[PATCH perf-tools-next v2 3/4] perf trace: Enhance BTF type formatting to symbolise kernel function pointers

When BTF (BPF Type Format) metadata is loaded from vmlinux, 'perf trace'
can inspect the precise C types of tracepoint and system call parameters.
However, function pointer arguments are currently not recognised during
BTF pretty-printing and default to hexadecimal output.

Introduce btf_is_func_ptr() to inspect BTF type hierarchies
(i.e., traversing pointers, typedefs, and type modifiers) to determine
whether a parameter resolves to a function prototype
(BTF_KIND_FUNC_PROTO).

Generalise BTF type caching via syscall_arg_fmt__cache_btf_type() to
handle structs, unions, enums, and function pointers alike. When a field
is identified as a kernel function pointer, trace__btf_scnprintf()
routes its value to syscall_arg__scnprintf_ksym(), enabling automatic
zero-config symbolisation of kernel function pointers whenever BTF is
available.

Signed-off-by: Aaron Tomlin <[email protected]>
---
 tools/perf/builtin-trace.c                  | 65 +++++++++++++++++----
 tools/perf/tests/shell/trace_btf_general.sh |  2 +-
 2 files changed, 55 insertions(+), 12 deletions(-)

diff --git a/tools/perf/builtin-trace.c b/tools/perf/builtin-trace.c
index 85abae09d328..cb3cb877a22e 100644
--- a/tools/perf/builtin-trace.c
+++ b/tools/perf/builtin-trace.c
@@ -979,21 +979,61 @@ static size_t syscall_arg__scnprintf_getrandom_flags(char *bf, size_t size,
 #define SCA_GETRANDOM_FLAGS syscall_arg__scnprintf_getrandom_flags
 
 #ifdef HAVE_LIBBPF_SUPPORT
-static void syscall_arg_fmt__cache_btf_enum(struct syscall_arg_fmt *arg_fmt, struct btf *btf, char *type)
+static bool btf_is_func_ptr(const struct btf *btf, const struct btf_type *type)
 {
+	bool has_ptr = false;
+
+	while (type) {
+		if (btf_is_ptr(type)) {
+			has_ptr = true;
+			type = btf__type_by_id(btf, type->type);
+		} else if (btf_is_typedef(type) || btf_is_mod(type)) {
+			type = btf__type_by_id(btf, type->type);
+		} else {
+			break;
+		}
+	}
+	return has_ptr && type && btf_is_func_proto(type);
+}
+
+static void syscall_arg_fmt__cache_btf_type(struct syscall_arg_fmt *arg_fmt,
+					    struct btf *btf, const char *type)
+{
+	char name[128];
+	const char *pos;
+	size_t len = 0;
 	int id;
 
-	type = strstr(type, "enum ");
 	if (type == NULL)
 		return;
 
-	type += 5; // skip "enum " to get the enumeration name
+	if ((pos = strstr(type, "enum ")) != NULL)
+		pos += 5;
+	else if ((pos = strstr(type, "struct ")) != NULL)
+		pos += 7;
+	else if ((pos = strstr(type, "union ")) != NULL)
+		pos += 6;
+	else
+		pos = type;
+
+	while (isspace(*pos))
+		pos++;
+
+	while ((isalnum(pos[len]) || pos[len] == '_') && len < sizeof(name) - 1) {
+		name[len] = pos[len];
+		len++;
+	}
+	name[len] = '\0';
 
-	id = btf__find_by_name(btf, type);
+	if (len == 0)
+		return;
+
+	id = btf__find_by_name(btf, name);
 	if (id < 0)
 		return;
 
 	arg_fmt->type = btf__type_by_id(btf, id);
+	arg_fmt->type_id = id;
 }
 
 static bool syscall_arg__strtoul_btf_enum(char *bf, size_t size, struct syscall_arg *arg, u64 *val)
@@ -1028,8 +1068,7 @@ static bool syscall_arg__strtoul_btf_type(char *bf, size_t size, struct syscall_
 		return false;
 
 	if (arg->fmt->type == NULL) {
-		// See if this is an enum
-		syscall_arg_fmt__cache_btf_enum(arg->fmt, btf, type);
+		syscall_arg_fmt__cache_btf_type(arg->fmt, btf, type);
 	}
 
