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)
--
2.34.1Hi Donglin,
kernel test robot noticed the following build errors:
[auto build test ERROR on trace/for-next]
[also build test ERROR on linus/master v6.18 next-20251212]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]
url: https://github.com/intel-lab-lkp/linux/commits/Donglin-Peng/fgraph-Enhance-funcgraph-retval-with-BTF-based-type-aware-output/20251209-201633
base: https://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace for-next
patch link: https://lore.kernel.org/r/20251209121349.525641-2-dolinux.peng%40gmail.com
patch subject: [PATCH v3 1/2] fgraph: Enhance funcgraph-retval with BTF-based type-aware output
config: arm-randconfig-002-20251214 (https://download.01.org/0day-ci/archive/20251214/202512140850.JdD1lPmn-lkp@intel.com/config)
compiler: arm-linux-gnueabi-gcc (GCC) 10.5.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251214/202512140850.JdD1lPmn-lkp@intel.com/reproduce)
If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp@intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202512140850.JdD1lPmn-lkp@intel.com/
All errors (new ones prefixed by >>):
arm-linux-gnueabi-ld: kernel/trace/trace_functions_graph.o: in function `trim_retval':
>> kernel/trace/trace_functions_graph.c:888: undefined reference to `btf_find_func_proto'
vim +888 kernel/trace/trace_functions_graph.c
872
873 static void trim_retval(unsigned long func, unsigned long *retval, bool *print_retval,
874 int *fmt)
875 {
876 const struct btf_type *t;
877 char name[KSYM_NAME_LEN];
878 struct btf *btf;
879 u32 v, msb;
880 int kind;
881
882 if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
883 return;
884
885 if (lookup_symbol_name(func, name))
886 return;
887
> 888 t = btf_find_func_proto(name, &btf);
889 if (IS_ERR_OR_NULL(t))
890 return;
891
892 t = btf_type_skip_modifiers(btf, t->type, NULL);
893 kind = t ? BTF_INFO_KIND(t->info) : BTF_KIND_UNKN;
894 switch (kind) {
895 case BTF_KIND_UNKN:
896 *print_retval = false;
897 break;
898 case BTF_KIND_STRUCT:
899 case BTF_KIND_UNION:
900 case BTF_KIND_ENUM:
901 case BTF_KIND_ENUM64:
902 if (kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION)
903 *fmt = RETVAL_FMT_HEX;
904 else
905 *fmt = RETVAL_FMT_DEC;
906
907 if (t->size > sizeof(unsigned long)) {
908 *fmt |= RETVAL_FMT_TRUNC;
909 } else {
910 msb = BITS_PER_BYTE * t->size - 1;
911 *retval &= GENMASK(msb, 0);
912 }
913 break;
914 case BTF_KIND_INT:
915 v = *(u32 *)(t + 1);
916 if (BTF_INT_ENCODING(v) == BTF_INT_BOOL) {
917 *fmt = RETVAL_FMT_BOOL;
918 msb = 0;
919 } else {
920 if (BTF_INT_ENCODING(v) == BTF_INT_SIGNED)
921 *fmt = RETVAL_FMT_DEC;
922 else
923 *fmt = RETVAL_FMT_HEX;
924
925 if (t->size > sizeof(unsigned long)) {
926 *fmt |= RETVAL_FMT_TRUNC;
927 msb = BITS_PER_LONG - 1;
928 } else {
929 msb = BTF_INT_BITS(v) - 1;
930 }
931 }
932 *retval &= GENMASK(msb, 0);
933 break;
934 default:
935 *fmt = RETVAL_FMT_HEX;
936 break;
937 }
938 }
939
--
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki
On Sun, Dec 14, 2025 at 9:15 AM kernel test robot <lkp@intel.com> wrote:
>
> Hi Donglin,
>
> kernel test robot noticed the following build errors:
>
> [auto build test ERROR on trace/for-next]
> [also build test ERROR on linus/master v6.18 next-20251212]
> [If your patch is applied to the wrong git tree, kindly drop us a note.
