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

[Francis Laniel <flaniel@linux.microsoft.com>]

Hi.

Le jeudi 17 août 2023, 17:13:03 CEST Steven Rostedt a écrit :
> On Thu, 17 Aug 2023 12:59:30 +0200
> 
> Francis Laniel <flaniel@linux.microsoft.com> wrote:
> > > Why can't you just pass in the address like sysfs does?
> > 
> > To get the addresses from /proc/kallsyms, you need to either have
> > CAP_SYSLOG or even CAP_SYS_ADMIN.
> > But to call perf_event_open(), you only need CAP_PERFMON.
> > This way, by giving only the name you can trace function with less
> > privileges (i.e. without CAP_SYS_ADMIN).
> > Please correct me if I am wrong as I am not an expert in knowing the
> > minimal set of capabilities you need to trace.
> 
> I wonder if we should add an option to put in the non-relocated address?
> One that can be acquired by debuginfo in the vmlinux. I'm assuming that the
> kernel has access to the added offset (I haven't looked). If it does, then
> we could allow users to just add something like "+@0xffffffffdeadbeef" and
> add the relocation offset to get to the mapped address of the function.
> 
> This would allow those without kallsym privileges to pass in kernel address
> for tracing.

This seems interesting but I am wondering about this when using KASLR.
Would it be possible to compute the final address as:
final_address = debuginfo_address + relocation_offset + kaslr_offset?
I will check regarding both the relocation offset and how KASLR works (I only 
know what it does, not how it does it).

Moreover, regarding accessing vmlinux, I can only think to access it through 
vmlinuz which is in /boot.
Sadly, you cannot read /boot/vmlinuz without being root on a several 
distributions.
Note that, the same occurs for /boot/System.map*.

> -- Steve

Best regards.