[PATCH v4 0/3] Use BTF to trim return values

[PATCH v4 0/3] Use BTF to trim return values

[Donglin Peng]

From: pengdonglin <pengdonglin@xiaomi.com>

This patch series addresses two limitations of the funcgraph-retval feature:

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.

By leveraging BTF to obtain precise return type information, we now:

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 */

Note: enabling funcgraph-retval now adds overhead due to repeated btf_find_by_name_kind()
calls during trace output. A separate series [1] optimizes this function with
binary search (O(log n) vs current O(n)), which will greatly reduce the impact.

Here is a performance comparison:

1. Original funcgraph-retval:
# time cat trace | wc -l
101024

real    0m0.682s
user    0m0.000s
sys     0m0.695s

2. Enhanced funcgraph-retval:
# time cat trace | wc -l
99326

real    0m12.886s
user    0m0.010s
sys     0m12.680s

3. Enhanced funcgraph-retval + optimizined btf_find_by_name_kind:
# time cat trace | wc -l
102922

real    0m0.794s
user    0m0.000s
sys     0m0.810s

We can see that after optimizing btf_find_by_name_kind, the overhead
becomes negligible.

Changelog:
v4:
- Build trace_btf.c only when CONFIG_DEBUG_INFO_BTF is enabled
- Remove the redundant dependency of CONFIG_PROBE_EVENTS_BTF_ARGS
- Update related documentation and the cover letter

v3:
- Link: https://lore.kernel.org/all/20251209121349.525641-1-dolinux.peng@gmail.com/
- Print the return value based on its type for human readability, thanks Masami
- Update documentation and cover letter

v2:
- Link: https://lore.kernel.org/all/20251208131917.2444620-1-dolinux.peng@gmail.com/
- Update the funcgraph-retval documentation
- Revise the cover letter

v1:
- Link: https://lore.kernel.org/all/20251207142742.229924-1-dolinux.peng@gmail.com/

[1] https://lore.kernel.org/all/20251208062353.1702672-1-dolinux.peng@gmail.com/

pengdonglin (3):
  ftrace: Build trace_btf.c when CONFIG_DEBUG_INFO_BTF is enabled
  fgraph: Enhance funcgraph-retval with BTF-based type-aware output
  tracing: Update funcgraph-retval documentation

 Documentation/trace/ftrace.rst       |  88 +++++++++++--------
 kernel/trace/Kconfig                 |   2 +-
 kernel/trace/Makefile                |   2 +-
 kernel/trace/trace_functions_graph.c | 124 ++++++++++++++++++++++++---
 4 files changed, 163 insertions(+), 53 deletions(-)

-- 
2.34.1

[PATCH v4 1/3] ftrace: Build trace_btf.c when CONFIG_DEBUG_INFO_BTF is enabled

[Donglin Peng]

From: pengdonglin <pengdonglin@xiaomi.com>

The trace_btf.c file provides BTF helper functions used by the ftrace
subsystem. This change makes its compilation solely dependent on
CONFIG_DEBUG_INFO_BTF, allowing features like funcgraph-retval to also
utilize these helpers.

Additionally, the redundant dependency on CONFIG_PROBE_EVENTS_BTF_ARGS
is removed, as CONFIG_DEBUG_INFO_BTF already depends on
CONFIG_BPF_SYSCALL.

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/Kconfig  | 2 +-
 kernel/trace/Makefile | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index e1214b9dc990..653c1fcefa4c 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -755,7 +755,7 @@ config FPROBE_EVENTS
 config PROBE_EVENTS_BTF_ARGS
 	depends on HAVE_FUNCTION_ARG_ACCESS_API
 	depends on FPROBE_EVENTS || KPROBE_EVENTS
-	depends on DEBUG_INFO_BTF && BPF_SYSCALL
+	depends on DEBUG_INFO_BTF
 	bool "Support BTF function arguments for probe events"
 	default y
 	help
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index fc5dcc888e13..6c4bf5a6c4f3 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -116,7 +116,7 @@ obj-$(CONFIG_KGDB_KDB) += trace_kdb.o
 endif
 obj-$(CONFIG_DYNAMIC_EVENTS) += trace_dynevent.o
 obj-$(CONFIG_PROBE_EVENTS) += trace_probe.o
-obj-$(CONFIG_PROBE_EVENTS_BTF_ARGS) += trace_btf.o
+obj-$(CONFIG_DEBUG_INFO_BTF) += trace_btf.o
 obj-$(CONFIG_UPROBE_EVENTS) += trace_uprobe.o
 obj-$(CONFIG_BOOTTIME_TRACING) += trace_boot.o
 obj-$(CONFIG_FTRACE_RECORD_RECURSION) += trace_recursion_record.o
-- 
2.34.1

