linux-kernel - [PATCH 2/2] selftests/nolibc: correctly report errors from printf() and friends

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Message-Id: <20250704-nolibc-printf-error-v1-2-74b7a092433b@linutronix.de>
Date: Fri, 04 Jul 2025 15:43:13 +0200
From: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
To: Willy Tarreau <w@....eu>, 
 Thomas Weißschuh <linux@...ssschuh.net>, 
 Shuah Khan <shuah@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>
Cc: linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: [PATCH 2/2] selftests/nolibc: correctly report errors from
 printf() and friends

When an error is encountered by printf() it needs to be reported.
errno() is already set by the callback.

sprintf() is different, but that keeps working and is already tested.

Also add a new test.

Fixes: 7e4346f4a3a6 ("tools/nolibc/stdio: add a minimal [vf]printf() implementation")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@...utronix.de>
---
 tools/include/nolibc/stdio.h                 |  4 ++--
 tools/testing/selftests/nolibc/nolibc-test.c | 23 +++++++++++++++++++++++
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index c470d334ef3f410d0584e01045d380347d61ad95..7630234408c584a560b059f0a7e30d407d186f17 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -358,11 +358,11 @@ int __nolibc_printf(__nolibc_printf_cb cb, intptr_t state, size_t n, const char
 				n -= w;
 				while (width-- > w) {
 					if (cb(state, " ", 1) != 0)
-						break;
+						return -1;
 					written += 1;
 				}
 				if (cb(state, outstr, w) != 0)
-					break;
+					return -1;
 			}
 
 			written += len;
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index 929b966099b369aeca6d43002ebb0a160674dddc..2835f65bda30ffabde749f42a6d3dfecc9b555fd 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -1661,6 +1661,28 @@ int test_strerror(void)
 	return 0;
 }
 
+static int test_printf_error(void)
+{
+	int fd, ret, saved_errno;
+
+	fd = open("/dev/full", O_RDWR);
+	if (fd == -1)
+		return 1;
+
+	errno = 0;
+	ret = dprintf(fd, "foo");
+	saved_errno = errno;
+	close(fd);
+
+	if (ret != -1)
+		return 2;
+
+	if (saved_errno != ENOSPC)
+		return 3;
+
+	return 0;
+}
+
 static int run_printf(int min, int max)
 {
 	int test;
@@ -1690,6 +1712,7 @@ static int run_printf(int min, int max)
 		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_TEST(printf_error); EXPECT_ZR(1, test_printf_error()); break;
 		case __LINE__:
 			return ret; /* must be last */
 		/* note: do not set any defaults so as to permit holes above */

-- 
2.50.0