perf symbols: Save DSO loading errno to better report errors

index 5fb8723..1cb3436 100644
@@ -757,8 +757,10 @@ static void perf_event__process_sample(struct perf_tool *tool,
 		    al.map == machine->vmlinux_maps[MAP__FUNCTION] &&
 		    RB_EMPTY_ROOT(&al.map->dso->symbols[MAP__FUNCTION])) {
 			if (symbol_conf.vmlinux_name) {
-				ui__warning("The %s file can't be used.\n%s",
-					    symbol_conf.vmlinux_name, msg);
+				char serr[256];
+				dso__strerror_load(al.map->dso, serr, sizeof(serr));
+				ui__warning("The %s file can't be used: %s\n%s",
+					    symbol_conf.vmlinux_name, serr, msg);
 			} else {
 				ui__warning("A vmlinux file was not found.\n%s",
 					    msg);

index 0d3667f..fc0ddd5 100644
@@ -1137,3 +1137,36 @@ enum dso_type dso__type(struct dso *dso, struct machine *machine)
 
 	return dso__type_fd(fd);
 }
+
+int dso__strerror_load(struct dso *dso, char *buf, size_t buflen)
+{
+	int idx, errnum = dso->load_errno;
+	/*
+	 * This must have a same ordering as the enum dso_load_errno.
+	 */
+	static const char *dso_load__error_str[] = {
+	"Internal tools/perf/ library error",
+	"Invalid ELF file",
+	"Can not read build id",
+	"Mismatching build id",
+	"Decompression failure",
+	};
+
+	BUG_ON(buflen == 0);
+
+	if (errnum >= 0) {
+		const char *err = strerror_r(errnum, buf, buflen);
+
+		if (err != buf)
+			scnprintf(buf, buflen, "%s", err);
+
+		return 0;
+	}
+
+	if (errnum <  __DSO_LOAD_ERRNO__START || errnum >= __DSO_LOAD_ERRNO__END)
+		return -1;
+
+	idx = errnum - __DSO_LOAD_ERRNO__START;
+	scnprintf(buf, buflen, "%s", dso_load__error_str[idx]);
+	return 0;
+}

index 88f345c..e0901b4 100644
@@ -60,6 +60,31 @@ enum dso_type {
 	DSO__TYPE_X32BIT,
 };
 
+enum dso_load_errno {
+	DSO_LOAD_ERRNO__SUCCESS		= 0,
+
+	/*
+	 * Choose an arbitrary negative big number not to clash with standard
+	 * errno since SUS requires the errno has distinct positive values.
+	 * See 'Issue 6' in the link below.
+	 *
+	 * http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/errno.h.html
+	 */
+	__DSO_LOAD_ERRNO__START		= -10000,
+
+	DSO_LOAD_ERRNO__INTERNAL_ERROR	= __DSO_LOAD_ERRNO__START,
+
+	/* for symsrc__init() */
+	DSO_LOAD_ERRNO__INVALID_ELF,
+	DSO_LOAD_ERRNO__CANNOT_READ_BUILDID,
+	DSO_LOAD_ERRNO__MISMATCHING_BUILDID,
+
+	/* for decompress_kmodule */
+	DSO_LOAD_ERRNO__DECOMPRESSION_FAILURE,
+
+	__DSO_LOAD_ERRNO__END,
+};
+
 #define DSO__SWAP(dso, type, val)			\
 ({							\
 	type ____r = val;				\

@@ -113,6 +138,7 @@ struct dso {
 	enum dso_swap_type	needs_swap;
 	enum dso_binary_type	symtab_type;
 	enum dso_binary_type	binary_type;
+	enum dso_load_errno	load_errno;
 	u8		 adjust_symbols:1;
 	u8		 has_build_id:1;
 	u8		 has_srcline:1;

@@ -294,4 +320,6 @@ void dso__free_a2l(struct dso *dso);
 
 enum dso_type dso__type(struct dso *dso, struct machine *machine);
 
