[PATCH v2] tracing: preserve module tracepoint strings

Cao Ruichuang
2026-04-10T05:19:24Z

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

include/linux/module.h | 2 ++
include/linux/tracepoint.h | 14 ++++++---
kernel/module/main.c | 4 +++
kernel/trace/trace_printk.c | 63 ++++++++++++++++++++++++++++---------
4 files changed, 63 insertions(+), 20 deletions(-)

diff --git a/include/linux/module.h b/include/linux/module.h
index 14f391b186c..e475466a785 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -515,6 +515,8 @@ struct module {
#ifdef CONFIG_TRACING
unsigned int num_trace_bprintk_fmt;
const char **trace_bprintk_fmt_start;
+ unsigned int num_tracepoint_strings;
+ const char **tracepoint_strings_start;
#endif
#ifdef CONFIG_EVENT_TRACING
struct trace_event_call **trace_events;
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 1d7f29f5e90..f14da542402 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -475,11 +475,15 @@ static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
* the ASCII strings they represent.
*
* 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.
+ * make sense to show a string that no longer exists.
+ *
+ * For built-in code, the tracing system uses the original string address.
+ * For modules, the tracing code saves tracepoint strings into
+ * tracing-managed storage when the module loads, so their mappings remain
+ * available through printk_formats and trace string checks even after the
+ * module's own memory goes away. As long as the string does not change
+ * during the life of the module, it is fine to use tracepoint_string()
+ * within a module.
*/
#define tracepoint_string(str) \
({ \
diff --git a/kernel/module/main.c b/kernel/module/main.c
index c3ce106c70a..d7d890138ac 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2672,6 +2672,10 @@ static int find_module_sections(struct module *mod, struct load_info *info)
mod->trace_bprintk_fmt_start = section_objs(info, "__trace_printk_fmt",
sizeof(*mod->trace_bprintk_fmt_start),
&mod->num_trace_bprintk_fmt);
+ mod->tracepoint_strings_start =
+ section_objs(info, "__tracepoint_str",
+ sizeof(*mod->tracepoint_strings_start),
+ &mod->num_tracepoint_strings);
#endif
#ifdef CONFIG_DYNAMIC_FTRACE
/* sechdrs[0].sh_size is always zero */
diff --git a/kernel/trace/trace_printk.c b/kernel/trace/trace_printk.c
index 5ea5e0d76f0..9f67ce42ef6 100644
--- a/kernel/trace/trace_printk.c
+++ b/kernel/trace/trace_printk.c
@@ -22,8 +22,9 @@
#ifdef CONFIG_MODULES

/*
- * modules trace_printk()'s formats are autosaved in struct trace_bprintk_fmt
- * which are queued on trace_bprintk_fmt_list.
+ * modules trace_printk() formats and tracepoint_string() strings are
+ * autosaved in struct trace_bprintk_fmt, which are queued on
+ * trace_bprintk_fmt_list.
*/
static LIST_HEAD(trace_bprintk_fmt_list);

@@ -33,8 +34,12 @@ static DEFINE_MUTEX(btrace_mutex);
struct trace_bprintk_fmt {
struct list_head list;
const char *fmt;
+ unsigned int type;
};

+#define TRACE_BPRINTK_TYPE BIT(0)
+#define TRACE_TRACEPOINT_TYPE BIT(1)
+
static inline struct trace_bprintk_fmt *lookup_format(const char *fmt)
{
struct trace_bprintk_fmt *pos;
@@ -49,22 +54,24 @@ static inline struct trace_bprintk_fmt *lookup_format(const char *fmt)
return NULL;
}

-static
-void hold_module_trace_bprintk_format(const char **start, const char **end)
+static void hold_module_trace_format(const char **start, const char **end,
+ unsigned int type)
{
const char **iter;
char *fmt;

