tools/nolibc: unistd: Add getcwd()

Add getcwd() for getting the current working directory.

The behaviour matches what musl is doing except for one
important difference: If the passed buf is NULL musl (and glibc)
uses a big buffer on the stack and that is then strdup()'d and
returned.

According to the man page for getcwd() this is a glibc extension
and I don't think we need it in nolibc.

Signed-off-by: Daniel Palmer <daniel@thingy.jp>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260702085101.3304547-2-daniel@thingy.jp
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>

tools/include/nolibc/unistd.h

@@ -73,6 +73,48 @@ int ftruncate(int fd, off_t length)
 	return __sysret(_sys_ftruncate(fd, length));
 }
 
+/*
+ * char *getcwd(char *buf, size_t size);
+ */
+
+static __attribute__((unused))
+int _sys_getcwd(char *buf, size_t size)
+{
+	return __nolibc_syscall2(__NR_getcwd, buf, size);
+}
+
+static __attribute__((unused))
+char *getcwd(char *buf, size_t size)
+{
+	int ret;
+
+	/* Unlike other libc's we don't handle passing NULL for buf */
+	if (!buf || !size) {
+		SET_ERRNO(EINVAL);
+		return NULL;
+	}
+
+	ret = __sysret(_sys_getcwd(buf, size));
+
+	/* On error return NULL, __sysret() above will have set errno */
+	if (ret < 0)
+		return NULL;
+
+	/* Handle no path being written or the kernel putting
+	 * "(unreachable)" into the buffer instead of a path.
+	 * This matches what musl is doing.
+	 */
+	if (ret == 0 || buf[0] != '/') {
+		SET_ERRNO(ENOENT);
+		return NULL;
+	}
+
+	/* ret must be the number of bytes written at this point,
+	 * so return the pointer to buf.
+	 */
+	return buf;
+}
+
 static __attribute__((unused))
 int msleep(unsigned int msecs)
 {