Re: [PATCH v2] tracing: preserve module tracepoint strings

Petr Pavlu
2026-04-13T09:40:14Z

On 4/10/26 7:18 AM, 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.
>
> Update the tracepoint_string() documentation to describe this
> module behavior explicitly, so the comment matches the preserved
> module-string mappings exported by tracing.
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=217196
> Signed-off-by: Cao Ruichuang <create0818@163.com>
> ---
> v2:
> - update tracepoint_string() documentation to describe the preserved
> module-string mapping explicitly
> - address Petr Pavlu's review about the comment not matching the
> implemented module behavior

I questioned in my previous comment whether the data associated with
tracepoint_string() could be dropped when the module that created it is
unloaded. Typically, modules should not leave any data behind when they
are removed. Note how kernel/trace/trace_events.c tracks event fields
using add_str_to_module() and releases them in
trace_module_remove_events(). In practice, I suppose this isn't a large
problem because the usage of tracepoint_string() is limited and one
won't typically load/unload different modules that use this facility.
Nonetheless, what is the reason for keeping the tracepoint_string()
data for unloaded modules?

--
Thanks,
Petr