Message-ID: <20251218215741.2446883-1-namhyung@kernel.org>
Date: Thu, 18 Dec 2025 13:57:41 -0800
From: Namhyung Kim <namhyung@...nel.org>
To: Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ian Rogers <irogers@...gle.com>,
	James Clark <james.clark@...aro.org>
Cc: Jiri Olsa <jolsa@...nel.org>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-perf-users@...r.kernel.org
Subject: [RFC/PATCH] perf inject: Add --convert-callchain option

There are applications not built with frame pointers, so DWARF is needed
to get the stack traces.  So `perf record --call-graph dwarf` saves the
stack and register data for each sample to get the stacktrace offline.
But sometimes those data may have sensitive information and we don't
want to keep them in the file.

This perf inject --convert-callchain option parses the callchains and
discard the stack and register after that.  This will save storage space
and processing time for the new data file.  Of course, users should
remove the original data file. :)

The down side is that it cannot handle inlined callchain entries as they
all have the same IPs.  Maybe we can add an option to perf report to
look up inlined functions using DWARF - IIUC it won't requires stack and
register data.

This is an example.

  $ perf record --call-graph dwarf -- perf test -w noploop

  $ perf report --stdio --no-children --percent-limit=0 > output-prev

  $ perf inject -i perf.data --convert-callchain -o perf.data.out

  $ perf report --stdio --no-children --percent-limit=0 -i perf.data.out > output-next

  $ diff -u output-prev output-next
  ...
        0.23%  perf          ld-linux-x86-64.so.2  [.] _dl_relocate_object_no_relro
               |
  -            ---elf_dynamic_do_Rela (inlined)
  -               _dl_relocate_object_no_relro
  +            ---_dl_relocate_object_no_relro
                  _dl_relocate_object
                  dl_main
                  _dl_sysdep_start
  -               _dl_start_final (inlined)
                  _dl_start
                  _start

Signed-off-by: Namhyung Kim <namhyung@...nel.org>
---
 tools/perf/Documentation/perf-inject.txt |   5 +
 tools/perf/builtin-inject.c              | 128 +++++++++++++++++++++++
 2 files changed, 133 insertions(+)

diff --git a/tools/perf/Documentation/perf-inject.txt b/tools/perf/Documentation/perf-inject.txt
index c972032f4ca0d248..95dfdf39666efe89 100644
--- a/tools/perf/Documentation/perf-inject.txt
+++ b/tools/perf/Documentation/perf-inject.txt
@@ -109,6 +109,11 @@ include::itrace.txt[]
 	should be used, and also --buildid-all and --switch-events may be
 	useful.
 
+--convert-callchain::
+	Parse DWARF callchains and convert them to usual callchains.  This also
+	discards stack and register data from the samples.  This will lose
+	inlined callchain entries.
+
 :GMEXAMPLECMD: inject
 :GMEXAMPLESUBCMD:
 include::guestmount.txt[]
diff --git a/tools/perf/builtin-inject.c b/tools/perf/builtin-inject.c
index 6080afec537d2178..2a2fcc8e3e9e5fe5 100644
--- a/tools/perf/builtin-inject.c
+++ b/tools/perf/builtin-inject.c
@@ -122,6 +122,7 @@ struct perf_inject {
 	bool			in_place_update;
 	bool			in_place_update_dry_run;
 	bool			copy_kcore_dir;
+	bool			convert_callchain;
 	const char		*input_name;
 	struct perf_data	output;
 	u64			bytes_written;
@@ -133,6 +134,7 @@ struct perf_inject {
 	struct guest_session	guest_session;
 	struct strlist		*known_build_ids;
 	const struct evsel	*mmap_evsel;
+	struct ip_callchain	*raw_callchain;
 };
 
 struct event_entry {
@@ -383,6 +385,89 @@ static int perf_event__repipe_sample(const struct perf_tool *tool,
 	return perf_event__repipe_synth(tool, event);
 }
 
