tools/nolibc: add fopen()

This is used in various selftests and will be handy when integrating
those with nolibc.

Only the standard POSIX modes are supported.
No extensions nor the (noop) "b" from ISO C are accepted.

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-v2-13-3c043eeab06c@linutronix.de
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

2 files changed, 51 insertions, 0 deletions

diff --git a/tools/include/nolibc/stdio.h b/tools/include/nolibc/stdio.h
index df5717d59182..c470d334ef3f 100644
--- a/tools/include/nolibc/stdio.h
+++ b/tools/include/nolibc/stdio.h
@@ -13,6 +13,7 @@
 #include "std.h"
 #include "arch.h"
 #include "errno.h"
+#include "fcntl.h"
 #include "types.h"
 #include "sys.h"
 #include "stdarg.h"
@@ -55,6 +56,32 @@ FILE *fdopen(int fd, const char *mode __attribute__((unused)))
 	return (FILE*)(intptr_t)~fd;
 }
 
+static __attribute__((unused))
+FILE *fopen(const char *pathname, const char *mode)
+{
+	int flags, fd;
+
+	switch (*mode) {
+	case 'r':
+		flags = O_RDONLY;
+		break;
+	case 'w':
+		flags = O_WRONLY | O_CREAT | O_TRUNC;
+		break;
+	case 'a':
+		flags = O_WRONLY | O_CREAT | O_APPEND;
+		break;
+	default:
+		SET_ERRNO(EINVAL); return NULL;
+	}
+
+	if (mode[1] == '+')
+		flags = (flags & ~(O_RDONLY | O_WRONLY)) | O_RDWR;
+
+	fd = open(pathname, flags, 0666);
+	return fdopen(fd, mode);
+}
+
 /* provides the fd of stream. */
 static __attribute__((unused))
 int fileno(FILE *stream)
diff --git a/tools/testing/selftests/nolibc/nolibc-test.c b/tools/testing/selftests/nolibc/nolibc-test.c
index d1157319b5d7..0391c7d01380 100644
--- a/tools/testing/selftests/nolibc/nolibc-test.c
+++ b/tools/testing/selftests/nolibc/nolibc-test.c
@@ -859,6 +859,29 @@ int test_getpagesize(void)
 	return !c;
 }
 
+int test_file_stream(void)
+{
+	FILE *f;
+	int r;
+
+	f = fopen("/dev/null", "r");
+	if (!f)
+		return -1;
+
+	errno = 0;
+	r = fwrite("foo", 1, 3, f);
+	if (r != 0 || errno != EBADF) {
+		fclose(f);
+		return -1;
+	}
+
+	r = fclose(f);
+	if (r == EOF)
+		return -1;
+
+	return 0;
+}
+
 int test_fork(void)
 {
 	int status;
@@ -1311,6 +1334,7 @@ int run_syscall(int min, int max)
 		CASE_TEST(dup3_0);            tmp = dup3(0, 100, 0);  EXPECT_SYSNE(1, tmp, -1); close(tmp); break;
 		CASE_TEST(dup3_m1);           tmp = dup3(-1, 100, 0); EXPECT_SYSER(1, tmp, -1, EBADF); if (tmp != -1) close(tmp); break;
 		CASE_TEST(execve_root);       EXPECT_SYSER(1, execve("/", (char*[]){ [0] = "/", [1] = NULL }, NULL), -1, EACCES); break;
+		CASE_TEST(file_stream);       EXPECT_SYSZR(1, test_file_stream()); break;
 		CASE_TEST(fork);              EXPECT_SYSZR(1, test_fork()); break;
 		CASE_TEST(getdents64_root);   EXPECT_SYSNE(1, test_getdents64("/"), -1); break;
 		CASE_TEST(getdents64_null);   EXPECT_SYSER(1, test_getdents64("/dev/null"), -1, ENOTDIR); break;