[RFC PATCH 1/4] tracing: add __print_sym() to replace __print_symbolic()

From: Johannes Berg
Date: Thu Sep 21 2023 - 13:26:03 EST

From: Johannes Berg <johannes.berg@xxxxxxxxx>

The way __print_symbolic() works is limited and inefficient
in multiple ways:
- you can only use it with a static list of symbols, but
e.g. the SKB dropreasons are now a dynamic list

- it builds the list in memory _three_ times, so it takes
a lot of memory:
- The print_fmt contains the list (since it's passed to
the macro there). This actually contains the names
_twice_, which is fixed up at runtime.
- TRACE_DEFINE_ENUM() puts a 24-byte struct trace_eval_map
for every entry, plus the string pointed to by it, which
cannot be deduplicated with the strings in the print_fmt
- The in-kernel symbolic printing creates yet another list
of struct trace_print_flags for trace_print_symbols_seq()

- it also requires runtime fixup during init, which is a lot
of string parsing due to the print_fmt fixup

Introduce __print_sym() to - over time - replace the old one.
We can easily extend this also to __print_flags later, but I
cared more about the SKB dropreasons for now.

This new __print_sym() requires only a single list of items,
created by TRACE_DEFINE_SYM_LIST(), for can even use another
already existing list by using TRACE_DEFINE_SYM_FNS() with
lookup and show methods.

Then, instead of doing an init-time fixup, just do this at the
time when userspace reads the print_fmt. This way, dynamically
updated lists are possible.

For userspace, nothing actually changes, because the print_fmt
is shown exactly the same way the old __print_symbolic() was.

This adds about 4k .text in my test builds, but that'll be
more than paid for by the actual conversions.

OPEN QUESTIONS:
- do we need TRACE_EVENT_FL_DYNPRINT or should we just always
go through show_print_fmt()? If we do, we can get rid of
set_event_dynprint() which is also a stupid function, and
somewhat inefficient.

- Is the WARN_ON() in show_print_fmt() correct? Can we have a
dynamic event using this infrastructure? I don't know about
dynamic events enough.

- Is the double-slash (//) thing a good approach? I was looking
for something that couldn't happen in C, and that's a comment
so it shouldn't be possible ... It's only internal though so
we could always change it. Parsing to the next , means we'd
have to parse a LOT of C, so that's out of the question.

- Is it correct that we can assume RCU critical section when in
the lookup function? The SKB code currently does, but I may
not ever have actually run this code yet.

- Where should all the functions be implemented :-)

Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx>
---
include/asm-generic/vmlinux.lds.h | 3 +-
include/linux/module.h | 2 +
include/linux/trace_events.h | 10 ++
include/linux/tracepoint.h | 20 ++++
include/trace/stages/init.h | 54 +++++++++
include/trace/stages/stage2_data_offsets.h | 6 +
include/trace/stages/stage3_trace_output.h | 9 ++
include/trace/stages/stage7_class_define.h | 4 +
kernel/module/main.c | 3 +
kernel/trace/trace_events.c | 131 ++++++++++++++++++++-
kernel/trace/trace_output.c | 43 +++++++
11 files changed, 282 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 9c59409104f6..b2b45ff0a701 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -263,7 +263,8 @@
#define FTRACE_EVENTS() \
. = ALIGN(8); \
BOUNDED_SECTION(_ftrace_events) \
- BOUNDED_SECTION_BY(_ftrace_eval_map, _ftrace_eval_maps)
+ BOUNDED_SECTION_BY(_ftrace_eval_map, _ftrace_eval_maps) \
+ BOUNDED_SECTION(_ftrace_sym_defs)
#else
#define FTRACE_EVENTS()
#endif
diff --git a/include/linux/module.h b/include/linux/module.h
index a98e188cf37b..bf14b74a872c 100644
--- a/include/linux/module.h
+++ b/include/linux/module.h
@@ -524,6 +524,8 @@ struct module {
unsigned int num_trace_events;
struct trace_eval_map **trace_evals;
unsigned int num_trace_evals;
+ struct trace_sym_def **trace_sym_defs;
+ unsigned int num_trace_sym_defs;
#endif
#ifdef CONFIG_FTRACE_MCOUNT_RECORD
unsigned int num_ftrace_callsites;
diff --git a/include/linux/trace_events.h b/include/linux/trace_events.h
index eb5c3add939b..9ab651a8fa0b 100644
--- a/include/linux/trace_events.h
+++ b/include/linux/trace_events.h
@@ -24,6 +24,13 @@ const char *trace_print_flags_seq(struct trace_seq *p, const char *delim,
const char *trace_print_symbols_seq(struct trace_seq *p, unsigned long val,
const struct trace_print_flags *symbol_array);

