seitan: find fd of seccomp noitifier from proc

Instead of assuming that the fd of the notifier is always 3, find the
correct fd from procfs.

Signed-off-by: Alice Frosi <afrosi@redhat.com>

1 files changed, 55 insertions, 1 deletions

diff --git a/seitan.c b/seitan.c
index 6c66593..dd4bd9c 100644
--- a/seitan.c
+++ b/seitan.c
@@ -19,12 +19,15 @@
 #include <unistd.h>
 #include <limits.h>
 #include <signal.h>
+#include <dirent.h>
 #include <sys/prctl.h>
 #include <sys/syscall.h>
 #include <sys/ioctl.h>
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <sys/epoll.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 #include <argp.h>
 #include <linux/netlink.h>
 #include <linux/connector.h>
@@ -181,6 +184,51 @@ static void unblock_eater(int pidfd){
 	}
 }
 
+static int find_fd_seccomp_notifier(int pid)
+{
+	char path[PATH_MAX + 1];
+	char entry[2*PATH_MAX + 1];
+	char buf[PATH_MAX + 1];
+	struct dirent *dp;
+	ssize_t nbytes;
+	struct stat sb;
+	DIR *dirp;
+
+	snprintf(path, sizeof(path), "/proc/%i/fd", pid);
+	if ((dirp = opendir(path)) == NULL) {
+		fprintf(stderr,"failed reading fds from proc \n");
+		return -1;
+	}
+	while ((dp = readdir (dirp)) != NULL) {
+		snprintf(entry, sizeof(entry), "%s/%s", path, dp->d_name);
+		if (lstat(entry, &sb) == -1) {
+			perror("lstat");
+		}
+		/* Skip the entry if it isn't a symbolic link */
+		if (!S_ISLNK(sb.st_mode))
+			continue;
+
+		nbytes = readlink(entry, buf, PATH_MAX);
+		if (nbytes == -1) {
+			perror("readlink");
+		}
+		if (nbytes == PATH_MAX) {
+			perror("buffer overflow");
+			continue;
+		}
+		/*
+		 * From man proc: For  file  descriptors  that  have no
+		 * corresponding inode (e.g., file descriptors produced by
+		 * bpf(2)..), the  entry  will be a symbolic link with contents
+		 * of the form:
+		 * 	anon_inode:<file-type>
+		 */
+		if (strcmp(buf, "anon_inode:seccomp notify") == 0)
+			return atoi(dp->d_name);
+	}
+	return -1;
+}
+
 int handle(struct seccomp_notif *req, int notifyfd)
 {
 	char path[PATH_MAX + 1];
@@ -235,6 +283,7 @@ int main(int argc, char **argv)
 	struct arguments arguments;
 	bool running = true;
 	int fd, epollfd;
+	int notifierfd;
 	int nevents,i;
 
 	arguments.pid = -1;
@@ -257,7 +306,12 @@ int main(int argc, char **argv)
 	}
 	sleep(1);
 
-	if ((notifier = syscall(SYS_pidfd_getfd, pidfd, 3, 0)) < 0) {
+	if ((notifierfd = find_fd_seccomp_notifier(ret)) < 0){
+		fprintf(stderr, "failed getting fd of the notifier\n");
+		exit(EXIT_FAILURE);
+	}
+	printf("fd notifier: %d \n", notifierfd);
+	if ((notifier = syscall(SYS_pidfd_getfd, pidfd, notifierfd, 0)) < 0) {
 		perror("pidfd_getfd");
 		exit(EXIT_FAILURE);
 	}