[PATCH V4 06/13] ftrace: Add symbols for ftrace trampolines

Adrian Hunter
2020-03-04T09:07:48Z

Symbols are needed for tools to describe instruction addresses. Pages
allocated for ftrace's purposes need symbols to be created for them.
Add such symbols to be visible via /proc/kallsyms.

Example on x86 with CONFIG_DYNAMIC_FTRACE=y

# echo function > /sys/kernel/debug/tracing/current_tracer
# cat /proc/kallsyms | grep '\[__builtin__ftrace\]'
ffffffffc0238000 t ftrace_trampoline [__builtin__ftrace]

Note: This patch adds "__builtin__ftrace" as a module name in /proc/kallsyms for
symbols for pages allocated for ftrace's purposes, even though "__builtin__ftrace"
is not a module.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
include/linux/ftrace.h | 12 ++++---
kernel/kallsyms.c | 5 +++
kernel/trace/ftrace.c | 77 ++++++++++++++++++++++++++++++++++++++++--
3 files changed, 88 insertions(+), 6 deletions(-)

diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
index db95244a62d4..ea726ad1fa83 100644
--- a/include/linux/ftrace.h
+++ b/include/linux/ftrace.h
@@ -58,9 +58,6 @@ struct ftrace_direct_func;
const char *
ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
unsigned long *off, char **modname, char *sym);
-int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *value,
- char *type, char *name,
- char *module_name, int *exported);
#else
static inline const char *
ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
@@ -68,6 +65,13 @@ ftrace_mod_address_lookup(unsigned long addr, unsigned long *size,
{
return NULL;
}
+#endif
+
+#if defined(CONFIG_FUNCTION_TRACER) && defined(CONFIG_DYNAMIC_FTRACE)
+int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *value,
+ char *type, char *name,
+ char *module_name, int *exported);
+#else
static inline int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *value,
char *type, char *name,
char *module_name, int *exported)
@@ -76,7 +80,6 @@ static inline int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *val
}
#endif

-
#ifdef CONFIG_FUNCTION_TRACER

extern int ftrace_enabled;
@@ -207,6 +210,7 @@ struct ftrace_ops {
struct ftrace_ops_hash old_hash;
unsigned long trampoline;
unsigned long trampoline_size;
+ struct list_head list;
#endif
};

diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
index 4a93511e6243..24638586a39e 100644
--- a/kernel/kallsyms.c
+++ b/kernel/kallsyms.c
@@ -483,6 +483,11 @@ static int get_ksymbol_mod(struct kallsym_iter *iter)
return 1;
}

+/*
+ * ftrace_mod_get_kallsym() may also get symbols for pages allocated for ftrace
+ * purposes. In that case "__builtin__ftrace" is used as a module name, even
+ * though "__builtin__ftrace" is not a module.
+ */
static int get_ksymbol_ftrace_mod(struct kallsym_iter *iter)
{
int ret = ftrace_mod_get_kallsym(iter->pos - iter->pos_mod_end,
diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index 9bf1f2cd515e..aa3149fd1fc2 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -2768,6 +2768,38 @@ void __weak arch_ftrace_trampoline_free(struct ftrace_ops *ops)
{
}

+/* List of trace_ops that have allocated trampolines */
+static LIST_HEAD(ftrace_ops_trampoline_list);
+
+static void ftrace_add_trampoline_to_kallsyms(struct ftrace_ops *ops)
+{
+ lockdep_assert_held(&ftrace_lock);
+ list_add_rcu(&ops->list, &ftrace_ops_trampoline_list);
+}
+
+static void ftrace_remove_trampoline_from_kallsyms(struct ftrace_ops *ops)
+{
+ lockdep_assert_held(&ftrace_lock);
+ list_del_rcu(&ops->list);
+}
+
+/*
+ * "__builtin__ftrace" is used as a module name in /proc/kallsyms for symbols
+ * for pages allocated for ftrace purposes, even though "__builtin__ftrace" is
+ * not a module.
+ */
+#define FTRACE_TRAMPOLINE_MOD "__builtin__ftrace"
+#define FTRACE_TRAMPOLINE_SYM "ftrace_trampoline"
+
+static void ftrace_trampoline_free(struct ftrace_ops *ops)
+{
+ if (ops && (ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP) &&
+ ops->trampoline)
+ ftrace_remove_trampoline_from_kallsyms(ops);
+
+ arch_ftrace_trampoline_free(ops);
+}
+
static void ftrace_startup_enable(int command)
{
if (saved_ftrace_func != ftrace_trace_function) {
@@ -2938,7 +2970,7 @@ int ftrace_shutdown(struct ftrace_ops *ops, int command)
synchronize_rcu_tasks();

free_ops:
- arch_ftrace_trampoline_free(ops);
+ ftrace_trampoline_free(ops);
}

return 0;
@@ -6174,6 +6206,27 @@ struct ftrace_mod_map {
unsigned int num_funcs;
};

+static int ftrace_get_trampoline_kallsym(unsigned int symnum,
+ unsigned long *value, char *type,
+ char *name, char *module_name,
+ int *exported)
+{
+ struct ftrace_ops *op;
+
+ list_for_each_entry_rcu(op, &ftrace_ops_trampoline_list, list) {
+ if (!op->trampoline || symnum--)
+ continue;
+ *value = op->trampoline;
+ *type = 't';
+ strlcpy(name, FTRACE_TRAMPOLINE_SYM, KSYM_NAME_LEN);
+ strlcpy(module_name, FTRACE_TRAMPOLINE_MOD, MODULE_NAME_LEN);
+ *exported = 0;
+ return 0;
+ }
+
+ return -ERANGE;
+}
+
#ifdef CONFIG_MODULES

#define next_to_ftrace_page(p) container_of(p, struct ftrace_page, next)
@@ -6510,6 +6563,7 @@ int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *value,
{
struct ftrace_mod_map *mod_map;
struct ftrace_mod_func *mod_func;
+ int ret;

preempt_disable();
list_for_each_entry_rcu(mod_map, &ftrace_mod_maps, list) {
@@ -6536,8 +6590,10 @@ int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *value,
WARN_ON(1);
break;
}
+ ret = ftrace_get_trampoline_kallsym(symnum, value, type, name,
+ module_name, exported);
preempt_enable();
- return -ERANGE;
+ return ret;
}

#else
@@ -6549,6 +6605,18 @@ allocate_ftrace_mod_map(struct module *mod,
{
return NULL;
}
+int ftrace_mod_get_kallsym(unsigned int symnum, unsigned long *value,
+ char *type, char *name, char *module_name,
+ int *exported)
+{
+ int ret;
+
+ preempt_disable();
+ ret = ftrace_get_trampoline_kallsym(symnum, value, type, name,
+ module_name, exported);
+ preempt_enable();
+ return ret;
+}
#endif /* CONFIG_MODULES */

struct ftrace_init_func {
@@ -6729,7 +6797,12 @@ void __weak arch_ftrace_update_trampoline(struct ftrace_ops *ops)

static void ftrace_update_trampoline(struct ftrace_ops *ops)
{
+ unsigned long trampoline = ops->trampoline;
+
arch_ftrace_update_trampoline(ops);
+ if (ops->trampoline && ops->trampoline != trampoline &&
+ (ops->flags & FTRACE_OPS_FL_ALLOC_TRAMP))
+ ftrace_add_trampoline_to_kallsyms(ops);
}

void ftrace_init_trace_array(struct trace_array *tr)
--
2.17.1