BuildSystem: Write my own disk image perm updater

If user's bash does not have bultin stat, updating image perms was
terribly slow. This patch adds a simple c program that does the job
without exec overhead

tools/update-image-perms.c

@@ -0,0 +1,60 @@
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+int main(int argc, char** argv)
+{
+	if (argc != 3)
+	{
+		fprintf(stderr, "usage: %s MOUNT_POINT PERMS_FILE\n", argv[0]);
+		return 1;
+	}
+
+	int mount_fd = open(argv[1], O_RDONLY | O_DIRECTORY);
+	if (mount_fd == -1)
+	{
+		fprintf(stderr, "could not open %s: %s\n", argv[1], strerror(errno));
+		return 1;
+	}
+
+	FILE* perm_fp = fopen(argv[2], "r");
+	if (perm_fp == NULL)
+	{
+		fprintf(stderr, "could not open %s: %s\n", argv[2], strerror(errno));
+		return 1;
+	}
+
+	char buffer[1024];
+	while (fgets(buffer, sizeof(buffer), perm_fp))
+	{
+		char path[PATH_MAX];
+		uid_t uid;
+		gid_t gid;
+		mode_t mode;
+
+		if (sscanf(buffer, "%[^|\n]|%d|%d|%o\n", path, &uid, &gid, &mode) != 4)
+			continue;
+
+		struct stat st;
+		if (fstatat(mount_fd, path, &st, AT_SYMLINK_NOFOLLOW) != 0)
+			continue;
+
+		if (st.st_uid != uid || st.st_gid != gid)
+			if (fchownat(mount_fd, path, uid, gid, AT_SYMLINK_NOFOLLOW) != 0)
+				fprintf(stderr, "fchownat: %s: %s\n", path, strerror(errno));
+
+		const mode_t mode_mask = S_ISUID | S_ISGID | S_ISVTX | 0777;
+		if (mode != (st.st_mode & mode_mask))
+			if (fchmodat(mount_fd, path, mode, AT_SYMLINK_NOFOLLOW) != 0)
+				fprintf(stderr, "fchmodat: %s: %s\n", path, strerror(errno));
+	}
+
+	fclose(perm_fp);
+	close(mount_fd);
+
+	return 0;
+}