perf symbols: Accept symbols starting at address 0

That is the case of _text on s390, and we have some functions that return an
address, using address zero to report problems, oops.

This would lead the symbol loading routines to not use "_text" as the reference
relocation symbol, or the first symbol for the kernel, but use instead
"_stext", that is at the same address on x86_64 and others, but not on s390:

  [acme@localhost perf-4.11.0-rc6]$ head -15 /proc/kallsyms
  0000000000000000 T _text
  0000000000000418 t iplstart
  0000000000000800 T start
  000000000000080a t .base
  000000000000082e t .sk8x8
  0000000000000834 t .gotr
  0000000000000842 t .cmd
  0000000000000846 t .parm
  000000000000084a t .lowcase
  0000000000010000 T startup
  0000000000010010 T startup_kdump
  0000000000010214 t startup_kdump_relocated
  0000000000011000 T startup_continue
  00000000000112a0 T _ehead
  0000000000100000 T _stext
  [acme@localhost perf-4.11.0-rc6]$

Which in turn would make 'perf test vmlinux' to fail because it wouldn't find
the symbols before "_stext" in kallsyms.

Fix it by using the return value only for errors and storing the
address, when the symbol is successfully found, in a provided pointer
arg.

Before this patch:

After:

  [acme@localhost perf-4.11.0-rc6]$ tools/perf/perf test -v 1
   1: vmlinux symtab matches kallsyms            :
  --- start ---
  test child forked, pid 40693
  Looking at the vmlinux_path (8 entries long)
  Using /usr/lib/debug/lib/modules/3.10.0-654.el7.s390x/vmlinux for symbols
  ERR : 0: _text not on kallsyms
  ERR : 0x418: iplstart not on kallsyms
  ERR : 0x800: start not on kallsyms
  ERR : 0x80a: .base not on kallsyms
  ERR : 0x82e: .sk8x8 not on kallsyms
  ERR : 0x834: .gotr not on kallsyms
  ERR : 0x842: .cmd not on kallsyms
  ERR : 0x846: .parm not on kallsyms
  ERR : 0x84a: .lowcase not on kallsyms
  ERR : 0x10000: startup not on kallsyms
  ERR : 0x10010: startup_kdump not on kallsyms
  ERR : 0x10214: startup_kdump_relocated not on kallsyms
  ERR : 0x11000: startup_continue not on kallsyms
  ERR : 0x112a0: _ehead not on kallsyms
  <SNIP warnings>
  test child finished with -1
  ---- end ----
  vmlinux symtab matches kallsyms: FAILED!
  [acme@localhost perf-4.11.0-rc6]$

After:

  [acme@localhost perf-4.11.0-rc6]$ tools/perf/perf test -v 1
   1: vmlinux symtab matches kallsyms            :
  --- start ---
  test child forked, pid 47160
  <SNIP warnings>
  test child finished with 0
  ---- end ----
  vmlinux symtab matches kallsyms: Ok
  [acme@localhost perf-4.11.0-rc6]$

Reported-by: Michael Petlan <mpetlan@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-9x9bwgd3btwdk1u51xie93fz@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-buildid-cache.c

@@ -49,19 +49,22 @@ static bool same_kallsyms_reloc(const char *from_dir, char *to_dir)
 	char to[PATH_MAX];
 	const char *name;
 	u64 addr1 = 0, addr2 = 0;
-	int i;
+	int i, err = -1;
 
 	scnprintf(from, sizeof(from), "%s/kallsyms", from_dir);
 	scnprintf(to, sizeof(to), "%s/kallsyms", to_dir);
 
 	for (i = 0; (name = ref_reloc_sym_names[i]) != NULL; i++) {
-		addr1 = kallsyms__get_function_start(from, name);
-		if (addr1)
+		err = kallsyms__get_function_start(from, name, &addr1);
+		if (!err)
 			break;
 	}
 
-	if (name)
-		addr2 = kallsyms__get_function_start(to, name);
+	if (err)
+		return false;
+
+	if (kallsyms__get_function_start(to, name, &addr2))
+		return false;
 
 	return addr1 == addr2;
 }

tools/perf/util/event.c

@@ -768,15 +768,16 @@ static int find_symbol_cb(void *arg, const char *name, char type,
 	return 1;
 }
 
-u64 kallsyms__get_function_start(const char *kallsyms_filename,
-				 const char *symbol_name)
+int kallsyms__get_function_start(const char *kallsyms_filename,
+				 const char *symbol_name, u64 *addr)
 {
 	struct process_symbol_args args = { .name = symbol_name, };
 
 	if (kallsyms__parse(kallsyms_filename, &args, find_symbol_cb) <= 0)
-		return 0;
+		return -1;
 
-	return args.start;
+	*addr = args.start;
+	return 0;
 }
 
 int perf_event__synthesize_kernel_mmap(struct perf_tool *tool,

tools/perf/util/event.h

@@ -675,8 +675,8 @@ size_t perf_event__fprintf_cpu_map(union perf_event *event, FILE *fp);
 size_t perf_event__fprintf_namespaces(union perf_event *event, FILE *fp);
 size_t perf_event__fprintf(union perf_event *event, FILE *fp);
 
