Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf_kprobe' PMU

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 18 Aug 2023 14:20:33 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 18 Aug 2023 20:13:43 +0200
> Francis Laniel <flaniel@linux.microsoft.com> wrote:
> 
> > Hi.
> > 
> > Le vendredi 18 août 2023, 17:41:41 CEST Steven Rostedt a écrit :
> > > On Fri, 18 Aug 2023 21:37:05 +0900
> > > 
> > > Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:  
> > > > That's why perf probe uses the offset from '_text'. Normal KASLR will just
> > > > moves all symbols. (Finer one will move all symbols randomely)
> > > > This should not need to access /proc/kallsyms but vmlinux or SystemMap.  
> > > 
> > > We could just pass in: "_text+offset" too.  
> > 
> > So, the idea would be to change the existing create_local_trace_kprobe() and 
> > above functions to indicate the user's offset is to be used against _text and 
> > not address?
> 
> No, not to modify that function, but if you know the offset from _text (via
> the vmlinux), you can easily calculate it for that function.

Note that the kprobe-event PMU interface itself allows you to specify
FUNC+OFFSET style;

https://lore.kernel.org/lkml/20171206224518.3598254-5-songliubraving@fb.com/

perf_event_attr::kprobe_func = "_text";
perf_event_attr::probe_offset = OFFSET;

Then, it should be able to specify the correct one. Of course you can use
other unique symbols around the target symbol.

Thank you,

> 
> I mentioned having a way to pass in the vmlinux debug info address and
> subtract the kaslr_offset from it. But that's actually unnecessary. If you
> have the address of the function you want, and the address of _text, both
> from the debug info of vmlinux, you can simply pass in "_text+offset", and
> then use kallsyms to give you _text, and add the offset to give you the
> address for create_local_trace_kprobe().
> 
> -- Steve


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>