tools/nolibc: add %m printf format

The %m format can be used to format the current errno.
It is non-standard but supported by other commonly used libcs like glibc and
musl, so applications do rely on them.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250428-nolibc-misc-v2-2-3c043eeab06c@linutronix.de
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/include/nolibc/stdio.h

@@ -20,6 +20,8 @@
 #include "string.h"
 #include "compiler.h"
 
+static const char *strerror(int errnum);
+
 #ifndef EOF
 #define EOF (-1)
 #endif
@@ -292,6 +294,11 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
 				if (!outstr)
 					outstr="(null)";
 			}
+#ifndef NOLIBC_IGNORE_ERRNO
+			else if (c == 'm') {
+				outstr = strerror(errno);
+			}
+#endif /* NOLIBC_IGNORE_ERRNO */
 			else if (c == '%') {
 				/* queue it verbatim */
 				continue;

tools/testing/selftests/nolibc/nolibc-test.c

@@ -1393,6 +1393,23 @@ static int test_scanf(void)
 	return 0;
 }
 
+int test_strerror(void)
+{
+	char buf[100];
+	ssize_t ret;
+
+	memset(buf, 'A', sizeof(buf));
+
+	errno = EINVAL;
+	ret = snprintf(buf, sizeof(buf), "%m");
+	if (is_nolibc) {
+		if (ret < 6 || memcmp(buf, "errno=", 6))
+			return 1;
+	}
+
+	return 0;
+}
+
 static int run_printf(int min, int max)
 {
 	int test;
@@ -1421,6 +1438,7 @@ static int run_printf(int min, int max)
 		CASE_TEST(number_width); EXPECT_VFPRINTF(10, "         1", "%10d", 1); break;
 		CASE_TEST(width_trunc);  EXPECT_VFPRINTF(25, "                    ", "%25d", 1); break;
 		CASE_TEST(scanf);        EXPECT_ZR(1, test_scanf()); break;
+		CASE_TEST(strerror);     EXPECT_ZR(1, test_strerror()); break;
 		case __LINE__:
 			return ret; /* must be last */
 		/* note: do not set any defaults so as to permit holes above */