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

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Message-ID: <YpnfldPKcEqesioK@krava>
Date:   Fri, 3 Jun 2022 12:16:53 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
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 Thu, Jun 02, 2022 at 03:52:03PM -0700, Andrii Nakryiko wrote:
> 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?

hum, this callback is called for each kallsyms symbol and there
are duplicates in /proc/kallsyms.. so even if we have just single
copy of such symbol in args->syms, bsearch will find this single
symbol for all the duplicates in /proc/kallsyms and we will endup
in here.. and it's still fine, we should continue

jirka

> 
> 
> >                 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
> >