[PATCH 0/2] selftests/nolibc: correctly report errors from printf() and friends

[PATCH 0/2] selftests/nolibc: correctly report errors from printf() and friends

[Thomas Weißschuh]

When an error is encountered by printf() it needs to be reported.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Thomas Weißschuh (2):
      selftests/nolibc: create /dev/full when running as PID 1
      selftests/nolibc: correctly report errors from printf() and friends

 tools/include/nolibc/stdio.h                 |  4 ++--
 tools/testing/selftests/nolibc/nolibc-test.c | 27 ++++++++++++++++++++++++++-
 2 files changed, 28 insertions(+), 3 deletions(-)
---
base-commit: 1536aa0fb1e09cb50f401ec4852c60f38173d751
change-id: 20250704-nolibc-printf-error-aa54f951c0a4

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>

[PATCH 1/2] selftests/nolibc: create /dev/full when running as PID 1

[Thomas Weißschuh]

An upcoming testcase will use /dev/full.

Make sure it is always present.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 tools/testing/selftests/nolibc/nolibc-test.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index b5bca1dcf36e95a576ca9ffba4f7c213978a3f35..929b966099b369aeca6d43002ebb0a160674dddc 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -1777,12 +1777,14 @@ int prepare(void)
 	if (stat("/dev/.", &stat_buf) == 0 || mkdir("/dev", 0755) == 0) {
 		if (stat("/dev/console", &stat_buf) != 0 ||
 		    stat("/dev/null", &stat_buf) != 0 ||
-		    stat("/dev/zero", &stat_buf) != 0) {
+		    stat("/dev/zero", &stat_buf) != 0 ||
+		    stat("/dev/full", &stat_buf) != 0) {
 			/* try devtmpfs first, otherwise fall back to manual creation */
 			if (mount("/dev", "/dev", "devtmpfs", 0, 0) != 0) {
 				mknod("/dev/console", 0600 | S_IFCHR, makedev(5, 1));
 				mknod("/dev/null",    0666 | S_IFCHR, makedev(1, 3));
 				mknod("/dev/zero",    0666 | S_IFCHR, makedev(1, 5));
+				mknod("/dev/full",    0666 | S_IFCHR, makedev(1, 7));
 			}
 		}
 	}

-- 
2.50.0

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

[Thomas Weißschuh]

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@linutronix.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

Re: [PATCH 0/2] selftests/nolibc: correctly report errors from printf() and friends

[Willy Tarreau]

On Fri, Jul 04, 2025 at 03:43:11PM +0200, Thomas Weißschuh wrote:
> When an error is encountered by printf() it needs to be reported.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Good catch!

Acked-by: Willy Tarreau <w@1wt.eu>

Willy