[PATCH v4 2/3] fgraph: Enhance funcgraph-retval with BTF-based type-aware output

[Donglin Peng]

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

[PATCH v4 3/3] tracing: Update funcgraph-retval documentation

[Donglin Peng]

From: pengdonglin <pengdonglin@xiaomi.com>

The existing documentation for funcgraph-retval is outdated and partially
incorrect, as it describes limitations that have now been resolved.

Recent changes (e.g., using BTF to obtain function return types) have
addressed key issues:
1. Return values are now printed only for non-void functions.
2. Values are trimmed to the correct width of the return type, avoiding
   garbage data from high bits.

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>
---
 Documentation/trace/ftrace.rst | 88 +++++++++++++++++++---------------
 1 file changed, 50 insertions(+), 38 deletions(-)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index d1f313a5f4ad..b231e80e6a4f 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -1454,6 +1454,10 @@ Options for function_graph tracer:
 	printed in hexadecimal format. By default, this option
 	is off.
 
+  funcgraph-retaddr
+	When set, the return address will always be printed.
+	By default, this option is off.
+
   sleep-time
 	When running function graph tracer, to include
 	the time a task schedules out in its function.
@@ -2800,7 +2804,7 @@ It is default disabled.
     0)   2.861 us    |      } /* putname() */
 
 The return value of each traced function can be displayed after
-an equal sign "=". When encountering system call failures, it
+an equal sign "ret =". When encountering system call failures, it
 can be very helpful to quickly locate the function that first
 returns an error code.
 
@@ -2810,16 +2814,16 @@ returns an error code.
   Example with funcgraph-retval::
 
     1)               |    cgroup_migrate() {
-    1)   0.651 us    |      cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
+    1)   0.651 us    |      cgroup_migrate_add_task(); /* ret=0xffff93fcfd346c00 */
     1)               |      cgroup_migrate_execute() {
     1)               |        cpu_cgroup_can_attach() {
     1)               |          cgroup_taskset_first() {
-    1)   0.732 us    |            cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
-    1)   1.232 us    |          } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
-    1)   0.380 us    |          sched_rt_can_attach(); /* = 0x0 */
-    1)   2.335 us    |        } /* cpu_cgroup_can_attach = -22 */
-    1)   4.369 us    |      } /* cgroup_migrate_execute = -22 */
-    1)   7.143 us    |    } /* cgroup_migrate = -22 */
+    1)   0.732 us    |            cgroup_taskset_next(); /* ret=0xffff93fc8fb20000 */
+    1)   1.232 us    |          } /* cgroup_taskset_first ret=0xffff93fc8fb20000 */
+    1)   0.380 us    |          sched_rt_can_attach(); /* ret=0x0 */
+    1)   2.335 us    |        } /* cpu_cgroup_can_attach ret=-22 */
+    1)   4.369 us    |      } /* cgroup_migrate_execute ret=-22 */
+    1)   7.143 us    |    } /* cgroup_migrate ret=-22 */
 
 The above example shows that the function cpu_cgroup_can_attach
 returned the error code -22 firstly, then we can read the code
@@ -2836,37 +2840,41 @@ printed in hexadecimal format.
   Example with funcgraph-retval-hex::
 
