Re: [PATCH] tracing: preserve module tracepoint strings

Petr Pavlu
2026-04-09T12:37:19Z

On 4/6/26 7:09 PM, Cao Ruichuang wrote:
> tracepoint_string() is documented as exporting constant strings
> through printk_formats, including when it is used from modules.
> That currently does not work.
>
> A small test module that calls
> tracepoint_string("tracepoint_string_test_module_string") loads
> successfully and gets a pointer back, but the string never appears
> in /sys/kernel/tracing/printk_formats. The loader only collects
> __trace_printk_fmt from modules and ignores __tracepoint_str.
>
> Collect module __tracepoint_str entries too, copy them to stable
> tracing-managed storage like module trace_printk formats, and let
> trace_is_tracepoint_string() recognize those copied strings. This
> makes module tracepoint strings visible through printk_formats and
> keeps them accepted by the trace string safety checks.

The documentation for tracepoint_string() in include/linux/tracepoint.h
contains:

* The @str used must be a constant string and persistent as it would not
* make sense to show a string that no longer exists. But it is still fine
* to be used with modules, because when modules are unloaded, if they
* had tracepoints, the ring buffers are cleared too. As long as the string
* does not change during the life of the module, it is fine to use
* tracepoint_string() within a module.

The implemented approach copies all strings referenced by
__tracepoint_str and keeps them for the kernel's lifetime. I wonder if
the code could directly reference the original data and handle its
removal when the module is unloaded, or if the quoted comment should be
updated to reflect the new behavior.

--
Thanks,
Petr