Re: [PATCH v5 2/9] fprobe: Add ftrace based probe APIs

Masami Hiramatsu
2022-01-26T02:50:33Z

On Tue, 25 Jan 2022 19:11:52 +0100
Jiri Olsa <jolsa@redhat.com> wrote:

> On Tue, Jan 25, 2022 at 05:41:24PM +0100, Jiri Olsa wrote:
> > On Tue, Jan 25, 2022 at 09:11:57PM +0900, Masami Hiramatsu wrote:
> >
> > SNIP
> >
> > > +
> > > +/* Convert ftrace location address from symbols */
> > > +static int convert_func_addresses(struct fprobe *fp)
> > > +{
> > > + unsigned long addr, size;
> > > + unsigned int i;
> > > +
> > > + /* Convert symbols to symbol address */
> > > + if (fp->syms) {
> > > + fp->addrs = kcalloc(fp->nentry, sizeof(*fp->addrs), GFP_KERNEL);
> > > + if (!fp->addrs)
> > > + return -ENOMEM;
> > > +
> > > + for (i = 0; i < fp->nentry; i++) {
> > > + fp->addrs[i] = kallsyms_lookup_name(fp->syms[i]);
> > > + if (!fp->addrs[i]) /* Maybe wrong symbol */
> > > + goto error;
> > > + }
> > > + }
> > > +
> > > + /* Convert symbol address to ftrace location. */
> > > + for (i = 0; i < fp->nentry; i++) {
> > > + if (!kallsyms_lookup_size_offset(fp->addrs[i], &size, NULL))
> > > + size = MCOUNT_INSN_SIZE;
> > > + addr = ftrace_location_range(fp->addrs[i], fp->addrs[i] + size);
> >
> > you need to substract 1 from 'end' in here, as explained in
> > __within_notrace_func comment:
> >
> > /*
> > * Since ftrace_location_range() does inclusive range check, we need
> > * to subtract 1 byte from the end address.
> > */
> >
> > like in the patch below

Good catch, I missed that point.

> > also this convert is for archs where address from kallsyms does not match
> > the real attach addresss, like for arm you mentioned earlier, right?

Yes, that's right.

> >
> > could we have that arch specific, so we don't have extra heavy search
> > loop for archs that do not need it?

Hmm, is that so heavy? Even though, this kind of convertion is better
to be implemented in the arch-specific ftrace. They may know the
best way to do, because the offset is fixed for each arch.

>
> one more question..
>
> I'm adding support for user to pass function symbols to bpf fprobe link
> and I thought I'd pass symbols array to register_fprobe, but I'd need to
> copy the whole array of strings from user space first, which could take
> lot of memory considering attachment of 10k+ functions
>
> so I'm thinking better way is to resolve symbols already in bpf fprobe
> link code and pass just addresses to register_fprobe

That is OK. Fprobe accepts either ::syms or ::addrs.

>
> I assume you want to keep symbol interface, right? could we have some
> flag ensuring the conversion code is skipped, so we don't go through
> it twice?

Yeah, we still have many unused bits in fprobe::flags. :)

Thank you,

> in any case I need addresses before I call register_fprobe, because
> of the bpf cookies setup
>
> thanks,
> jirka
>

--
Masami Hiramatsu <mhiramat@kernel.org>