+const char *trace_print_sym_seq(struct trace_seq *p, unsigned long long val,
+ const char *(*lookup)(unsigned long long val));
+const char *trace_sym_lookup(const struct trace_sym_entry *list,
+ size_t len, unsigned long long value);
+void trace_sym_show(struct seq_file *m,
+ const struct trace_sym_entry *list, size_t len);
+
#if BITS_PER_LONG == 32
const char *trace_print_flags_seq_u64(struct trace_seq *p, const char *delim,
unsigned long long flags,
@@ -331,6 +338,7 @@ enum {
TRACE_EVENT_FL_EPROBE_BIT,
TRACE_EVENT_FL_FPROBE_BIT,
TRACE_EVENT_FL_CUSTOM_BIT,
+ TRACE_EVENT_FL_DYNPRINT_BIT,
};

/*
@@ -348,6 +356,7 @@ enum {
* CUSTOM - Event is a custom event (to be attached to an exsiting tracepoint)
* This is set when the custom event has not been attached
* to a tracepoint yet, then it is cleared when it is.
+ * DYNPRINT - Event has dynamic symbol prints
*/
enum {
TRACE_EVENT_FL_FILTERED = (1 << TRACE_EVENT_FL_FILTERED_BIT),
@@ -361,6 +370,7 @@ enum {
TRACE_EVENT_FL_EPROBE = (1 << TRACE_EVENT_FL_EPROBE_BIT),
TRACE_EVENT_FL_FPROBE = (1 << TRACE_EVENT_FL_FPROBE_BIT),
TRACE_EVENT_FL_CUSTOM = (1 << TRACE_EVENT_FL_CUSTOM_BIT),
+ TRACE_EVENT_FL_DYNPRINT = (1 << TRACE_EVENT_FL_DYNPRINT_BIT),
};

#define TRACE_EVENT_FL_UKPROBE (TRACE_EVENT_FL_KPROBE | TRACE_EVENT_FL_UPROBE)
diff --git a/include/linux/tracepoint.h b/include/linux/tracepoint.h
index 88c0ba623ee6..7650019f4347 100644
--- a/include/linux/tracepoint.h
+++ b/include/linux/tracepoint.h
@@ -31,6 +31,24 @@ struct trace_eval_map {
unsigned long eval_value;
};

+struct trace_sym_def {
+ const char *system;
+ const char *symbol_id;
+ /* may return NULL */
+ const char * (*lookup)(unsigned long long);
+ /*
+ * Must print the list: ', { val, "name"}, ...'
+ * with no trailing comma, but with the leading ', '
+ * to simplify things:
+ */
+ void (*show)(struct seq_file *);
+};
+
+struct trace_sym_entry {
+ unsigned long long value;
+ const char *name;
+};
+
#define TRACEPOINT_DEFAULT_PRIO 10

extern struct srcu_struct tracepoint_srcu;
@@ -109,6 +127,8 @@ extern void syscall_unregfunc(void);

#define TRACE_DEFINE_ENUM(x)
#define TRACE_DEFINE_SIZEOF(x)
+#define TRACE_DEFINE_SYM_FNS(...)
+#define TRACE_DEFINE_SYM_LIST(...)

#ifdef CONFIG_HAVE_ARCH_PREL32_RELOCATIONS
static inline struct tracepoint *tracepoint_ptr_deref(tracepoint_ptr_t *p)
diff --git a/include/trace/stages/init.h b/include/trace/stages/init.h
index 000bcfc8dd2e..251ab22485d1 100644
--- a/include/trace/stages/init.h
+++ b/include/trace/stages/init.h
@@ -23,6 +23,60 @@ TRACE_MAKE_SYSTEM_STR();
__section("_ftrace_eval_map") \
*TRACE_SYSTEM##_##a = &__##TRACE_SYSTEM##_##a

