[PATCH bpf-next v1 1/4] selftests/bpf: NUL-terminate bpftool command output

[Ihor Solodrai <ihor.solodrai@linux.dev>]

run_command() writes a bpftool command's output into a caller-supplied
buffer, and every caller treats that buffer as a C string.

However fread() reports a byte count and doesn't terminate the
string. The helper does not terminate either, so callers have to zero
the buffer first. prog_tests/bpftool_metadata.c does not, for example.

Read one byte less and terminate in the helper.

Fixes: f21fae577446 ("selftests/bpf: Add a few helpers for bpftool testing")
Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
---
 tools/testing/selftests/bpf/bpftool_helpers.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/bpftool_helpers.c b/tools/testing/selftests/bpf/bpftool_helpers.c
index 0a2a4f0a2794..c49fdd90eb03 100644
--- a/tools/testing/selftests/bpf/bpftool_helpers.c
+++ b/tools/testing/selftests/bpf/bpftool_helpers.c
@@ -53,6 +53,7 @@ static int run_command(char *args, char *output_buf, size_t output_max_len)
 	static char bpftool_path[PATH_MAX] = {};
 	bool suppress_output = !(output_buf && output_max_len);
 	char command[BPFTOOL_FULL_CMD_MAX_LEN];
+	size_t n;
 	FILE *f;
 	int ret;
 
@@ -68,8 +69,10 @@ static int run_command(char *args, char *output_buf, size_t output_max_len)
 	if (!f)
 		return 1;
 
-	if (!suppress_output)
-		fread(output_buf, 1, output_max_len, f);
+	if (!suppress_output) {
+		n = fread(output_buf, 1, output_max_len - 1, f);
+		output_buf[n] = '\0';
+	}
 	ret = pclose(f);
 
 	return ret;
-- 
2.55.0