> And when submitting patch, we suggest to use '--base' as documented in
> https://git-scm.com/docs/git-format-patch#_base_tree_information]
>
> url: https://github.com/intel-lab-lkp/linux/commits/Donglin-Peng/fgraph-Enhance-funcgraph-retval-with-BTF-based-type-aware-output/20251209-201633
> base: https://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace for-next
> patch link: https://lore.kernel.org/r/20251209121349.525641-2-dolinux.peng%40gmail.com
> patch subject: [PATCH v3 1/2] fgraph: Enhance funcgraph-retval with BTF-based type-aware output
> config: arm-randconfig-002-20251214 (https://download.01.org/0day-ci/archive/20251214/202512140850.JdD1lPmn-lkp@intel.com/config)
> compiler: arm-linux-gnueabi-gcc (GCC) 10.5.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20251214/202512140850.JdD1lPmn-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202512140850.JdD1lPmn-lkp@intel.com/
>
> All errors (new ones prefixed by >>):
>
> arm-linux-gnueabi-ld: kernel/trace/trace_functions_graph.o: in function `trim_retval':
> >> kernel/trace/trace_functions_graph.c:888: undefined reference to `btf_find_func_proto'
Thanks. I will address this in the next version. The issue occurs because
CONFIG_FUNCTION_GRAPH_RETVAL and CONFIG_DEBUG_INFO_BTF
are enabled, but CONFIG_PROBE_EVENTS_BTF_ARGS is disabled.
This prevents trace_btf.c from being compiled, while the function
btf_find_func_proto
it provides is still required.
Thanks,
Donglin
>
>
> vim +888 kernel/trace/trace_functions_graph.c
>
> 872
> 873 static void trim_retval(unsigned long func, unsigned long *retval, bool *print_retval,
> 874 int *fmt)
> 875 {
> 876 const struct btf_type *t;
> 877 char name[KSYM_NAME_LEN];
> 878 struct btf *btf;
> 879 u32 v, msb;
> 880 int kind;
> 881
> 882 if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF))
> 883 return;
> 884
> 885 if (lookup_symbol_name(func, name))
> 886 return;
> 887
> > 888 t = btf_find_func_proto(name, &btf);
> 889 if (IS_ERR_OR_NULL(t))
> 890 return;
> 891
> 892 t = btf_type_skip_modifiers(btf, t->type, NULL);
> 893 kind = t ? BTF_INFO_KIND(t->info) : BTF_KIND_UNKN;
> 894 switch (kind) {
> 895 case BTF_KIND_UNKN:
> 896 *print_retval = false;
> 897 break;
> 898 case BTF_KIND_STRUCT:
> 899 case BTF_KIND_UNION:
> 900 case BTF_KIND_ENUM:
> 901 case BTF_KIND_ENUM64:
> 902 if (kind == BTF_KIND_STRUCT || kind == BTF_KIND_UNION)
> 903 *fmt = RETVAL_FMT_HEX;
> 904 else
> 905 *fmt = RETVAL_FMT_DEC;
> 906
> 907 if (t->size > sizeof(unsigned long)) {
> 908 *fmt |= RETVAL_FMT_TRUNC;
> 909 } else {
> 910 msb = BITS_PER_BYTE * t->size - 1;
> 911 *retval &= GENMASK(msb, 0);
> 912 }
> 913 break;
> 914 case BTF_KIND_INT:
> 915 v = *(u32 *)(t + 1);
> 916 if (BTF_INT_ENCODING(v) == BTF_INT_BOOL) {
> 917 *fmt = RETVAL_FMT_BOOL;
> 918 msb = 0;
> 919 } else {
> 920 if (BTF_INT_ENCODING(v) == BTF_INT_SIGNED)
> 921 *fmt = RETVAL_FMT_DEC;
> 922 else
> 923 *fmt = RETVAL_FMT_HEX;
> 924
> 925 if (t->size > sizeof(unsigned long)) {
> 926 *fmt |= RETVAL_FMT_TRUNC;
> 927 msb = BITS_PER_LONG - 1;
> 928 } else {
> 929 msb = BTF_INT_BITS(v) - 1;
> 930 }
> 931 }
> 932 *retval &= GENMASK(msb, 0);
> 933 break;
> 934 default:
> 935 *fmt = RETVAL_FMT_HEX;
> 936 break;
> 937 }
> 938 }
> 939
>
> --
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki
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