     1)               |      cgroup_migrate() {
-    1)   0.651 us    |        cgroup_migrate_add_task(); /* = 0xffff93fcfd346c00 */
+    1)   0.651 us    |        cgroup_migrate_add_task(); /* ret=0xffff93fcfd346c00 */
     1)               |        cgroup_migrate_execute() {
     1)               |          cpu_cgroup_can_attach() {
     1)               |            cgroup_taskset_first() {
-    1)   0.732 us    |              cgroup_taskset_next(); /* = 0xffff93fc8fb20000 */
-    1)   1.232 us    |            } /* cgroup_taskset_first = 0xffff93fc8fb20000 */
-    1)   0.380 us    |            sched_rt_can_attach(); /* = 0x0 */
-    1)   2.335 us    |          } /* cpu_cgroup_can_attach = 0xffffffea */
-    1)   4.369 us    |        } /* cgroup_migrate_execute = 0xffffffea */
-    1)   7.143 us    |      } /* cgroup_migrate = 0xffffffea */
-
-At present, there are some limitations when using the funcgraph-retval
-option, and these limitations will be eliminated in the future:
-
-- Even if the function return type is void, a return value will still
-  be printed, and you can just ignore it.
-
-- Even if return values are stored in multiple registers, only the
-  value contained in the first register will be recorded and printed.
-  To illustrate, in the x86 architecture, eax and edx are used to store
-  a 64-bit return value, with the lower 32 bits saved in eax and the
-  upper 32 bits saved in edx. However, only the value stored in eax
-  will be recorded and printed.
-
-- In certain procedure call standards, such as arm64's AAPCS64, when a
-  type is smaller than a GPR, it is the responsibility of the consumer
-  to perform the narrowing, and the upper bits may contain UNKNOWN values.
-  Therefore, it is advisable to check the code for such cases. For instance,
-  when using a u8 in a 64-bit GPR, bits [63:8] may contain arbitrary values,
-  especially when larger types are truncated, whether explicitly or implicitly.
-  Here are some specific cases to illustrate this point:
+    1)   0.732 us    |              cgroup_taskset_next(); /* ret=0xffff93fc8fb20000 */
+    1)   1.232 us    |            } /* cgroup_taskset_first ret=0xffff93fc8fb20000 */
+    1)   0.380 us    |            sched_rt_can_attach(); /* ret=0x0 */
+    1)   2.335 us    |          } /* cpu_cgroup_can_attach ret=0xffffffea */
+    1)   4.369 us    |        } /* cgroup_migrate_execute ret=0xffffffea */
+    1)   7.143 us    |      } /* cgroup_migrate ret=0xffffffea */
+
+Note that there are some limitations when using the funcgraph-retval
+option:
+
+- If CONFIG_DEBUG_INFO_BTF is disabled (n), a return value is printed even for
+  functions with a void return type. When CONFIG_DEBUG_INFO_BTF is enabled (y),
+  the return value is printed only for non-void functions.
+
+- If a return value occupies multiple registers, only the value in the first
+  register is recorded and printed. For example, on the x86 architecture, a
+  64-bit return value is stored across eax (lower 32 bits) and edx (upper 32 bits),
+  but only the contents of eax are captured. If CONFIG_DEBUG_INFO_BTF is enabled,
+  the suffix "(trunc)" is appended to the printed value to indicate that the
+  output may be truncated because high-order register contents are omitted.
+
+- Under certain procedure-call standards (e.g., arm64's AAPCS64), when the return
+  type is smaller than a general-purpose register (GPR), the caller is responsible
+  for narrowing the value; the upper bits of the register may contain undefined data.
+  For instance, when a u8 is returned in 64-bit GPR, bits [63:8] can hold arbitrary
+  values, especially when larger types are truncated (explicitly or implicitly). It
+  is therefore advisable to inspect the code in such cases. If CONFIG_DEBUG_INFO_BTF
+  is enabled (y), the return value is automatically trimmed to the width of the return
+  type.
+
+  The following examples illustrate the behavior:
 
   **Case One**:
 
@@ -2885,7 +2893,9 @@ option, and these limitations will be eliminated in the future:
 		RET
 
   If you pass 0x123456789abcdef to this function and want to narrow it,
-  it may be recorded as 0x123456789abcdef instead of 0xef.
+  it may be recorded as 0x123456789abcdef instead of 0xef. When
+  CONFIG_DEBUG_INFO_BTF is enabled, the value will be correctly truncated
+  to 0xef based on the size constraints of the u8 type.
 
   **Case Two**:
 
@@ -2910,7 +2920,9 @@ option, and these limitations will be eliminated in the future:
 		RET
 
   When passing 0x2_0000_0000 to it, the return value may be recorded as
-  0x2_0000_0000 instead of 0.
+  0x2_0000_0000 instead of 0. When CONFIG_DEBUG_INFO_BTF is enabled, the
+  value will be correctly truncated to 0 based on the size constraints of
+  the int type.
 
 You can put some comments on specific functions by using
 trace_printk() For example, if you want to put a comment inside
-- 
2.34.1