+static int perf_event__convert_sample_callchain(const struct perf_tool *tool,
+						union perf_event *event,
+						struct perf_sample *sample,
+						struct evsel *evsel,
+						struct machine *machine)
+{
+	struct perf_inject *inject = container_of(tool, struct perf_inject, tool);
+	struct callchain_cursor *cursor = get_tls_callchain_cursor();
+	union perf_event *event_copy = (void *)inject->event_copy;
+	struct callchain_cursor_node *node;
+	struct thread *thread;
+	u64 sample_type = evsel->core.attr.sample_type;
+	u32 sample_size = event->header.size;
+	u64 i, k;
+	int ret;
+
+	if (event_copy == NULL) {
+		inject->event_copy = malloc(PERF_SAMPLE_MAX_SIZE);
+		if (!inject->event_copy)
+			return -ENOMEM;
+
+		event_copy = (void *)inject->event_copy;
+	}
+
+	if (cursor == NULL)
+		return perf_event__repipe_synth(tool, event);
+
+	callchain_cursor_reset(cursor);
+
+	thread = machine__find_thread(machine, -1, sample->pid);
+	if (thread == NULL)
+		return perf_event__repipe_synth(tool, event);
+
+	/* this will parse DWARF using stack and register data */
+	ret = thread__resolve_callchain(thread, cursor, evsel, sample,
+					/*parent=*/NULL, /*root_al=*/NULL,
+					PERF_MAX_STACK_DEPTH);
+	thread__put(thread);
+	if (ret != 0)
+		return perf_event__repipe_synth(tool, event);
+
+	/* copy kernel callchain and context entries */
+	for (i = 0; i < sample->callchain->nr; i++) {
+		inject->raw_callchain->ips[i] = sample->callchain->ips[i];
+		if (sample->callchain->ips[i] == PERF_CONTEXT_USER) {
+			i++;
+			break;
+		}
+	}
+	if (i == 0 || inject->raw_callchain->ips[i - 1] != PERF_CONTEXT_USER)
+		inject->raw_callchain->ips[i++] = PERF_CONTEXT_USER;
+
+	node = cursor->first;
+	for (k = 0; k < cursor->nr && i < PERF_MAX_STACK_DEPTH; k++) {
+		if (node->ms.map && __map__is_kernel(node->ms.map))
+			/* kernel IPs were added already */;
+		else if (node->ms.sym && node->ms.sym->inlined)
+			/* we don't handle inlined symbols */;
+		else
+			inject->raw_callchain->ips[i++] = node->ip;
+
+		node = node->next;
+	}
+
+	inject->raw_callchain->nr = i;
+	sample->callchain = inject->raw_callchain;
+
+	memcpy(event_copy, event, sizeof(event->header));
+
+	/* adjust sample size for stack and regs */
+	sample_size -= sample->user_stack.size;
+	sample_size -= (hweight64(evsel->core.attr.sample_regs_user) + 1) * sizeof(u64);
+	sample_size += (sample->callchain->nr + 1) * sizeof(u64);
+	event_copy->header.size = sample_size;
+
+	/* remove sample_type {STACK,REGS}_USER for synthesize */
+	sample_type &= ~(PERF_SAMPLE_STACK_USER | PERF_SAMPLE_REGS_USER);
+
+	perf_event__synthesize_sample(event_copy, sample_type,
+				      evsel->core.attr.read_format, sample);
+	return perf_event__repipe_synth(tool, event_copy);
+}
+
 static struct dso *findnew_dso(int pid, int tid, const char *filename,
 			       const struct dso_id *id, struct machine *machine)
 {
@@ -2270,6 +2355,13 @@ static int __cmd_inject(struct perf_inject *inject)
 		/* Allow space in the header for guest attributes */
 		output_data_offset += gs->session->header.data_offset;
 		output_data_offset = roundup(output_data_offset, 4096);
+	} else if (inject->convert_callchain) {
+		inject->tool.sample	= perf_event__convert_sample_callchain;
+		inject->tool.fork	= perf_event__repipe_fork;
+		inject->tool.comm	= perf_event__repipe_comm;
+		inject->tool.exit	= perf_event__repipe_exit;
+		inject->tool.mmap	= perf_event__repipe_mmap;
+		inject->tool.mmap2	= perf_event__repipe_mmap2;
 	}
 
 	if (!inject->itrace_synth_opts.set)
@@ -2322,6 +2414,23 @@ static int __cmd_inject(struct perf_inject *inject)
 				perf_header__set_feat(&session->header,
 						      HEADER_BRANCH_STACK);
 		}
+
+		/*
+		 * The converted data file won't have stack and registers.
+		 * Update the perf_event_attr to remove them before writing.
+		 */
+		if (inject->convert_callchain) {
+			struct evsel *evsel;
+
+			evlist__for_each_entry(session->evlist, evsel) {
+				evsel__reset_sample_bit(evsel, REGS_USER);
+				evsel__reset_sample_bit(evsel, STACK_USER);
+				evsel->core.attr.sample_regs_user = 0;
+				evsel->core.attr.sample_stack_user = 0;
+				evsel->core.attr.exclude_callchain_user = 0;
+			}
+		}
+
 		session->header.data_offset = output_data_offset;
 		session->header.data_size = inject->bytes_written;
 		perf_session__inject_header(session, session->evlist, fd, &inj_fc.fc,
@@ -2414,6 +2523,8 @@ int cmd_inject(int argc, const char **argv)
 		OPT_STRING(0, "guestmount", &symbol_conf.guestmount, "directory",
 			   "guest mount directory under which every guest os"
 			   " instance has a subdir"),
+		OPT_BOOLEAN(0, "convert-callchain", &inject.convert_callchain,
+			    "Generate callchains using DWARF and drop register/stack data"),
 		OPT_END()
 	};
 	const char * const inject_usage[] = {
@@ -2429,6 +2540,9 @@ int cmd_inject(int argc, const char **argv)
 
 #ifndef HAVE_JITDUMP
 	set_option_nobuild(options, 'j', "jit", "NO_LIBELF=1", true);
+#endif
+#ifndef HAVE_LIBDW_SUPPORT
+	set_option_nobuild(options, 0, "convert-callchain", "NO_LIBDW=1", true);
 #endif
 	argc = parse_options(argc, argv, options, inject_usage, 0);
 
@@ -2588,6 +2702,19 @@ int cmd_inject(int argc, const char **argv)
 		}
 	}
 
+	if (inject.convert_callchain) {
+		if (inject->output.is_pipe || inject->session->data->is_pipe) {
+			pr_err("--convert-callchain cannot work with pipe\n");
+			goto out_delete;
+		}
+
+		inject.raw_callchain = calloc(PERF_MAX_STACK_DEPTH, sizeof(u64));
+		if (inject.raw_callchain == NULL) {
+			pr_err("callchain allocation failed\n");
+			goto out_delete;
+		}
+	}
+
 #ifdef HAVE_JITDUMP
 	if (inject.jit_mode) {
 		inject.tool.mmap2	   = perf_event__repipe_mmap2;
@@ -2618,5 +2745,6 @@ int cmd_inject(int argc, const char **argv)
 	free(inject.itrace_synth_opts.vm_tm_corr_args);
 	free(inject.event_copy);
 	free(inject.guest_session.ev.event_buf);
+	free(inject.raw_callchain);
 	return ret;
 }
-- 
2.52.0.322.g1dd061c0dc-goog

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