-u64 kallsyms__get_function_start(const char *kallsyms_filename,
-				 const char *symbol_name);
+int kallsyms__get_function_start(const char *kallsyms_filename,
+				 const char *symbol_name, u64 *addr);
 
 void *cpu_map_data__alloc(struct cpu_map *map, size_t *size, u16 *type, int *max);
 void  cpu_map_data__synthesize(struct cpu_map_data *data, struct cpu_map *map,

tools/perf/util/machine.c

@@ -796,11 +796,11 @@ const char *ref_reloc_sym_names[] = {"_text", "_stext", NULL};
  * Returns the name of the start symbol in *symbol_name. Pass in NULL as
  * symbol_name if it's not that important.
  */
-static u64 machine__get_running_kernel_start(struct machine *machine,
-					     const char **symbol_name)
+static int machine__get_running_kernel_start(struct machine *machine,
+					     const char **symbol_name, u64 *start)
 {
 	char filename[PATH_MAX];
-	int i;
+	int i, err = -1;
 	const char *name;
 	u64 addr = 0;
 
@@ -810,21 +810,28 @@ static u64 machine__get_running_kernel_start(struct machine *machine,
 		return 0;
 
 	for (i = 0; (name = ref_reloc_sym_names[i]) != NULL; i++) {
-		addr = kallsyms__get_function_start(filename, name);
-		if (addr)
+		err = kallsyms__get_function_start(filename, name, &addr);
+		if (!err)
 			break;
 	}
 
+	if (err)
+		return -1;
+
 	if (symbol_name)
 		*symbol_name = name;
 
-	return addr;
+	*start = addr;
+	return 0;
 }
 
 int __machine__create_kernel_maps(struct machine *machine, struct dso *kernel)
 {
 	int type;
-	u64 start = machine__get_running_kernel_start(machine, NULL);
+	u64 start = 0;
+
+	if (machine__get_running_kernel_start(machine, NULL, &start))
+		return -1;
 
 	/* In case of renewal the kernel map, destroy previous one */
 	machine__destroy_kernel_maps(machine);
@@ -1185,8 +1192,8 @@ static int machine__create_modules(struct machine *machine)
 int machine__create_kernel_maps(struct machine *machine)
 {
 	struct dso *kernel = machine__get_kernel(machine);
-	const char *name;
-	u64 addr;
+	const char *name = NULL;
+	u64 addr = 0;
 	int ret;
 
 	if (kernel == NULL)
@@ -1211,8 +1218,7 @@ int machine__create_kernel_maps(struct machine *machine)
 	 */
 	map_groups__fixup_end(&machine->kmaps);
 
-	addr = machine__get_running_kernel_start(machine, &name);
-	if (!addr) {
+	if (machine__get_running_kernel_start(machine, &name, &addr)) {
 	} else if (maps__set_kallsyms_ref_reloc_sym(machine->vmlinux_maps, name, addr)) {
 		machine__destroy_kernel_maps(machine);
 		return -1;

tools/perf/util/symbol.c

@@ -466,7 +466,7 @@ void dso__insert_symbol(struct dso *dso, enum map_type type, struct symbol *sym)
 struct symbol *dso__find_symbol(struct dso *dso,
 				enum map_type type, u64 addr)
 {
-	if (dso->last_find_result[type].addr != addr) {
+	if (dso->last_find_result[type].addr != addr || dso->last_find_result[type].symbol == NULL) {
 		dso->last_find_result[type].addr   = addr;
 		dso->last_find_result[type].symbol = symbols__find(&dso->symbols[type], addr);
 	}
@@ -1075,8 +1075,9 @@ static int validate_kcore_addresses(const char *kallsyms_filename,
 	if (kmap->ref_reloc_sym && kmap->ref_reloc_sym->name) {
 		u64 start;
 
-		start = kallsyms__get_function_start(kallsyms_filename,
-						     kmap->ref_reloc_sym->name);
+		if (kallsyms__get_function_start(kallsyms_filename,
+						 kmap->ref_reloc_sym->name, &start))
+			return -ENOENT;
 		if (start != kmap->ref_reloc_sym->addr)
 			return -EINVAL;
 	}
@@ -1248,9 +1249,7 @@ static int kallsyms__delta(struct map *map, const char *filename, u64 *delta)
 	if (!kmap->ref_reloc_sym || !kmap->ref_reloc_sym->name)
 		return 0;
 
-	addr = kallsyms__get_function_start(filename,
-					    kmap->ref_reloc_sym->name);
-	if (!addr)
+	if (kallsyms__get_function_start(filename, kmap->ref_reloc_sym->name, &addr))
 		return -1;
 
 	*delta = addr - kmap->ref_reloc_sym->addr;