epoll: Use kcmp to find proper target file

When target file obtained from epoll fdinfo (internally the
kernel keeps only file _number_ inside) we have to check its
identity to make sure it is exactly one which has been added
into epoll engine. The only proper way is to use kcmp syscall.
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/eventpoll.c

@@ -24,6 +24,8 @@
 #include "log.h"
 #include "pstree.h"
 #include "parasite.h"
+#include "kerndat.h"
+#include "kcmp.h"
 
 #include "protobuf.h"
 #include "images/eventpoll.pb-c.h"
@@ -53,12 +55,81 @@ static void pr_info_eventpoll(char *action, EventpollFileEntry *e)
 	pr_info("%seventpoll: id %#08x flags %#04x\n", action, e->id, e->flags);
 }
 
+static int tfd_cmp(const void *a, const void *b)
+{
+	if (((int *)a)[0] > ((int *)b)[0])
+		return 1;
+	if (((int *)a)[0] < ((int *)b)[0])
+		return -1;
+	return 0;
+}
+
+/*
+ * fds in fd_parms are sorted so we can use binary search
+ * for better performance.
+ */
+static int find_tfd(pid_t pid, int efd, int fds[], size_t nr_fds, int tfd)
+{
+	kcmp_epoll_slot_t slot = {
+		.efd	= efd,
+		.tfd	= tfd,
+		.toff	= 0,
+	};
+	int *tfd_found;
+	size_t i;
+
+	pr_debug("find_tfd: pid %d efd %d tfd %d\n", pid, efd, tfd);
+
+	/*
+	 * Optimistic case: the target fd belongs to us
+	 * and wasn't dup'ed.
+	 */
+	tfd_found = bsearch(&tfd, fds, nr_fds, sizeof(int), tfd_cmp);
+	if (tfd_found) {
+		if (kdat.has_kcmp_epoll_tfd) {
+			if (syscall(SYS_kcmp, pid, pid, KCMP_EPOLL_TFD, tfd, &slot) == 0) {
+				pr_debug("find_tfd (kcmp-yes): bsearch match pid %d efd %d tfd %d\n",
+					 pid, efd, tfd);
+				return tfd;
+			}
+		} else {
+			pr_debug("find_tfd (kcmp-no): bsearch match pid %d efd %d tfd %d\n",
+				 pid, efd, tfd);
+			return tfd;
+		}
+	}
+
+	/*
+	 * Pessimistic case: the file has been dup'ed, we have to walk
+	 * over all files and find one which is suitable via series of
+	 * the kcmp syscalls.
+	 */
+
+	if (!kdat.has_kcmp_epoll_tfd) {
+		pr_debug("find_tfd (kcmp-no): no match pid %d efd %d tfd %d\n",
+			 pid, efd, tfd);
+		return -1;
+	}
+
+	for (i = 0; i < nr_fds; i++) {
+		if (syscall(SYS_kcmp, pid, pid, KCMP_EPOLL_TFD, fds[i], &slot) == 0) {
+			pr_debug("find_tfd (kcmp-yes): nsearch match pid %d efd %d tfd %d -> %d\n",
+				 pid, efd, tfd, fds[i]);
+			return fds[i];
+		}
+	}
+
+	pr_debug("find_tfd (kcmp-yes): no match pid %d efd %d tfd %d\n",
+		 pid, efd, tfd);
+	return -1;
+}
+
 static int dump_one_eventpoll(int lfd, u32 id, const struct fd_parms *p)
 {
 	FileEntry fe = FILE_ENTRY__INIT;
 	EventpollFileEntry e = EVENTPOLL_FILE_ENTRY__INIT;
 	EventpollTfdEntry **tfd_cpy = NULL;
-	size_t i, j, k, n_tfd_cpy;
+	size_t i, j, n_tfd_cpy;
 	int ret = -1;
 
 	e.id = id;
@@ -87,21 +158,18 @@ static int dump_one_eventpoll(int lfd, u32 id, const struct fd_parms *p)
 	 */
 	if (p->dfds) {
 		for (i = j = 0; i < e.n_tfd; i++) {
-			for (k = 0; k < p->dfds->nr_fds; k++) {
-				if (p->dfds->fds[k] == e.tfd[i]->tfd)
-					break;
-			}
-
-			if (k >= p->dfds->nr_fds) {
+			int tfd = find_tfd(p->pid, p->fd, p->dfds->fds,
+					   p->dfds->nr_fds, e.tfd[i]->tfd);
+			if (tfd == -1) {
 				pr_warn("Escaped/closed fd descriptor %d on pid %d, ignoring\n",
 					e.tfd[i]->tfd, p->pid);
 				continue;
 			}
-
-			e.tfd[j++] = e.tfd[i];
+			e.tfd[j++]->tfd = tfd;
 		}
-		e.n_tfd = j; /* New amount of "semi-valid" fds */
-	}
+		e.n_tfd = j; /* New amount of "valid" fds */
+	} else
+		pr_warn_once("Unix SCM files are not verified\n");
 
 	pr_info_eventpoll("Dumping ", &e);
 	ret = pb_write_one(img_from_set(glob_imgset, CR_FD_FILES), &fe, PB_FILE);