+/*
+ * Define a symbol for __print_sym by giving lookup and
+ * show functions. See &struct trace_sym_def.
+ */
+#define TRACE_DEFINE_SYM_FNS(_symbol_id, _lookup, _show) \
+ _TRACE_DEFINE_SYM_FNS(TRACE_SYSTEM, _symbol_id, _lookup, _show)
+#define _TRACE_DEFINE_SYM_FNS(_system, _symbol_id, _lookup, _show) \
+ __TRACE_DEFINE_SYM_FNS(_system, _symbol_id, _lookup, _show)
+#define __TRACE_DEFINE_SYM_FNS(_system, _symbol_id, _lookup, _show) \
+ ___TRACE_DEFINE_SYM_FNS(_system ## _ ## _symbol_id, _symbol_id, \
+ _lookup, _show)
+#define ___TRACE_DEFINE_SYM_FNS(_name, _symbol_id, _lookup, _show) \
+ static struct trace_sym_def \
+ __trace_sym_def_ ## _name = { \
+ .system = TRACE_SYSTEM_STRING, \
+ /* need the ) for later strcmp */ \
+ .symbol_id = #_symbol_id ")", \
+ .lookup = _lookup, \
+ .show = _show, \
+ }; \
+ static struct trace_sym_def \
+ __section("_ftrace_sym_defs") \
+ *__trace_sym_def_p_ ## _name = &__trace_sym_def_ ## _name
+
+/*
+ * Define a symbol for __print_sym by giving lookup and
+ * show functions. See &struct trace_sym_def.
+ */
+#define TRACE_DEFINE_SYM_LIST(_symbol_id, ...) \
+ _TRACE_DEFINE_SYM_LIST(TRACE_SYSTEM, _symbol_id, __VA_ARGS__)
+#define _TRACE_DEFINE_SYM_LIST(_system, _symbol_id, ...) \
+ __TRACE_DEFINE_SYM_LIST(_system, _symbol_id, __VA_ARGS__)
+#define __TRACE_DEFINE_SYM_LIST(_system, _symbol_id, ...) \
+ ___TRACE_DEFINE_SYM_LIST(_system ## _ ## _symbol_id, _symbol_id,\
+ __VA_ARGS__)
+#define ___TRACE_DEFINE_SYM_LIST(_name, _symbol_id, ...) \
+ static struct trace_sym_entry \
+ __trace_sym_list_ ## _name[] = { __VA_ARGS__ }; \
+ static const char * \
+ __trace_sym_lookup_ ## _name(unsigned long long value) \
+ { \
+ return trace_sym_lookup(__trace_sym_list_ ## _name, \
+ ARRAY_SIZE(__trace_sym_list_ ## _name), value); \
+ } \
+ static void \
+ __trace_sym_show_ ## _name(struct seq_file *m) \
+ { \
+ trace_sym_show(m, __trace_sym_list_ ## _name, \
+ ARRAY_SIZE(__trace_sym_list_ ## _name)); \
+ } \
+ ___TRACE_DEFINE_SYM_FNS(_name, _symbol_id, \
+ __trace_sym_lookup_ ## _name, \
+ __trace_sym_show_ ## _name)
+
#undef TRACE_DEFINE_SIZEOF
#define TRACE_DEFINE_SIZEOF(a) \
static struct trace_eval_map __used __initdata \
diff --git a/include/trace/stages/stage2_data_offsets.h b/include/trace/stages/stage2_data_offsets.h
index 469b6a64293d..83a287f78501 100644
--- a/include/trace/stages/stage2_data_offsets.h
+++ b/include/trace/stages/stage2_data_offsets.h
@@ -5,6 +5,12 @@
#undef TRACE_DEFINE_ENUM
#define TRACE_DEFINE_ENUM(a)

+#undef TRACE_DEFINE_SYM_FNS
+#define TRACE_DEFINE_SYM_FNS(_symbol_id, _lookup, _show)
+
+#undef TRACE_DEFINE_SYM_LIST
+#define TRACE_DEFINE_SYM_LIST(_symbol_id, ...)
+
#undef TRACE_DEFINE_SIZEOF
#define TRACE_DEFINE_SIZEOF(a)

diff --git a/include/trace/stages/stage3_trace_output.h b/include/trace/stages/stage3_trace_output.h
index c1fb1355d309..d2c6458b62dc 100644
--- a/include/trace/stages/stage3_trace_output.h
+++ b/include/trace/stages/stage3_trace_output.h
@@ -79,6 +79,15 @@
trace_print_symbols_seq(p, value, symbols); \
})

