[PATCH bpf] samples/bpf: suppress compiler warning

[Matteo Croce]

GCC 9 fails to calculate the size of local constant strings and produces a
false positive:

samples/bpf/task_fd_query_user.c: In function ‘test_debug_fs_uprobe’:
samples/bpf/task_fd_query_user.c:242:67: warning: ‘%s’ directive output may be 
truncated writing up to 255 bytes into a region of size 215 
[-Wformat-truncation=]
  242 |  snprintf(buf, sizeof(buf), 
"/sys/kernel/debug/tracing/events/%ss/%s/id",
      |                                                                   ^~
  243 |    event_type, event_alias);
      |                ~~~~~~~~~~~
samples/bpf/task_fd_query_user.c:242:2: note: ‘snprintf’ output between 45 and 
300 bytes into a destination of size 256
  242 |  snprintf(buf, sizeof(buf), 
"/sys/kernel/debug/tracing/events/%ss/%s/id",
      |  
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  243 |    event_type, event_alias);
      |    ~~~~~~~~~~~~~~~~~~~~~~~~

Workaround this by lowering the buffer size to a reasonable value.
Related GCC Bugzilla: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83431

Signed-off-by: Matteo Croce <mcr...@redhat.com>
---
 samples/bpf/task_fd_query_user.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/samples/bpf/task_fd_query_user.c b/samples/bpf/task_fd_query_user.c
index aff2b4ae914e..e39938058223 100644
--- a/samples/bpf/task_fd_query_user.c
+++ b/samples/bpf/task_fd_query_user.c
@@ -216,7 +216,7 @@ static int test_debug_fs_uprobe(char *binary_path, long 
offset, bool is_return)
 {
        const char *event_type = "uprobe";
        struct perf_event_attr attr = {};
-       char buf[256], event_alias[256];
+       char buf[256], event_alias[sizeof("test_1234567890")];
        __u64 probe_offset, probe_addr;
        __u32 len, prog_id, fd_type;
        int err, res, kfd, efd;
-- 
2.21.0