[bpf-next 1/2] samples: bpf: Fix tracex7 error raised on the missing argument

[Juhee Kang <claudiajkang@gmail.com>]

The current behavior of 'tracex7' doesn't consist with other bpf samples
tracex{1..6}. Other samples do not require any argument to run with, but
tracex7 should be run with btrfs device argument. (it should be executed
with test_override_return.sh)

Currently, tracex7 doesn't have any description about how to run this
program and raises an unexpected error. And this result might be
confusing since users might not have a hunch about how to run this
program.

    // Current behavior
    # ./tracex7
    sh: 1: Syntax error: word unexpected (expecting ")")
    // Fixed behavior
    # ./tracex7
    ERROR: Run with the btrfs device argument!

In order to fix this error, this commit adds logic to report a message
and exit when running this program with a missing argument.

Additionally in test_override_return.sh, there is a problem with
multiple directory(tmpmnt) creation. So in this commit adds a line with
removing the directory with every execution.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
---
 samples/bpf/test_override_return.sh | 1 +
 samples/bpf/tracex7_user.c          | 5 +++++
 2 files changed, 6 insertions(+)

diff --git a/samples/bpf/test_override_return.sh b/samples/bpf/test_override_return.sh
index e68b9ee6814b..6480b55502c7 100755
--- a/samples/bpf/test_override_return.sh
+++ b/samples/bpf/test_override_return.sh
@@ -1,5 +1,6 @@
 #!/bin/bash
 
+rm -rf tmpmnt
 rm -f testfile.img
 dd if=/dev/zero of=testfile.img bs=1M seek=1000 count=1
 DEVICE=$(losetup --show -f testfile.img)
diff --git a/samples/bpf/tracex7_user.c b/samples/bpf/tracex7_user.c
index fdcd6580dd73..8be7ce18d3ba 100644
--- a/samples/bpf/tracex7_user.c
+++ b/samples/bpf/tracex7_user.c
@@ -14,6 +14,11 @@ int main(int argc, char **argv)
 	int ret = 0;
 	FILE *f;
 
+	if (!argv[1]) {
+		fprintf(stderr, "ERROR: Run with the btrfs device argument!\n");
+		return 0;
+	}
+
 	snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
 	obj = bpf_object__open_file(filename, NULL);
 	if (libbpf_get_error(obj)) {
-- 
2.27.0