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

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Date:   Fri, 3 Jun 2022 12:12:17 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Jiri Olsa <olsajiri@...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 Fri, Jun 3, 2022 at 3:16 AM Jiri Olsa <olsajiri@...il.com> wrote:
>
> 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
>

ah, ok, duplicate kallsyms entries, right, never mind then

> 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
> > >