linux-kernel - Re: [PATCHv2 bpf-next 1/4] kallsyms: Add kallsyms_lookup

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Date:   Tue, 26 Apr 2022 14:27:04 +0200
From:   Jiri Olsa <olsajiri@...il.com>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, netdev@...r.kernel.org,
        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>
Subject: Re: [PATCHv2 bpf-next 1/4] kallsyms: Add kallsyms_lookup_names
 function

On Tue, Apr 26, 2022 at 07:01:08PM +0900, Masami Hiramatsu wrote:
> Hi Jiri,
> 
> Sorry for replying late.
> 
> On Fri, 22 Apr 2022 08:47:13 +0200
> Jiri Olsa <olsajiri@...il.com> wrote:
> 
> > On Tue, Apr 19, 2022 at 10:26:05AM +0200, Jiri Olsa wrote:
> > 
> > SNIP
> > 
> > > > > +static int kallsyms_callback(void *data, const char *name,
> > > > > +			     struct module *mod, unsigned long addr)
> > > > > +{
> > > > > +	struct kallsyms_data *args = data;
> > > > > +
> > > > > +	if (!bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp))
> > > > > +		return 0;
> > > > > +
> > > > > +	addr = ftrace_location(addr);
> > > > > +	if (!addr)
> > > > > +		return 0;
> > > > 
> > > > Ooops, wait. Did you do this last version? I missed this point.
> > > > This changes the meanings of the kernel function.
> > > 
> > > yes, it was there before ;-) and you're right.. so some archs can
> > > return different address, I did not realize that
> > > 
> > > > 
> > > > > +
> > > > > +	args->addrs[args->found++] = addr;
> > > > > +	return args->found == args->cnt ? 1 : 0;
> > > > > +}
> > > > > +
> > > > > +/**
> > > > > + * kallsyms_lookup_names - Lookup addresses for array of symbols
> > > > 
> > > > More correctly "Lookup 'ftraced' addresses for array of sorted symbols", right?
> > > > 
> > > > I'm not sure, we can call it as a 'kallsyms' API, since this is using
> > > > kallsyms but doesn't return symbol address, but ftrace address.
> > > > I think this name misleads user to expect returning symbol address.
> > > > 
> > > > > + *
> > > > > + * @syms: array of symbols pointers symbols to resolve, must be
> > > > > + * alphabetically sorted
> > > > > + * @cnt: number of symbols/addresses in @syms/@...rs arrays
> > > > > + * @addrs: array for storing resulting addresses
> > > > > + *
> > > > > + * This function looks up addresses for array of symbols provided in
> > > > > + * @syms array (must be alphabetically sorted) and stores them in
> > > > > + * @addrs array, which needs to be big enough to store at least @cnt
> > > > > + * addresses.
> > > > 
> > > > Hmm, sorry I changed my mind. I rather like to expose kallsyms_on_each_symbol()
> > > > and provide this API from fprobe or ftrace, because this returns ftrace address
> > > > and thus this is only used from fprobe.
> > > 
> > > ok, so how about:
> > > 
> > >   int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs);
> > 
> > quick question.. is it ok if it stays in kalsyms.c object?
> 
> I think if this is for the ftrace API, I think it should be in the ftrace.c, and
> it can remove unneeded #ifdefs in C code.
> 
> > 
> > so we don't need to expose kallsyms_on_each_symbol,
> > and it stays in 'kalsyms' place
> 
> We don't need to expose it to modules, but just make it into a global scope.
> I don't think that doesn't cause a problem.

np, will move it to ftrace

thanks,
jirka