perf unwind: Refactor get_entries to allow dynamic libdw/libunwind selection

Currently, both libdw and libunwind define 'unwind__get_entries'. This
causes a duplicate symbol build failure when both are compiled into
perf.

This commit refactors the DWARF unwind post-processing to be
configurable at runtime via the .perfconfig file option
'unwind.style', or using the argument '--unwind-style' in the commands
'perf report', 'perf script' and 'perf inject', in a similar manner to
the addr2line or the disassembler style.

The file 'tools/perf/util/unwind.c' adds the top-level dispatch
function 'unwind__get_entries'. The backend implementations are
renamed to 'libdw__get_entries' and 'libunwind__get_entries'. Both are
attempted as fallbacks if not configured, or if the primary backend
fails.

Fixes: 2e9191573a ("perf build: Remove NO_LIBDW_DWARF_UNWIND option")
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Andrew Jones <andrew.jones@oss.qualcomm.com>
Cc: Athira Rajeev <atrajeev@linux.ibm.com>
Cc: Dapeng Mi <dapeng1.mi@linux.intel.com>
Cc: Dmitrii Dolgov <9erthalion6@gmail.com>
Cc: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: Howard Chu <howardchu95@gmail.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: libunwind-devel@nongnu.org
Cc: Li Guan <guanli.oerv@isrc.iscas.ac.cn>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shimin Guo <shimin.guo@skydio.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Tomas Glozar <tglozar@redhat.com>
Cc: Will Deacon <will@kernel.org>
[ Don't mix declarations and code, move 'entries' variable to the start of scope ]
[ Use pr_warning_once() instead of pr_err() in stubs for get_entries(), suggested by a local sashiko instance ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-inject.c

@@ -26,6 +26,7 @@
 #include "util/synthetic-events.h"
 #include "util/thread.h"
 #include "util/namespaces.h"
+#include "util/unwind.h"
 #include "util/util.h"
 #include "util/tsc.h"
 
@@ -2563,6 +2564,9 @@ 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_CALLBACK(0, "unwind-style", NULL, "unwind style",
+			     "unwind styles (libdw,libunwind)",
+			     unwind__option),
 		OPT_BOOLEAN(0, "convert-callchain", &inject.convert_callchain,
 			    "Generate callchains using DWARF and drop register/stack data"),
 		OPT_END()

tools/perf/builtin-report.c

@@ -48,6 +48,7 @@
 #include "util/time-utils.h"
 #include "util/auxtrace.h"
 #include "util/units.h"
+#include "util/unwind.h"
 #include "util/util.h" // perf_tip()
 #include "ui/ui.h"
 #include "ui/progress.h"
@@ -1449,6 +1450,9 @@ int cmd_report(int argc, const char **argv)
 	OPT_CALLBACK(0, "addr2line-style", NULL, "addr2line style",
 		     "addr2line styles (libdw,llvm,libbfd,addr2line)",
 		     report_parse_addr2line_config),
+	OPT_CALLBACK(0, "unwind-style", NULL, "unwind style",
+		     "unwind styles (libdw,libunwind)",
+		     unwind__option),
 	OPT_BOOLEAN(0, "demangle", &symbol_conf.demangle,
 		    "Symbol demangling. Enabled by default, use --no-demangle to disable."),
 	OPT_BOOLEAN(0, "demangle-kernel", &symbol_conf.demangle_kernel,

tools/perf/builtin-script.c

@@ -63,6 +63,7 @@
 #include <linux/err.h>
 #include "util/dlfilter.h"
 #include "util/record.h"
+#include "util/unwind.h"
 #include "util/util.h"
 #include "util/cgroup.h"
 #include "util/annotate.h"
@@ -4159,6 +4160,9 @@ int cmd_script(int argc, const char **argv)
 			"Enable symbol demangling"),
 	OPT_BOOLEAN(0, "demangle-kernel", &symbol_conf.demangle_kernel,
 			"Enable kernel symbol demangling"),
+	OPT_CALLBACK(0, "unwind-style", NULL, "unwind style",
+		     "unwind styles (libdw,libunwind)",
+		     unwind__option),
 	OPT_STRING(0, "addr2line", &symbol_conf.addr2line_path, "path",
 			"addr2line binary to use for line numbers"),
 	OPT_STRING(0, "time", &script.time_str, "str",

tools/perf/util/Build

@@ -216,6 +216,7 @@ ifndef CONFIG_SETNS
 perf-util-y += setns.o
 endif
 
+perf-util-y += unwind.o
 perf-util-$(CONFIG_LIBDW) += probe-finder.o
 perf-util-$(CONFIG_LIBDW) += dwarf-aux.o
 perf-util-$(CONFIG_LIBDW) += dwarf-regs.o

tools/perf/util/config.c

@@ -23,6 +23,7 @@
 #include "build-id.h"
 #include "debug.h"
 #include "config.h"
+#include "unwind.h"
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <stdlib.h>
@@ -525,6 +526,9 @@ int perf_default_config(const char *var, const char *value,
 	if (strstarts(var, "addr2line."))
 		return addr2line_configure(var, value, dummy);
 
