feat(fs): add eventfd syscall support (#858)

* feat(fs): add eventfd syscall support

user/apps/test_eventfd/.gitignore

@@ -0,0 +1 @@
+test_eventfd

user/apps/test_eventfd/Makefile

@@ -0,0 +1,20 @@
+ifeq ($(ARCH), x86_64)
+	CROSS_COMPILE=x86_64-linux-musl-
+else ifeq ($(ARCH), riscv64)
+	CROSS_COMPILE=riscv64-linux-musl-
+endif
+
+CC=$(CROSS_COMPILE)gcc
+
+.PHONY: all
+all: main.c
+	$(CC) -static -o test_eventfd main.c
+
+.PHONY: install clean
+install: all
+	mv test_eventfd $(DADK_CURRENT_BUILD_DIR)/test_eventfd
+
+clean:
+	rm test_eventfd *.o
+
+fmt:

user/apps/test_eventfd/main.c

@@ -0,0 +1,52 @@
+#include <err.h>
+#include <inttypes.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/eventfd.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+int
+main(int argc, char *argv[])
+{
+    int       efd;
+    uint64_t  u;
+    ssize_t   s;
+
+    if (argc < 2) {
+        fprintf(stderr, "Usage: %s <num>...\n", argv[0]);
+        exit(EXIT_FAILURE);
+    }
+
+    efd = eventfd(0, 0);
+    if (efd == -1)
+        err(EXIT_FAILURE, "eventfd");
+
+    switch (fork()) {
+        case 0:
+            for (size_t j = 1; j < argc; j++) {
+                printf("Child writing %s to efd\n", argv[j]);
+                u = strtoull(argv[j], NULL, 0);
+                /* strtoull() allows various bases */
+                s = write(efd, &u, sizeof(uint64_t));
+                if (s != sizeof(uint64_t))
+                    err(EXIT_FAILURE, "write");
+            }
+            printf("Child completed write loop\n");
+
+            exit(EXIT_SUCCESS);
+
+        default:
+            sleep(2);
+
+            printf("Parent about to read\n");
+            s = read(efd, &u, sizeof(uint64_t));
+            if (s != sizeof(uint64_t))
+                err(EXIT_FAILURE, "read");
+            printf("Parent read %"PRIu64" (%#"PRIx64") from efd\n", u, u);
+            exit(EXIT_SUCCESS);
+
+        case -1:
+            err(EXIT_FAILURE, "fork");
+    }
+}