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 <[e-mail]>
Acked-by: Willy Tarreau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Thomas Weißschuh <[e-mail]>

tools/include/nolibc/stdio.h

···
 #include "std.h"
 #include "arch.h"
 #include "errno.h"
+#include "fcntl.h"
 #include "types.h"
 #include "sys.h"
 #include "stdarg.h"
···
 		return NULL;
 	}
 	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. */

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

···
 	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;
···
 		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;