+	if (strstarts(var, "unwind."))
+		return unwind__configure(var, value, dummy);
+
 	/* Add other config variables here. */
 	return 0;
 }

tools/perf/util/symbol_conf.h

@@ -9,6 +9,15 @@
 struct strlist;
 struct intlist;
 
+enum unwind_style {
+	UNWIND_STYLE_UNKNOWN = 0,
+	UNWIND_STYLE_LIBDW,
+	UNWIND_STYLE_LIBUNWIND,
+};
+
+#define MAX_UNWIND_STYLE (UNWIND_STYLE_LIBUNWIND + 1)
+
+
 enum a2l_style {
 	A2L_STYLE_UNKNOWN = 0,
 	A2L_STYLE_LIBDW,
@@ -81,6 +90,7 @@ struct symbol_conf {
 	const char		*addr2line_path;
 	enum a2l_style	addr2line_style[MAX_A2L_STYLE];
 	int             addr2line_timeout_ms;
+	enum unwind_style unwind_style[MAX_UNWIND_STYLE];
 	unsigned long	time_quantum;
        struct strlist	*dso_list,
 			*comm_list,

tools/perf/util/unwind-libdw.c

@@ -339,7 +339,7 @@ frame_callback(Dwfl_Frame *state, void *arg)
 	       DWARF_CB_ABORT : DWARF_CB_OK;
 }
 
-int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
+int libdw__get_entries(unwind_entry_cb_t cb, void *arg,
 			struct thread *thread,
 			struct perf_sample *data,
 			int max_stack,
@@ -353,10 +353,10 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 	static struct unwind_info *ui;
 	Dwfl *dwfl;
 	Dwarf_Word ip;
-	int err = -EINVAL, i;
+	int err = -EINVAL, i, entries;
 
 	if (!data->user_regs || !data->user_regs->regs)
-		return -EINVAL;
+		return 0;
 
 	ui = zalloc(sizeof(*ui) + sizeof(ui->entries[0]) * max_stack);
 	if (!ui)
@@ -430,6 +430,18 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 		map_symbol__exit(&ui->entries[i].ms);
 
 	dwfl_ui_ti->ui = NULL;
+	entries = (int)ui->idx;
 	free(ui);
-	return 0;
+	/*
+	 * Unwinder return contract:
+	 *  > 0 : unwinding succeeded (stops fallback). If we found frames but hit an error
+	 *        (e.g. truncated stack), report success to preserve existing frames.
+	 *    0 : unwinding failed without yielding frames. Ignore non-fatal errors
+	 *        (e.g. missing debug info, DWARF corruption) to allow fallback unwinder or
+	 *        kernel callchain resolution to proceed.
+	 *  < 0 : fatal error (e.g. -ENOMEM). Aborts unwinding entirely.
+	 */
+	if (err)
+		return (err == -ENOMEM) ? -ENOMEM : (entries > 0 ? 1 : 0);
+	return entries;
 }

tools/perf/util/unwind-libunwind-local.c

@@ -744,7 +744,7 @@ static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
 	ret = perf_reg_value(&val, perf_sample__user_regs(ui->sample),
 			     perf_arch_reg_ip(e_machine));
 	if (ret)
-		return ret;
+		return 0;
 
 	ips[i++] = (unw_word_t) val;
 
@@ -757,7 +757,7 @@ static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
 		addr_space = maps__addr_space(thread__maps(ui->thread));
 
 		if (addr_space == NULL)
-			return -1;
+			return 0;
 
 		ret = unw_init_remote(&c, addr_space, ui);
 		if (ret && !ui->best_effort)
@@ -785,15 +785,30 @@ static int get_entries(struct unwind_info *ui, unwind_entry_cb_t cb,
 	/*
 	 * Display what we got based on the order setup.
 	 */
+	int entries = 0;
 	for (i = 0; i < max_stack && !ret; i++) {
 		int j = i;
 
 		if (callchain_param.order == ORDER_CALLER)
 			j = max_stack - i - 1;
-		ret = ips[j] ? entry(ips[j], ui->thread, cb, arg) : 0;
+		if (ips[j]) {
+			ret = entry(ips[j], ui->thread, cb, arg);
+			if (ret)
+				break;
+			entries++;
+		}
 	}
 
