tools/nolibc: add support for directory access

Add an implementation for directory access operations.
To keep nolibc itself allocation-free, a "DIR *" does not point to any
data, but directly encodes a filedescriptor number, equivalent to "FILE *".
Without any per-directory storage it is not possible to implement
readdir() POSIX confirming. Instead only readdir_r() is provided.
While readdir_r() is deprecated in glibc, the reasons for that are
not applicable to nolibc.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://lore.kernel.org/r/20250209-nolibc-dir-v2-2-57cc1da8558b@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

3 files changed, 100 insertions, 0 deletions

diff --git a/tools/include/nolibc/Makefile b/tools/include/nolibc/Makefile
index a1f55fb24bb3..dceec0e1a135 100644
--- a/tools/include/nolibc/Makefile
+++ b/tools/include/nolibc/Makefile
@@ -29,6 +29,7 @@ all_files := \
 		compiler.h \
 		crt.h \
 		ctype.h \
+		dirent.h \
 		errno.h \
 		nolibc.h \
 		signal.h \
diff --git a/tools/include/nolibc/dirent.h b/tools/include/nolibc/dirent.h
new file mode 100644
index 000000000000..c5c30d0dd680
--- /dev/null
+++ b/tools/include/nolibc/dirent.h
@@ -0,0 +1,98 @@
+/* SPDX-License-Identifier: LGPL-2.1 OR MIT */
+/*
+ * Directory access for NOLIBC
+ * Copyright (C) 2025 Thomas Weißschuh <linux@weissschuh.net>
+ */
+
+#ifndef _NOLIBC_DIRENT_H
+#define _NOLIBC_DIRENT_H
+
+#include "stdint.h"
+#include "types.h"
+
+#include <linux/limits.h>
+
+struct dirent {
+	ino_t	d_ino;
+	char	d_name[NAME_MAX + 1];
+};
+
+/* See comment of FILE in stdio.h */
+typedef struct {
+	char dummy[1];
+} DIR;
+
+static __attribute__((unused))
+DIR *fdopendir(int fd)
+{
+	if (fd < 0) {
+		SET_ERRNO(EBADF);
+		return NULL;
+	}
+	return (DIR *)(intptr_t)~fd;
+}
+
+static __attribute__((unused))
+DIR *opendir(const char *name)
+{
+	int fd;
+
+	fd = open(name, O_RDONLY);
+	if (fd == -1)
+		return NULL;
+	return fdopendir(fd);
+}
+
+static __attribute__((unused))
+int closedir(DIR *dirp)
+{
+	intptr_t i = (intptr_t)dirp;
+
+	if (i >= 0) {
+		SET_ERRNO(EBADF);
+		return -1;
+	}
+	return close(~i);
+}
+
+static __attribute__((unused))
+int readdir_r(DIR *dirp, struct dirent *entry, struct dirent **result)
+{
+	char buf[sizeof(struct linux_dirent64) + NAME_MAX + 1];
+	struct linux_dirent64 *ldir = (void *)buf;
+	intptr_t i = (intptr_t)dirp;
+	int fd, ret;
+
+	if (i >= 0)
+		return EBADF;
+
+	fd = ~i;
+
+	ret = sys_getdents64(fd, ldir, sizeof(buf));
+	if (ret < 0)
+		return -ret;
+	if (ret == 0) {
+		*result = NULL;
+		return 0;
+	}
+
+	/*
+	 * getdents64() returns as many entries as fit the buffer.
+	 * readdir() can only return one entry at a time.
+	 * Make sure the non-returned ones are not skipped.
+	 */
+	ret = lseek(fd, ldir->d_off, SEEK_SET);
+	if (ret == -1)
+		return errno;
+
+	entry->d_ino = ldir->d_ino;
+	/* the destination should always be big enough */
+	strlcpy(entry->d_name, ldir->d_name, sizeof(entry->d_name));
+	*result = entry;
+	return 0;
+}
+
+/* make sure to include all global symbols */
+#include "nolibc.h"
+
+#endif /* _NOLIBC_DIRENT_H */
diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 92436b1e4441..05d92afedb72 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -105,6 +105,7 @@
 #include "string.h"
 #include "time.h"
 #include "stackprotector.h"
+#include "dirent.h"
 
 /* Used by programs to avoid std includes */
 #define NOLIBC