netdev - Re: [PATCH bpf-next 2/3] ftrace: Keep address offset in ftrace_lookup

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Date:   Thu, 2 Jun 2022 15:52:03 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <jolsa@...nel.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH bpf-next 2/3] ftrace: Keep address offset in ftrace_lookup_symbols

On Fri, May 27, 2022 at 1:56 PM Jiri Olsa <jolsa@...nel.org> wrote:
>
> We want to store the resolved address on the same index as
> the symbol string, because that's the user (bpf kprobe link)
> code assumption.
>
> Also making sure we don't store duplicates that might be
> present in kallsyms.
>
> Fixes: bed0d9a50dac ("ftrace: Add ftrace_lookup_symbols function")
> Signed-off-by: Jiri Olsa <jolsa@...nel.org>
> ---
>  kernel/trace/ftrace.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 674add0aafb3..00d0ba6397ed 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -7984,15 +7984,23 @@ static int kallsyms_callback(void *data, const char *name,
>                              struct module *mod, unsigned long addr)
>  {
>         struct kallsyms_data *args = data;
> +       const char **sym;
> +       int idx;
>
> -       if (!bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp))
> +       sym = bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp);
> +       if (!sym)
> +               return 0;
> +
> +       idx = sym - args->syms;
> +       if (args->addrs[idx])

if we have duplicated symbols we won't increment args->found here,
right? So we won't stop early. But we also don't want to increment
args->found here because we use it to check that we don't have
duplicates (in addition to making sure we resolved all the unique
symbols), right?

So I wonder if in this situation should we return some error code to
signify that we encountered symbol duplicate?


>                 return 0;
>
>         addr = ftrace_location(addr);
>         if (!addr)
>                 return 0;
>
> -       args->addrs[args->found++] = addr;
> +       args->addrs[idx] = addr;
> +       args->found++;
>         return args->found == args->cnt ? 1 : 0;
>  }
>
> @@ -8017,6 +8025,7 @@ int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *a
>         struct kallsyms_data args;
>         int err;
>
> +       memset(addrs, 0x0, sizeof(*addrs) * cnt);
>         args.addrs = addrs;
>         args.syms = sorted_syms;
>         args.cnt = cnt;
> --
> 2.35.3
>