-	return ret;
+	/*
+	 * Unwinder return contract:
+	 *  > 0 : unwinding succeeded (stops fallback).
+	 *    0 : unwinding failed without yielding frames. Ignore non-fatal errors
+	 *        (e.g. stepping failure) to allow fallback unwinder or kernel callchains.
+	 *  < 0 : fatal error (e.g. -ENOMEM). Aborts unwinding entirely.
+	 */
+	if (ret == -ENOMEM)
+		return -ENOMEM;
+	return (entries > 0 || ret == 0) ? entries : 0;
 }
 
 static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
@@ -809,10 +824,10 @@ static int _unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 	};
 
 	if (!data->user_regs || !data->user_regs->regs)
-		return -EINVAL;
+		return 0;
 
 	if (max_stack <= 0)
-		return -EINVAL;
+		return 0;
 
 	return get_entries(&ui, cb, arg, max_stack);
 }

tools/perf/util/unwind-libunwind.c

@@ -79,7 +79,7 @@ void unwind__finish_access(struct maps *maps)
 		ops->finish_access(maps);
 }
 
-int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
+int libunwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			 struct thread *thread,
 			 struct perf_sample *data, int max_stack,
 			 bool best_effort)

tools/perf/util/unwind.c

@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "debug.h"
+#include "symbol_conf.h"
+#include "unwind.h"
+#include <linux/string.h>
+#include <string.h>
+#include <stdlib.h>
+
+int unwind__get_entries(unwind_entry_cb_t cb __maybe_unused, void *arg __maybe_unused,
+			struct thread *thread __maybe_unused,
+			struct perf_sample *data __maybe_unused,
+			int max_stack __maybe_unused,
+			bool best_effort __maybe_unused)
+{
+	int ret = 0;
+
+#if defined(HAVE_LIBDW_SUPPORT) || defined(HAVE_LIBUNWIND_SUPPORT)
+	if (symbol_conf.unwind_style[0] == UNWIND_STYLE_UNKNOWN) {
+		int i = 0;
+#ifdef HAVE_LIBDW_SUPPORT
+		symbol_conf.unwind_style[i++] = UNWIND_STYLE_LIBDW;
+#endif
+#ifdef HAVE_LIBUNWIND_SUPPORT
+		symbol_conf.unwind_style[i++] = UNWIND_STYLE_LIBUNWIND;
+#endif
+	}
+#endif //defined(HAVE_LIBDW_SUPPORT) || defined(HAVE_LIBUNWIND_SUPPORT)
+
+	for (size_t i = 0; i < ARRAY_SIZE(symbol_conf.unwind_style); i++) {
+		switch (symbol_conf.unwind_style[i]) {
+		case UNWIND_STYLE_LIBDW:
+			ret = libdw__get_entries(cb, arg, thread, data, max_stack, best_effort);
+			break;
+		case UNWIND_STYLE_LIBUNWIND:
+			ret = libunwind__get_entries(cb, arg, thread, data, max_stack, best_effort);
+			break;
+		case UNWIND_STYLE_UNKNOWN:
+		default:
+#if !defined(HAVE_LIBDW_SUPPORT) && !defined(HAVE_LIBUNWIND_SUPPORT)
+			pr_warning_once(
+				"Error: dwarf unwinding not supported, build perf with libdw or libunwind.\n");
+#endif
+			ret = 0;
+			break;
+		}
+		if (ret > 0) {
+			ret = 0;
+			break;
+		}
+		if (ret < 0)
+			break;
+	}
+	return ret;
+}
+
+int unwind__configure(const char *var, const char *value, void *cb __maybe_unused)
+{
+	static const char * const unwind_style_names[] = {
+		[UNWIND_STYLE_LIBDW] = "libdw",
+		[UNWIND_STYLE_LIBUNWIND] = "libunwind",
+		NULL
+	};
+	char *s, *p, *saveptr;
+	size_t i = 0;
+
+	if (strcmp(var, "unwind.style"))
+		return 0;
+
+	if (!value)
+		return -1;
+
+	s = strdup(value);
+	if (!s)
+		return -1;
+
+	memset(symbol_conf.unwind_style, 0, sizeof(symbol_conf.unwind_style));
+
+	p = strtok_r(s, ",", &saveptr);
+	while (p && i < ARRAY_SIZE(symbol_conf.unwind_style)) {
+		bool found = false;
+		char *q = strim(p);
+
+		for (size_t j = UNWIND_STYLE_LIBDW; j < MAX_UNWIND_STYLE; j++) {
+			if (!strcasecmp(q, unwind_style_names[j])) {
+				symbol_conf.unwind_style[i++] = j;
+				found = true;
+				break;
+			}
+		}
+		if (!found)
+			pr_warning("Unknown unwind style: %s\n", q);
+		p = strtok_r(NULL, ",", &saveptr);
+	}
+
+	free(s);
+	return 0;
+}
+
+int unwind__option(const struct option *opt __maybe_unused,
+		   const char *arg,
+		   int unset __maybe_unused)
+{
+	return unwind__configure("unwind.style", arg, NULL);
+}