+int dso__strerror_load(struct dso *dso, char *buf, size_t buflen);
+
 #endif /* __PERF_DSO */

index 78ffde9..476268c 100644
@@ -595,10 +595,13 @@ static int decompress_kmodule(struct dso *dso, const char *name,
 		return -1;
 
 	fd = mkstemp(tmpbuf);
-	if (fd < 0)
+	if (fd < 0) {
+		dso->load_errno = errno;
 		goto out;
+	}
 
 	if (!decompress_to_file(m.ext, name, fd)) {
+		dso->load_errno = DSO_LOAD_ERRNO__DECOMPRESSION_FAILURE;
 		close(fd);
 		fd = -1;
 	}

@@ -635,37 +638,49 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
 	Elf *elf;
 	int fd;
 
-	if (dso__needs_decompress(dso))
+	if (dso__needs_decompress(dso)) {
 		fd = decompress_kmodule(dso, name, type);
-	else
+		if (fd < 0)
+			return -1;
+	} else {
 		fd = open(name, O_RDONLY);
-
-	if (fd < 0)
-		return -1;
+		if (fd < 0) {
+			dso->load_errno = errno;
+			return -1;
+		}
+	}
 
 	elf = elf_begin(fd, PERF_ELF_C_READ_MMAP, NULL);
 	if (elf == NULL) {
 		pr_debug("%s: cannot read %s ELF file.\n", __func__, name);
+		dso->load_errno = DSO_LOAD_ERRNO__INVALID_ELF;
 		goto out_close;
 	}
 
 	if (gelf_getehdr(elf, &ehdr) == NULL) {
+		dso->load_errno = DSO_LOAD_ERRNO__INVALID_ELF;
 		pr_debug("%s: cannot get elf header.\n", __func__);
 		goto out_elf_end;
 	}
 
-	if (dso__swap_init(dso, ehdr.e_ident[EI_DATA]))
+	if (dso__swap_init(dso, ehdr.e_ident[EI_DATA])) {
+		dso->load_errno = DSO_LOAD_ERRNO__INTERNAL_ERROR;
 		goto out_elf_end;
+	}
 
 	/* Always reject images with a mismatched build-id: */
 	if (dso->has_build_id) {
 		u8 build_id[BUILD_ID_SIZE];
 
-		if (elf_read_build_id(elf, build_id, BUILD_ID_SIZE) < 0)
+		if (elf_read_build_id(elf, build_id, BUILD_ID_SIZE) < 0) {
+			dso->load_errno = DSO_LOAD_ERRNO__CANNOT_READ_BUILDID;
 			goto out_elf_end;
+		}
 
-		if (!dso__build_id_equal(dso, build_id))
+		if (!dso__build_id_equal(dso, build_id)) {
+			dso->load_errno = DSO_LOAD_ERRNO__MISMATCHING_BUILDID;
 			goto out_elf_end;
+		}
 	}
 
 	ss->is_64_bit = (gelf_getclass(elf) == ELFCLASS64);

@@ -701,8 +716,10 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
 	}
 
 	ss->name   = strdup(name);
-	if (!ss->name)
+	if (!ss->name) {
+		dso->load_errno = errno;
 		goto out_elf_end;
+	}
 
 	ss->elf    = elf;
 	ss->fd     = fd;

index d7efb03..fd8477c 100644
@@ -246,13 +246,12 @@ out:
 	return ret;
 }
 
-int symsrc__init(struct symsrc *ss, struct dso *dso __maybe_unused,
-		 const char *name,
+int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
 	         enum dso_binary_type type)
 {
 	int fd = open(name, O_RDONLY);
 	if (fd < 0)
-		return -1;
+		goto out_errno;
 
 	ss->name = strdup(name);
 	if (!ss->name)

@@ -264,6 +263,8 @@ int symsrc__init(struct symsrc *ss, struct dso *dso __maybe_unused,
 	return 0;
 out_close:
 	close(fd);
+out_errno:
+	dso->load_errno = errno;
 	return -1;
 }