Re: [RFC PATCH 0/3] tracing: Introduce relative stacktrace

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2025-01-28 10:36, Masami Hiramatsu (Google) wrote:
> Hi,
> 
> This introduces relative stacktrace, which records stacktrace entry as
> the offset from _stext instead of raw address. User can enable this
> format by setting options/relative-stacktrace.
> 
> Basically, this does not change anything for users who are using ftrace
> with 'trace' text-formatted interface. This changes how each stacktrace
> entry address is stored, so users who is using 'trace_pipe_raw' needs
> to change how to decode the stacktrace.
> 
> Currently, the stacktrace is stored as raw kernel address. Thus, for
> decoding the binary trace data, we need to refer the kallsyms. But this
> is not useful on the platform which prohibits to access /proc/kallsyms
> for security reason. Since KASLR will change the kernel text address,
> we can not decode symbols without kallsyms in userspace.
> 
> On the other hand, if we record the stacktrace entries in the offset
> from _stext, we can use System.map file to decode it. This is also good
> for the stacktrace in the persistent ring buffer, because we don't need
> to save the kallsyms before crash anymore.
> 
> The problem is to decode the address in the modules because it will be
> loaded in the different place. To solve this issue, I also introduced
> 'module_text_offsets' event, which records module's text and init_text
> info as the offset from _stext when loading it. User can store this
> event in the (another) persistent ring buffer for decoding.

This does not handle the situation where a module is already loaded
before tracing starts. In LTTng we have a statedump facility for this,
where we can iterate on all modules at trace start and dump the relevant
information.

You may want to consider a similar approach for other tracers.

Thanks,

Mathieu

> 
> Thank you,
> 
> ---
> 
> Masami Hiramatsu (Google) (3):
>        tracing: Record stacktrace as the offset from _stext
>        tracing: Introduce "rel_stack" option
>        modules: tracing: Add module_text_offsets event
> 
> 
>   include/trace/events/module.h |   40 ++++++++++++++++++++++++++++++++++++++++
>   kernel/module/main.c          |    1 +
>   kernel/trace/trace.c          |   11 ++++++++++-
>   kernel/trace/trace.h          |    2 ++
>   kernel/trace/trace_entries.h  |   22 ++++++++++++++++++++++
>   kernel/trace/trace_output.c   |   35 +++++++++++++++++++++++++++++++----
>   6 files changed, 106 insertions(+), 5 deletions(-)
> 
> --
> Signature

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com