Re: [PATCHv4 bpf-next 7/8] libbpf: Allow specification of "kprobe/function+offset"

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Jiri Olsa <jolsa@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andriin@fb.com>,
	Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	Yonghong Song <yhs@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Alan Maguire <alan.maguire@oracle.com>
Subject: Re: [PATCHv4 bpf-next 7/8] libbpf: Allow specification of "kprobe/function+offset"
Date: Fri, 16 Jul 2021 18:42:05 -0700	[thread overview]
Message-ID: <CAEf4BzYELMgTv_RhW7qWNgOYc_mCyh8-VX0FUYabi_TU3OiGKw@mail.gmail.com> (raw)
In-Reply-To: <20210714094400.396467-8-jolsa@kernel.org>

On Wed, Jul 14, 2021 at 2:45 AM Jiri Olsa <jolsa@redhat.com> wrote:
>
> From: Alan Maguire <alan.maguire@oracle.com>
>
> kprobes can be placed on most instructions in a function, not
> just entry, and ftrace and bpftrace support the function+offset
> notification for probe placement.  Adding parsing of func_name
> into func+offset to bpf_program__attach_kprobe() allows the
> user to specify
>
> SEC("kprobe/bpf_fentry_test5+0x6")
>
> ...for example, and the offset can be passed to perf_event_open_probe()
> to support kprobe attachment.
>
> [jolsa: changed original code to use bpf_program__attach_kprobe_opts
> and use dynamic allocation in sscanf]
>
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  tools/lib/bpf/libbpf.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d93a6f9408d1..abe6d4842bb0 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -10348,6 +10348,7 @@ static int perf_event_open_probe(bool uprobe, bool retprobe, const char *name,
>
>  struct bpf_program_attach_kprobe_opts {
>         bool retprobe;
> +       unsigned long offset;
>  };
>
>  static struct bpf_link*
> @@ -10360,7 +10361,7 @@ bpf_program__attach_kprobe_opts(struct bpf_program *prog,
>         int pfd, err;
>
>         pfd = perf_event_open_probe(false /* uprobe */, opts->retprobe, func_name,
> -                                   0 /* offset */, -1 /* pid */);
> +                                   opts->offset, -1 /* pid */);
>         if (pfd < 0) {
>                 pr_warn("prog '%s': failed to create %s '%s' perf event: %s\n",
>                         prog->name, opts->retprobe ? "kretprobe" : "kprobe", func_name,
> @@ -10394,12 +10395,31 @@ static struct bpf_link *attach_kprobe(const struct bpf_sec_def *sec,
>                                       struct bpf_program *prog)
>  {
>         struct bpf_program_attach_kprobe_opts opts;
> +       unsigned long offset = 0;
> +       struct bpf_link *link;
>         const char *func_name;
> +       char *func;
> +       int n, err;
>
>         func_name = prog->sec_name + sec->len;
>         opts.retprobe = strcmp(sec->sec, "kretprobe/") == 0;
>
> -       return bpf_program__attach_kprobe_opts(prog, func_name, &opts);
> +       n = sscanf(func_name, "%m[a-zA-Z0-9_.]+%lx", &func, &offset);

could have used %li here to support both +0xabc and +123 forms

> +       if (n < 1) {
> +               err = -EINVAL;
> +               pr_warn("kprobe name is invalid: %s\n", func_name);
> +               return libbpf_err_ptr(err);
> +       }
> +       if (opts.retprobe && offset != 0) {
> +               err = -EINVAL;

leaking func here


> +               pr_warn("kretprobes do not support offset specification\n");
> +               return libbpf_err_ptr(err);
> +       }
> +
> +       opts.offset = offset;
> +       link = bpf_program__attach_kprobe_opts(prog, func, &opts);
> +       free(func);
> +       return link;
>  }
>
>  struct bpf_link *bpf_program__attach_uprobe(struct bpf_program *prog,
> --
> 2.31.1
>