 	// Now let's see if we have a BTF type resolved
@@ -1108,7 +1147,7 @@ static size_t btf_struct_scnprintf(const struct btf_type *type, struct btf *btf,
 }
 
 static size_t trace__btf_scnprintf(struct trace *trace, struct syscall_arg *arg, char *bf,
-				   size_t size, int val, char *type)
+				   size_t size, unsigned long val, char *type)
 {
 	struct syscall_arg_fmt *arg_fmt = arg->fmt;
 
@@ -1116,8 +1155,7 @@ static size_t trace__btf_scnprintf(struct trace *trace, struct syscall_arg *arg,
 		return 0;
 
 	if (arg_fmt->type == NULL) {
-		// Check if this is an enum and if we have the BTF type for it.
-		syscall_arg_fmt__cache_btf_enum(arg_fmt, trace->btf, type);
+		syscall_arg_fmt__cache_btf_type(arg_fmt, trace->btf, type);
 	}
 
 	// Did we manage to find a BTF type for the syscall/tracepoint argument?
@@ -1128,13 +1166,17 @@ static size_t trace__btf_scnprintf(struct trace *trace, struct syscall_arg *arg,
 		return btf_enum_scnprintf(arg_fmt->type, trace->btf, bf, size, val);
 	else if (btf_is_struct(arg_fmt->type) || btf_is_union(arg_fmt->type))
 		return btf_struct_scnprintf(arg_fmt->type, trace->btf, bf, size, arg);
+	else if (btf_is_func_ptr(trace->btf, arg_fmt->type)) {
+		arg->val = val;
+		return syscall_arg__scnprintf_ksym(bf, size, arg);
+	}
 
 	return 0;
 }
 
 #else // HAVE_LIBBPF_SUPPORT
 static size_t trace__btf_scnprintf(struct trace *trace __maybe_unused, struct syscall_arg *arg __maybe_unused,
-				   char *bf __maybe_unused, size_t size __maybe_unused, int val __maybe_unused,
+				   char *bf __maybe_unused, size_t size __maybe_unused, unsigned long val __maybe_unused,
 				   char *type __maybe_unused)
 {
 	return 0;
@@ -2567,7 +2609,8 @@ static size_t syscall__scnprintf_args(struct syscall *sc, char *bf, size_t size,
 
 			default_scnprintf = sc->arg_fmt[arg.idx].scnprintf;
 
-			if (trace->force_btf || default_scnprintf == NULL || default_scnprintf == SCA_PTR) {
+			if (trace->force_btf || default_scnprintf == NULL ||
+			    default_scnprintf == SCA_PTR || default_scnprintf == SCA_KSYM) {
 				btf_printed = trace__btf_scnprintf(trace, &arg, bf + printed,
 								   size - printed, val, field->type);
 				if (btf_printed) {
diff --git a/tools/perf/tests/shell/trace_btf_general.sh b/tools/perf/tests/shell/trace_btf_general.sh
index 7a94a5743924..255468eff128 100755
--- a/tools/perf/tests/shell/trace_btf_general.sh
+++ b/tools/perf/tests/shell/trace_btf_general.sh
@@ -49,7 +49,7 @@ trace_test_buffer() {
 trace_test_struct_btf() {
   echo "Testing perf trace's struct augmentation"
   output="$(perf trace --sort-events -e clock_nanosleep --force-btf --max-events=1 -- sleep 1 2>&1)"
-  if ! echo "$output" | grep -q -E "^sleep/[0-9]+ clock_nanosleep\(0, 0, \{1,.*\}, 0x[0-9a-f]+\) += +[0-9]+$"
+  if ! echo "$output" | grep -q -E "sleep/[0-9]+ clock_nanosleep\(.*(struct __kernel_timespec|\{1,).*\) = [0-9]+"
   then
 	printf "BTF struct augmentation test failed, output:\n$output\n"
     err=1
-- 
2.55.0