Messages in this thread | | | From | Francis Laniel <> | Subject | Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf_kprobe' PMU | Date | Fri, 18 Aug 2023 11:01:19 +0200 |
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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.
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