tools/perf/util/unwind.h

@@ -4,9 +4,10 @@
 
 #include <linux/compiler.h>
 #include <linux/types.h>
-#include "util/map_symbol.h"
+#include "map_symbol.h"
 
 struct maps;
+struct option;
 struct perf_sample;
 struct thread;
 
@@ -26,7 +27,9 @@ struct unwind_libunwind_ops {
 			   struct perf_sample *data, int max_stack, bool best_effort);
 };
 
-#ifdef HAVE_DWARF_UNWIND_SUPPORT
+int unwind__configure(const char *var, const char *value, void *cb);
+int unwind__option(const struct option *opt, const char *arg, int unset);
+
 /*
  * When best_effort is set, don't report errors and fail silently. This could
  * be expanded in the future to be more permissive about things other than
@@ -36,8 +39,31 @@ int unwind__get_entries(unwind_entry_cb_t cb, void *arg,
 			struct thread *thread,
 			struct perf_sample *data, int max_stack,
 			bool best_effort);
-/* libunwind specific */
+
+#ifdef HAVE_LIBDW_SUPPORT
+int libdw__get_entries(unwind_entry_cb_t cb, void *arg,
+		       struct thread *thread,
+		       struct perf_sample *data, int max_stack,
+		       bool best_effort);
+#else
+#include "debug.h"
+static inline int libdw__get_entries(unwind_entry_cb_t cb __maybe_unused, void *arg __maybe_unused,
+				     struct thread *thread __maybe_unused,
+				     struct perf_sample *data __maybe_unused,
+				     int max_stack __maybe_unused,
+				     bool best_effort __maybe_unused)
+{
+	pr_warning_once("Error: libdw dwarf unwinding not built into perf\n");
+	return 0;
+}
+#endif
+
 #ifdef HAVE_LIBUNWIND_SUPPORT
+/* libunwind specific */
+int libunwind__get_entries(unwind_entry_cb_t cb, void *arg,
+			   struct thread *thread,
+			   struct perf_sample *data, int max_stack,
+			   bool best_effort);
 #ifndef LIBUNWIND__ARCH_REG_ID
 #define LIBUNWIND__ARCH_REG_ID(regnum) libunwind__arch_reg_id(regnum)
 #endif
@@ -47,6 +73,18 @@ int unwind__prepare_access(struct maps *maps, struct map *map, bool *initialized
 void unwind__flush_access(struct maps *maps);
 void unwind__finish_access(struct maps *maps);
 #else
+#include "debug.h"
+static inline int libunwind__get_entries(unwind_entry_cb_t cb __maybe_unused,
+					 void *arg __maybe_unused,
+					 struct thread *thread __maybe_unused,
+					 struct perf_sample *data __maybe_unused,
+					 int max_stack __maybe_unused,
+					 bool best_effort __maybe_unused)
+{
+	pr_warning_once("Error: libunwind dwarf unwinding not built into perf\n");
+	return 0;
+}
+
 static inline int unwind__prepare_access(struct maps *maps __maybe_unused,
 					 struct map *map __maybe_unused,
 					 bool *initialized __maybe_unused)
@@ -57,26 +95,5 @@ static inline int unwind__prepare_access(struct maps *maps __maybe_unused,
 static inline void unwind__flush_access(struct maps *maps __maybe_unused) {}
 static inline void unwind__finish_access(struct maps *maps __maybe_unused) {}
 #endif
-#else
-static inline int
-unwind__get_entries(unwind_entry_cb_t cb __maybe_unused,
-		    void *arg __maybe_unused,
-		    struct thread *thread __maybe_unused,
-		    struct perf_sample *data __maybe_unused,
-		    int max_stack __maybe_unused,
-		    bool best_effort __maybe_unused)
-{
-	return 0;
-}
 
-static inline int unwind__prepare_access(struct maps *maps __maybe_unused,
-					 struct map *map __maybe_unused,
-					 bool *initialized __maybe_unused)
-{
-	return 0;
-}
-
-static inline void unwind__flush_access(struct maps *maps __maybe_unused) {}
-static inline void unwind__finish_access(struct maps *maps __maybe_unused) {}
-#endif /* HAVE_DWARF_UNWIND_SUPPORT */
 #endif /* __UNWIND_H */