+#undef __print_sym
+#define __print_sym(value, symbol_id) \
+ ___print_sym(TRACE_SYSTEM, value, symbol_id)
+#define ___print_sym(sys, value, symbol_id) \
+ ____print_sym(sys, value, symbol_id)
+#define ____print_sym(sys, value, symbol_id) \
+ trace_print_sym_seq(p, value, \
+ __trace_sym_def_p_ ## sys ## _ ## symbol_id->lookup)
+
#undef __print_flags_u64
#undef __print_symbolic_u64
#if BITS_PER_LONG == 32
diff --git a/include/trace/stages/stage7_class_define.h b/include/trace/stages/stage7_class_define.h
index bcb960d16fc0..ce802172b5d7 100644
--- a/include/trace/stages/stage7_class_define.h
+++ b/include/trace/stages/stage7_class_define.h
@@ -25,6 +25,10 @@
#undef __print_hex_dump
#undef __get_buf

+#undef __print_sym
+/* use // as the separator since it'd be a comment in C */
+#define __print_sym(value, symbol_id) __print_sym(value\/\/symbol_id)
+
/*
* The below is not executed in the kernel. It is only what is
* displayed in the print format for userspace to parse.
diff --git a/kernel/module/main.c b/kernel/module/main.c
index 98fedfdb8db5..7752c3883cd2 100644
--- a/kernel/module/main.c
+++ b/kernel/module/main.c
@@ -2161,6 +2161,9 @@ static int find_module_sections(struct module *mod, struct load_info *info)
mod->trace_evals = section_objs(info, "_ftrace_eval_map",
sizeof(*mod->trace_evals),
&mod->num_trace_evals);
+ mod->trace_sym_defs = section_objs(info, "_ftrace_sym_defs",
+ sizeof(*mod->trace_sym_defs),
+ &mod->num_trace_sym_defs);
#endif
#ifdef CONFIG_TRACING
mod->trace_bprintk_fmt_start = section_objs(info, "__trace_printk_fmt",
diff --git a/kernel/trace/trace_events.c b/kernel/trace/trace_events.c
index ed367d713be0..38eaa7dce078 100644
--- a/kernel/trace/trace_events.c
+++ b/kernel/trace/trace_events.c
@@ -1559,6 +1559,104 @@ static void *f_next(struct seq_file *m, void *v, loff_t *pos)
return node;
}

+extern struct trace_sym_def *__start_ftrace_sym_defs[];
+extern struct trace_sym_def *__stop_ftrace_sym_defs[];
+
+static void show_sym_list(struct seq_file *m, struct trace_event_call *call,
+ const char *name)
+{
+ struct trace_sym_def **sym_defs;
+ unsigned int n_sym_defs, i;
+
+ if (call->module) {
+ struct module *mod = call->module;
+
+ sym_defs = mod->trace_sym_defs;
+ n_sym_defs = mod->num_trace_sym_defs;
+ } else {
+ sym_defs = __start_ftrace_sym_defs;
+ n_sym_defs = __stop_ftrace_sym_defs - __start_ftrace_sym_defs;
+ }
+
+ for (i = 0; i < n_sym_defs; i++) {
+ if (!sym_defs[i])
+ continue;
+ if (sym_defs[i]->system != call->class->system)
+ continue;
+ if (strncmp(name, sym_defs[i]->symbol_id,
+ strlen(sym_defs[i]->symbol_id)))
+ continue;
+ if (sym_defs[i]->show)
+ sym_defs[i]->show(m);
+ break;
+ }
+}
+
+static void show_print_fmt(struct seq_file *m, struct trace_event_call *call)
+{
+ char *ptr = call->print_fmt;
+ int quote = 0;
+ int wait_for_doubleslash = 0;
+
+ if (!(call->flags & TRACE_EVENT_FL_DYNPRINT) ||
+ /* WARN because dynamic events have no module */
+ WARN_ON_ONCE(call->flags & TRACE_EVENT_FL_DYNAMIC)) {
+ seq_printf(m, "\nprint fmt: %s\n", call->print_fmt);
+ return;
+ }
+
+ seq_puts(m, "\nprint fmt: ");
+ while (*ptr) {
+ if (*ptr == '\\') {
+ seq_putc(m, *ptr);
+ ptr++;
+ /* paranoid */
+ if (!*ptr)
+ break;
+ goto next;
+ }
+ if (*ptr == '"') {
+ quote ^= 1;
+ goto next;
+ }
+ if (quote)
+ goto next;
+
+ if (wait_for_doubleslash == 1 && *ptr == '/') {
+ wait_for_doubleslash = 2;
+ ptr++;
+ continue;
+ } else if (wait_for_doubleslash == 2 && *ptr != '/') {
+ seq_putc(m, '/');
+ wait_for_doubleslash = 1;
+ goto next;
+ } else if (wait_for_doubleslash == 2 && *ptr == '/') {
+ const char *name;
+
+ ptr++;
+ name = ptr;
+ /* skip the name */
+ while (*ptr && *ptr != ')')
+ ptr++;
+ wait_for_doubleslash = 0;
+ show_sym_list(m, call, name);
+ continue;
+ }
+
+ if (strncmp(ptr, "__print_sym(", 12) == 0) {
+ ptr += 12;
+ seq_puts(m, "__print_symbolic(");
+ wait_for_doubleslash = 1;
+ continue;
+ }
+next:
+ seq_putc(m, *ptr);
+ ptr++;
+ }
+
+ seq_putc(m, '\n');
+}
+
static int f_show(struct seq_file *m, void *v)
{
struct trace_event_call *call = event_file_data(m->private);
@@ -1577,8 +1675,7 @@ static int f_show(struct seq_file *m, void *v)
return 0;

