kselftest: Add a ksft_perror() helper

The standard library perror() function provides a convenient way to print
an error message based on the current errno but this doesn't play nicely
with KTAP output. Provide a helper which does an equivalent thing in a KTAP
compatible format.

nolibc doesn't have a strerror() and adding the table of strings required
doesn't seem like a good fit for what it's trying to do so when we're using
that only print the errno.
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/kselftest.h

@@ -48,6 +48,7 @@
 #include <stdlib.h>
 #include <unistd.h>
 #include <stdarg.h>
+#include <string.h>
 #include <stdio.h>
 #endif
 
@@ -155,6 +156,19 @@ static inline void ksft_print_msg(const char *msg, ...)
 	va_end(args);
 }
 
+static inline void ksft_perror(const char *msg)
+{
+#ifndef NOLIBC
+	ksft_print_msg("%s: %s (%d)\n", msg, strerror(errno), errno);
+#else
+	/*
+	 * nolibc doesn't provide strerror() and it seems
+	 * inappropriate to add one, just print the errno.
+	 */
+	ksft_print_msg("%s: %d)\n", msg, errno);
+#endif
+}
+
 static inline void ksft_test_result_pass(const char *msg, ...)
 {
 	int saved_errno = errno;