[PATCH v3 1/2] fgraph: Enhance funcgraph-retval with BTF-based type-aware output

[Donglin Peng <dolinux.peng@gmail.com>]

From: pengdonglin <pengdonglin@xiaomi.com>

The current funcgraph-retval implementation suffers from two accuracy
issues:

1. Void-returning functions still print a return value, creating
   misleading noise in the trace output.

2. For functions returning narrower types (e.g., char, short), the
   displayed value can be incorrect because high bits of the register
   may contain undefined data.

This patch addresses both problems by leveraging BTF to obtain the exact
return type of each traced kernel function. The key changes are:

1. Void function filtering: Functions with void return type no longer
   display any return value in the trace output, eliminating unnecessary
   clutter.

2. Type-aware value formatting: The return value is now properly truncated
   to match the actual width of the return type before being displayed.
   Additionally, the value is formatted according to its type for better
   human readability.

Here is an output comparison:

Before:
 # perf ftrace -G vfs_read --graph-opts retval
 ...
 1)               |   touch_atime() {
 1)               |     atime_needs_update() {
 1)   0.069 us    |       make_vfsuid(); /* ret=0x0 */
 1)   0.067 us    |       make_vfsgid(); /* ret=0x0 */
 1)               |       current_time() {
 1)   0.197 us    |         ktime_get_coarse_real_ts64_mg(); /* ret=0x187f886aec3ed6f5 */
 1)   0.352 us    |       } /* current_time ret=0x69380753 */
 1)   0.792 us    |     } /* atime_needs_update ret=0x0 */
 1)   0.937 us    |   } /* touch_atime ret=0x0 */

After:
 # perf ftrace -G vfs_read --graph-opts retval
 ...
 2)               |   touch_atime() {
 2)               |     atime_needs_update() {
 2)   0.070 us    |       make_vfsuid(); /* ret=0x0 */
 2)   0.070 us    |       make_vfsgid(); /* ret=0x0 */
 2)               |       current_time() {
 2)   0.162 us    |         ktime_get_coarse_real_ts64_mg();
 2)   0.312 us    |       } /* current_time ret=0x69380649(trunc) */
 2)   0.753 us    |     } /* atime_needs_update ret=false */
 2)   0.899 us    |   } /* touch_atime */

Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Xiaoqin Zhang <zhangxiaoqin@xiaomi.com>
Signed-off-by: pengdonglin <pengdonglin@xiaomi.com>
---
 kernel/trace/trace_functions_graph.c | 124 ++++++++++++++++++++++++---
 1 file changed, 111 insertions(+), 13 deletions(-)

diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 17c75cf2348e..46b66b1cfc16 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -15,6 +15,7 @@
 
 #include "trace.h"
 #include "trace_output.h"
+#include "trace_btf.h"
 
 /* When set, irq functions might be ignored */
 static int ftrace_graph_skip_irqs;
@@ -120,6 +121,13 @@ enum {
 	FLAGS_FILL_END   = 3 << TRACE_GRAPH_PRINT_FILL_SHIFT,
 };
 
+enum {
+	RETVAL_FMT_HEX   = BIT(0),
+	RETVAL_FMT_DEC   = BIT(1),
+	RETVAL_FMT_BOOL  = BIT(2),
+	RETVAL_FMT_TRUNC = BIT(3),
+};
+
 static void
 print_graph_duration(struct trace_array *tr, unsigned long long duration,
 		     struct trace_seq *s, u32 flags);
@@ -865,6 +873,73 @@ static void print_graph_retaddr(struct trace_seq *s, struct fgraph_retaddr_ent_e
 
 #if defined(CONFIG_FUNCTION_GRAPH_RETVAL) || defined(CONFIG_FUNCTION_GRAPH_RETADDR)
 
