samples, bpf: suppress compiler warning [Linux 5.2]

samples, bpf: suppress compiler warning [Linux 5.2]

This Linux kernel change "samples, bpf: suppress compiler warning" is included in the Linux 5.2 release. This change is authored by Matteo Croce <mcroce [at] redhat.com> on Mon May 20 23:49:38 2019 +0200. The commit for this change in Linux stable tree is a195cef (patch).

samples, bpf: suppress compiler warning

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 <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>

There are 2 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 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 aff2b4a..e399380 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;

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