[PATCH] tracing/synthetic: avoid comparing string against NULL

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

gcc-10 warns about passing a NULL argument into __synth_event_add_val(),
which uses it in an unchecked strcmp():

In function '__synth_event_add_val',
    inlined from 'synth_event_add_next_val' at kernel/trace/trace_events_synth.c:2162:9:
kernel/trace/trace_events_synth.c:2076:8: error: argument 2 null where non-null expected [-Werror=nonnull]
 2076 |    if (strcmp(field->name, field_name) == 0)
      |        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from include/linux/string.h:65,
                 from kernel/trace/trace_events_synth.c:8:
kernel/trace/trace_events_synth.c: In function 'synth_event_add_next_val':
arch/s390/include/asm/string.h:39:5: note: in a call to function 'strcmp' declared here
   39 | int strcmp(const char *s1, const char *s2);
      |     ^~~~~~

The code is actually correct here, as the trace_state->add_name
does check for it, but this is hard for the compiler to prove.

Avoid the problem by repeating the check where the compiler
sees it.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 kernel/trace/trace_events_synth.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_synth.c b/kernel/trace/trace_events_synth.c
index dc15658a887c..c1287ca95aff 100644
--- a/kernel/trace/trace_events_synth.c
+++ b/kernel/trace/trace_events_synth.c
@@ -2073,7 +2073,8 @@ static int __synth_event_add_val(const char *field_name, u64 val,
 	if (trace_state->add_name) {
 		for (i = 0; i < event->n_fields; i++) {
 			field = event->fields[i];
-			if (strcmp(field->name, field_name) == 0)
+			if (!field_name ||
+			    strcmp(field->name, field_name) == 0)
 				break;
 		}
 		if (!field) {
-- 
2.53.0