+static void trim_retval(unsigned long func, unsigned long *retval, bool *print_retval,
+			int *fmt)
+{
+	const struct btf_type *t;
+	char name[KSYM_NAME_LEN];
+	struct btf *btf;
+	u32 v, msb;
+	int kind;
+
+	if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
+		return;
+
+	if (lookup_symbol_name(func, name))
+		return;
+
+	t = btf_find_func_proto(name, &btf);
+	if (IS_ERR_OR_NULL(t))
+		return;
+
+	t = btf_type_skip_modifiers(btf, t->type, NULL);
+	kind = t ? BTF_INFO_KIND(t->info) : BTF_KIND_UNKN;
+	switch (kind) {
+	case BTF_KIND_UNKN:
+		*print_retval = false;
+		break;
+	case BTF_KIND_STRUCT:
+	case BTF_KIND_UNION:
+	case BTF_KIND_ENUM:
+	case BTF_KIND_ENUM64:
+		if (kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION)
+			*fmt = RETVAL_FMT_HEX;
+		else
+			*fmt = RETVAL_FMT_DEC;
+
+		if (t->size > sizeof(unsigned long)) {
+			*fmt |= RETVAL_FMT_TRUNC;
+		} else {
+			msb = BITS_PER_BYTE * t->size - 1;
+			*retval &= GENMASK(msb, 0);
+		}
+		break;
+	case BTF_KIND_INT:
+		v = *(u32 *)(t + 1);
+		if (BTF_INT_ENCODING(v) == BTF_INT_BOOL) {
+			*fmt = RETVAL_FMT_BOOL;
+			msb = 0;
+		} else {
+			if (BTF_INT_ENCODING(v) == BTF_INT_SIGNED)
+				*fmt = RETVAL_FMT_DEC;
+			else
+				*fmt = RETVAL_FMT_HEX;
+
+			if (t->size > sizeof(unsigned long)) {
+				*fmt |= RETVAL_FMT_TRUNC;
+				msb = BITS_PER_LONG - 1;
+			} else {
+				msb = BTF_INT_BITS(v) - 1;
+			}
+		}
+		*retval &= GENMASK(msb, 0);
+		break;
+	default:
+		*fmt = RETVAL_FMT_HEX;
+		break;
+	}
+}
+
 static void print_graph_retval(struct trace_seq *s, struct ftrace_graph_ent_entry *entry,
 				struct ftrace_graph_ret *graph_ret, void *func,
 				u32 opt_flags, u32 trace_flags, int args_size)
@@ -873,7 +948,7 @@ static void print_graph_retval(struct trace_seq *s, struct ftrace_graph_ent_entr
 	unsigned long retval = 0;
 	bool print_retaddr = false;
 	bool print_retval = false;
-	bool hex_format = !!(opt_flags & TRACE_GRAPH_PRINT_RETVAL_HEX);
+	int retval_fmt = 0;
 
 #ifdef CONFIG_FUNCTION_GRAPH_RETVAL
 	retval = graph_ret->retval;
@@ -884,17 +959,35 @@ static void print_graph_retval(struct trace_seq *s, struct ftrace_graph_ent_entr
 	print_retaddr = !!(opt_flags & TRACE_GRAPH_PRINT_RETADDR);
 #endif
 
-	if (print_retval && retval && !hex_format) {
-		/* Check if the return value matches the negative format */
-		if (IS_ENABLED(CONFIG_64BIT) && (retval & BIT(31)) &&
-			(((u64)retval) >> 32) == 0) {
-			err_code = sign_extend64(retval, 31);
-		} else {
-			err_code = retval;
+	if (print_retval) {
+		int fmt = RETVAL_FMT_HEX;
+
+		trim_retval((unsigned long)func, &retval, &print_retval, &fmt);
+		if (print_retval) {
+			if (opt_flags & TRACE_GRAPH_PRINT_RETVAL_HEX)
+				retval_fmt = RETVAL_FMT_HEX;
+
+			if (retval && retval_fmt != RETVAL_FMT_HEX) {
+				/* Check if the return value matches the negative format */
+				if (IS_ENABLED(CONFIG_64BIT) && (retval & BIT(31)) &&
+					(((u64)retval) >> 32) == 0) {
+					err_code = sign_extend64(retval, 31);
+				} else {
+					err_code = retval;
+				}
+
+				if (!IS_ERR_VALUE(err_code))
+					err_code = 0;
+			}
+
+			if (retval_fmt == RETVAL_FMT_HEX) {
+				retval_fmt |= (fmt & RETVAL_FMT_TRUNC);
+			} else {
+				if (err_code && fmt & RETVAL_FMT_HEX)
+					fmt = (fmt & ~RETVAL_FMT_HEX) | RETVAL_FMT_DEC;
+				retval_fmt = fmt;
+			}
 		}
-
-		if (!IS_ERR_VALUE(err_code))
-			err_code = 0;
 	}
 
 	if (entry) {
@@ -921,10 +1014,15 @@ static void print_graph_retval(struct trace_seq *s, struct ftrace_graph_ent_entr
 				    trace_flags, false);
 
 	if (print_retval) {
-		if (hex_format || (err_code == 0))
+		if (retval_fmt & RETVAL_FMT_HEX)
 			trace_seq_printf(s, " ret=0x%lx", retval);
+		else if (retval_fmt & RETVAL_FMT_BOOL)
+			trace_seq_printf(s, " ret=%s", retval ? "true" : "false");
 		else
-			trace_seq_printf(s, " ret=%ld", err_code);
+			trace_seq_printf(s, " ret=%ld", err_code ?: retval);
+
+		if (retval_fmt & RETVAL_FMT_TRUNC)
+			trace_seq_printf(s, "(trunc)");
 	}
 
 	if (!entry || print_retval || print_retaddr)
-- 
2.34.1