev <sdf@...ichev.me>, Song Liu <song@...nel.org>, 
	Yonghong Song <yonghong.song@...ux.dev>
Subject: Re: [PATCH v2] Add BPF Kernel Function bpf_ptrace_vprintk

On Thu, Sep 26, 2024 at 12:28 AM Eric Yan <eric.yan@...o.com> wrote:
>
> add a kfunc 'bpf_ptrace_vprintk' printing bpf msg with trace_marker
> format requirement so that these msgs can be retrieved by android
> perfetto by default and well represented in perfetto UI.
>
> [testing prog]
> const volatile bool ptrace_enabled = true;
> extern int bpf_ptrace_vprintk(char *fmt, u32 fmt_size, const void *args, u32 args__sz) __ksym;
>
> ({                                    \
>     if (!ptrace_enabled) { \
>         bpf_printk(fmt, __VA_ARGS__);     \
>     } else {                              \
>         char __fmt[] = fmt;               \
>         _Pragma("GCC diagnostic push")    \
>         _Pragma("GCC diagnostic ignored \"-Wint-conversion\"")  \
>         u64 __params[] = { __VA_ARGS__ }; \
>         _Pragma("GCC diagnostic pop")     \
>         bpf_ptrace_vprintk(__fmt, sizeof(__fmt), __params, sizeof(__params)); \
>     }                                  \
> })
>
> SEC("perf_event")
> int do_sample(struct bpf_perf_event_data *ctx)
> {
>         u64 ip = PT_REGS_IP(&ctx->regs);
>         u64 id = bpf_get_current_pid_tgid();
>         s32 pid = id >> 32;
>         s32 tid = id;
>         debug_printk("N|%d|BPRF-%d|BPRF:%llx", pid, tid, ip);
>         return 0;
> }
>
> [output]:
>        app-3151    [000] d.h1.  6059.904239: tracing_mark_write: N|2491|BPRF-3151|BPRF:58750d0eec
>
> Signed-off-by: Eric Yan <eric.yan@...o.com>
> ---
>  kernel/bpf/helpers.c | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 1a43d06eab28..1e37dae74ca6 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2521,6 +2521,39 @@ __bpf_kfunc struct task_struct *bpf_task_from_pid(s32 pid)
>         return p;
>  }
>
> +static noinline void tracing_mark_write(char *buf)
> +{
> +       trace_printk(buf);
> +}
> +
> +/* same as bpf_trace_vprintk, only with a trace_marker format requirement
> + * @fmt: Format string, e.g. <B|E|C|N>|<%d:pid>|<%s:TAG>...
> + */
> +__bpf_kfunc int bpf_ptrace_vprintk(char *fmt, u32 fmt_size, const void *args, u32 args__sz)
> +{
> +       struct bpf_bprintf_data data = {
> +               .get_bin_args   = true,
> +               .get_buf        = true,
> +       };
> +       int ret, num_args;
> +
> +       if (args__sz & 7 || args__sz > MAX_BPRINTF_VARARGS * 8 || (args__sz && !args))
> +               return -EINVAL;
> +       num_args = args__sz / 8;
> +
> +       ret = bpf_bprintf_prepare(fmt, fmt_size, args, num_args, &data);
> +       if (ret < 0)
> +               return ret;
> +
> +       ret = bstr_printf(data.buf, MAX_BPRINTF_BUF, fmt, data.bin_args);
> +
> +       tracing_mark_write(data.buf);
> +
> +       bpf_bprintf_cleanup(&data);
> +
> +       return ret;
> +}
> +
>  /**
>   * bpf_dynptr_slice() - Obtain a read-only pointer to the dynptr data.
>   * @p: The dynptr whose data slice to retrieve
> @@ -3090,6 +3123,7 @@ BTF_ID_FLAGS(func, bpf_iter_bits_new, KF_ITER_NEW)
>  BTF_ID_FLAGS(func, bpf_iter_bits_next, KF_ITER_NEXT | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_iter_bits_destroy, KF_ITER_DESTROY)
>  BTF_ID_FLAGS(func, bpf_copy_from_user_str, KF_SLEEPABLE)
> +BTF_ID_FLAGS(func, bpf_ptrace_vprintk)
>  BTF_KFUNCS_END(common_btf_ids)

Why new kfunc?
Use bpf_snprintf() and follow with bpf_trace_printk() ?