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Mon, 08 Dec 2025 05:19:24 -0800 (PST) From: Donglin Peng To: rostedt@goodmis.org Cc: linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, pengdonglin , Masami Hiramatsu , Xiaoqin Zhang Subject: [PATCH v2 1/2] fgraph: Use BTF to trim and filter return values Date: Mon, 8 Dec 2025 21:19:16 +0800 Message-Id: <20251208131917.2444620-2-dolinux.peng@gmail.com> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20251208131917.2444620-1-dolinux.peng@gmail.com> References: <20251208131917.2444620-1-dolinux.peng@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" From: pengdonglin The current funcgraph-retval implementation has two limitations: 1. It prints a return value even when the traced function returns void. 2. When the return type is narrower than a register, the printed value may be incorrect because high bits can contain undefined data. Both issues are addressed by using BTF to obtain the precise return type of each traced function: - Return values are now printed only for functions whose return type is not void. - The value is truncated to the actual width of the return type, ensuring correct representation. These changes make the funcgraph-retval output more accurate and remove noise from void functions. Cc: Steven Rostedt (Google) Cc: Masami Hiramatsu Cc: Xiaoqin Zhang Signed-off-by: pengdonglin --- kernel/trace/trace_functions_graph.c | 64 +++++++++++++++++++++++----- 1 file changed, 54 insertions(+), 10 deletions(-) diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_func= tions_graph.c index 17c75cf2348e..9e63665c81e2 100644 --- a/kernel/trace/trace_functions_graph.c +++ b/kernel/trace/trace_functions_graph.c @@ -15,6 +15,7 @@ =20 #include "trace.h" #include "trace_output.h" +#include "trace_btf.h" =20 /* When set, irq functions might be ignored */ static int ftrace_graph_skip_irqs; @@ -865,6 +866,46 @@ static void print_graph_retaddr(struct trace_seq *s, s= truct fgraph_retaddr_ent_e =20 #if defined(CONFIG_FUNCTION_GRAPH_RETVAL) || defined(CONFIG_FUNCTION_GRAPH= _RETADDR) =20 +static void trim_retval(unsigned long func, unsigned long *retval, bool *p= rint_retval) +{ + const struct btf_type *t; + char name[KSYM_NAME_LEN]; + struct btf *btf; + u32 v, msb; + + if (!IS_ENABLED(CONFIG_DEBUG_INFO_BTF)) + return; + + if (lookup_symbol_name(func, name)) + return; + + t =3D btf_find_func_proto(name, &btf); + if (IS_ERR_OR_NULL(t)) + return; + + t =3D btf_type_skip_modifiers(btf, t->type, NULL); + switch (t ? BTF_INFO_KIND(t->info) : BTF_KIND_UNKN) { + case BTF_KIND_UNKN: + *print_retval =3D false; + break; + case BTF_KIND_ENUM: + case BTF_KIND_ENUM64: + msb =3D BITS_PER_BYTE * t->size - 1; + *retval &=3D GENMASK(msb, 0); + break; + case BTF_KIND_INT: + v =3D *(u32 *)(t + 1); + if (BTF_INT_ENCODING(v) =3D=3D BTF_INT_BOOL) + msb =3D 0; + else + msb =3D BTF_INT_BITS(v) - 1; + *retval &=3D GENMASK(msb, 0); + break; + default: + break; + } +} + static void print_graph_retval(struct trace_seq *s, struct ftrace_graph_en= t_entry *entry, struct ftrace_graph_ret *graph_ret, void *func, u32 opt_flags, u32 trace_flags, int args_size) @@ -884,17 +925,20 @@ static void print_graph_retval(struct trace_seq *s, s= truct ftrace_graph_ent_entr print_retaddr =3D !!(opt_flags & TRACE_GRAPH_PRINT_RETADDR); #endif =20 - 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) =3D=3D 0) { - err_code =3D sign_extend64(retval, 31); - } else { - err_code =3D retval; + if (print_retval) { + trim_retval((unsigned long)func, &retval, &print_retval); + 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) =3D=3D 0) { + err_code =3D sign_extend64(retval, 31); + } else { + err_code =3D retval; + } + + if (!IS_ERR_VALUE(err_code)) + err_code =3D 0; } - - if (!IS_ERR_VALUE(err_code)) - err_code =3D 0; } =20 if (entry) { --=20 2.34.1