/* allocate the trace_printk per cpu buffers */
- if (start != end)
+ if ((type & TRACE_BPRINTK_TYPE) && start != end)
trace_printk_init_buffers();

mutex_lock(&btrace_mutex);
for (iter = start; iter < end; iter++) {
struct trace_bprintk_fmt *tb_fmt = lookup_format(*iter);
if (tb_fmt) {
- if (!IS_ERR(tb_fmt))
+ if (!IS_ERR(tb_fmt)) {
+ tb_fmt->type |= type;
*iter = tb_fmt->fmt;
+ }
continue;
}

@@ -76,6 +83,7 @@ void hold_module_trace_bprintk_format(const char **start, const char **end)
list_add_tail(&tb_fmt->list, &trace_bprintk_fmt_list);
strcpy(fmt, *iter);
tb_fmt->fmt = fmt;
+ tb_fmt->type = type;
} else
kfree(tb_fmt);
}
@@ -85,17 +93,28 @@ void hold_module_trace_bprintk_format(const char **start, const char **end)
mutex_unlock(&btrace_mutex);
}

-static int module_trace_bprintk_format_notify(struct notifier_block *self,
- unsigned long val, void *data)
+static int module_trace_format_notify(struct notifier_block *self,
+ unsigned long val, void *data)
{
struct module *mod = data;
+
+ if (val != MODULE_STATE_COMING)
+ return NOTIFY_OK;
+
if (mod->num_trace_bprintk_fmt) {
const char **start = mod->trace_bprintk_fmt_start;
const char **end = start + mod->num_trace_bprintk_fmt;

- if (val == MODULE_STATE_COMING)
- hold_module_trace_bprintk_format(start, end);
+ hold_module_trace_format(start, end, TRACE_BPRINTK_TYPE);
+ }
+
+ if (mod->num_tracepoint_strings) {
+ const char **start = mod->tracepoint_strings_start;
+ const char **end = start + mod->num_tracepoint_strings;
+
+ hold_module_trace_format(start, end, TRACE_TRACEPOINT_TYPE);
}
+
return NOTIFY_OK;
}

@@ -171,8 +190,8 @@ static void format_mod_stop(void)

#else /* !CONFIG_MODULES */
__init static int
-module_trace_bprintk_format_notify(struct notifier_block *self,
- unsigned long val, void *data)
+module_trace_format_notify(struct notifier_block *self,
+ unsigned long val, void *data)
{
return NOTIFY_OK;
}
@@ -193,8 +212,8 @@ void trace_printk_control(bool enabled)
}

__initdata_or_module static
-struct notifier_block module_trace_bprintk_format_nb = {
- .notifier_call = module_trace_bprintk_format_notify,
+struct notifier_block module_trace_format_nb = {
+ .notifier_call = module_trace_format_notify,
};

int __trace_bprintk(unsigned long ip, const char *fmt, ...)
@@ -254,11 +273,25 @@ EXPORT_SYMBOL_GPL(__ftrace_vprintk);
bool trace_is_tracepoint_string(const char *str)
{
const char **ptr = __start___tracepoint_str;
+#ifdef CONFIG_MODULES
+ struct trace_bprintk_fmt *tb_fmt;
+#endif

for (ptr = __start___tracepoint_str; ptr < __stop___tracepoint_str; ptr++) {
if (str == *ptr)
return true;
}
+
+#ifdef CONFIG_MODULES
+ mutex_lock(&btrace_mutex);
+ list_for_each_entry(tb_fmt, &trace_bprintk_fmt_list, list) {
+ if ((tb_fmt->type & TRACE_TRACEPOINT_TYPE) && str == tb_fmt->fmt) {
+ mutex_unlock(&btrace_mutex);
+ return true;
+ }
+ }
+ mutex_unlock(&btrace_mutex);
+#endif
return false;
}

@@ -824,7 +857,7 @@ fs_initcall(init_trace_printk_function_export);

static __init int init_trace_printk(void)
{
- return register_module_notifier(&module_trace_bprintk_format_nb);
+ return register_module_notifier(&module_trace_format_nb);
}

early_initcall(init_trace_printk);
--
2.39.5 (Apple Git-154)