case FORMAT_PRINTFMT:
- seq_printf(m, "\nprint fmt: %s\n",
- call->print_fmt);
+ show_print_fmt(m, call);
return 0;
}

@@ -2552,6 +2649,32 @@ static int event_init(struct trace_event_call *call)
return ret;
}

+static void set_event_dynprint(struct trace_event_call *call)
+{
+ char *ptr;
+ int quote = 0;
+
+ for (ptr = call->print_fmt; *ptr; ptr++) {
+ if (*ptr == '\\') {
+ ptr++;
+ /* paranoid */
+ if (!*ptr)
+ break;
+ continue;
+ }
+ if (*ptr == '"') {
+ quote ^= 1;
+ continue;
+ }
+ if (quote)
+ continue;
+ if (strncmp(ptr, "__print_sym(", 12) == 0) {
+ call->flags |= TRACE_EVENT_FL_DYNPRINT;
+ break;
+ }
+ }
+}
+
static int
__register_event(struct trace_event_call *call, struct module *mod)
{
@@ -2567,6 +2690,8 @@ __register_event(struct trace_event_call *call, struct module *mod)
else
call->module = mod;

+ set_event_dynprint(call);
+
return 0;
}

@@ -3810,6 +3935,8 @@ static __init int event_trace_enable(void)
ret = event_init(call);
if (!ret)
list_add(&call->list, &ftrace_events);
+
+ set_event_dynprint(call);
}

register_trigger_cmds();
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index db575094c498..9196f408a1d2 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -124,6 +124,49 @@ trace_print_symbols_seq(struct trace_seq *p, unsigned long val,
}
EXPORT_SYMBOL(trace_print_symbols_seq);

+const char *trace_sym_lookup(const struct trace_sym_entry *list,
+ size_t len, unsigned long long value)
+{
+ size_t i;
+
+ for (i = 0; i < len; i++) {
+ if (list[i].value == value)
+ return list[i].name;
+ }
+ return NULL;
+}
+EXPORT_SYMBOL(trace_sym_lookup);
+
+void trace_sym_show(struct seq_file *m,
+ const struct trace_sym_entry *list, size_t len)
+{
+ size_t i;
+
+ for (i = 0; i < len; i++)
+ seq_printf(m, ", { %lld, \"%s\" }",
+ list[i].value, list[i].name);
+}
+EXPORT_SYMBOL(trace_sym_show);
+
+const char *
+trace_print_sym_seq(struct trace_seq *p, unsigned long long val,
+ const char *(*lookup)(unsigned long long val))
+{
+ const char *ret = trace_seq_buffer_ptr(p);
+ const char *name;
+
+ name = lookup(val);
+ if (name)
+ trace_seq_puts(p, name);
+ else
+ trace_seq_printf(p, "0x%llx", val);
+
+ trace_seq_putc(p, 0);
+
+ return ret;
+}
+EXPORT_SYMBOL(trace_print_sym_seq);
+
#if BITS_PER_LONG == 32
const char *
trace_print_flags_seq_u64(struct trace_seq *p, const char *delim,
--
2.41.0