[PATCH] perf jvmti: Fix gcc string overflow warning

[Jiri Olsa <jolsa@kernel.org>]

We are getting fake gcc warning when we compile with gcc9 (9.1.1):

     CC       jvmti/libjvmti.o
   In file included from /usr/include/string.h:494,
                    from jvmti/libjvmti.c:5:
   In function ‘strncpy’,
       inlined from ‘copy_class_filename.constprop’ at jvmti/libjvmti.c:166:3:
   /usr/include/bits/string_fortified.h:106:10: error: ‘__builtin_strncpy’ specified bound depends on the length of the source argument [-Werror=stringop-overflow=]
     106 |   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
         |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   jvmti/libjvmti.c: In function ‘copy_class_filename.constprop’:
   jvmti/libjvmti.c:165:26: note: length computed here
     165 |   size_t file_name_len = strlen(file_name);
         |                          ^~~~~~~~~~~~~~~~~
   cc1: all warnings being treated as errors

First I wanted to disable the check, but now I think the code
could be more straight forward. There's no need to check the
source size, strncpy will do that. We just need to make sure
the string is correctly terminated.

Cc: Ben Gainey <ben.gainey@arm.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-sve3b63c550wr907e6ui6gx5@git.kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 tools/perf/jvmti/libjvmti.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/jvmti/libjvmti.c b/tools/perf/jvmti/libjvmti.c
index aea7b1fe85aa..00fa0b7f1ad9 100644
--- a/tools/perf/jvmti/libjvmti.c
+++ b/tools/perf/jvmti/libjvmti.c
@@ -162,8 +162,8 @@ copy_class_filename(const char * class_sign, const char * file_name, char * resu
 		result[i] = '\0';
 	} else {
 		/* fallback case */
-		size_t file_name_len = strlen(file_name);
-		strncpy(result, file_name, file_name_len < max_length ? file_name_len : max_length);
+		strncpy(result, file_name, max_length - 1);
+		result[max_length - 1] = 0;
 	}
 }
 
-- 
2.21.0