[PATCH v3] perf data convert: Fix segfault when converting to json on arm64

[Ilkka Koskinen <ilkka@os.amperecomputing.com>]

Arm64 doesn't have Model in /proc/cpuinfo and, thus, cpu_desc doesn't get
assigned.

Running
	$ perf data convert --to-json perf.data.json

ends up calling output_json_string() with NULL pointer, which causes a
segmentation fault.

Signed-off-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
---
v1:
    - https://lore.kernel.org/all/20240111232923.8138-1-ilkka@os.amperecomputing.com/
v2:
    - Changed the patch based on James's comments.
v3:
    - The architecture is checked from the actual data file to allow one to do
      conversion on another system. (thanks to James for the feedback)
---
tools/perf/util/data-convert-json.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/data-convert-json.c b/tools/perf/util/data-convert-json.c
index 5bb3c2ba95ca..405c38371870 100644
--- a/tools/perf/util/data-convert-json.c
+++ b/tools/perf/util/data-convert-json.c
@@ -284,7 +284,13 @@ static void output_headers(struct perf_session *session, struct convert_json *c)
 	output_json_key_string(out, true, 2, "os-release", header->env.os_release);
 	output_json_key_string(out, true, 2, "arch", header->env.arch);
 
-	output_json_key_string(out, true, 2, "cpu-desc", header->env.cpu_desc);
+	/*
+	 * Arm64 doesn't have Model section in /proc/cpuinfo and, thus, cpu-desc
+	 * is not set.
+	 */
+	if (strncmp(header->env.arch, "aarch64", 7))
+		output_json_key_string(out, true, 2, "cpu-desc", header->env.cpu_desc);
+
 	output_json_key_string(out, true, 2, "cpuid", header->env.cpuid);
 	output_json_key_format(out, true, 2, "nrcpus-online", "%u", header->env.nr_cpus_online);
 	output_json_key_format(out, true, 2, "nrcpus-avail", "%u", header->env.nr_cpus_avail);
-- 
2.43.0