From a0b0d8fbbf063c9a2bb0b719e5fbd4cb766f2f6d Mon Sep 17 00:00:00 2001
From: smitsohu <smitsohu@gmail.com>
Date: Mon, 23 Nov 2020 18:24:14 +0100
Subject: use openat2 syscall when available

---
 src/firejail/util.c | 63 +++++++++++++++++++++++++++++++----------------------
 1 file changed, 37 insertions(+), 26 deletions(-)

diff --git a/src/firejail/util.c b/src/firejail/util.c
index 02befdc12..e8b35a64b 100644
--- a/src/firejail/util.c
+++ b/src/firejail/util.c
@@ -29,6 +29,7 @@
 #include <sys/ioctl.h>
 #include <termios.h>
 #include <sys/wait.h>
+#include <sys/syscall.h>
 #include <limits.h>
 
 #include <fcntl.h>
@@ -36,6 +37,10 @@
 #define O_PATH 010000000
 #endif
 
+#ifdef __NR_openat2
+#include <linux/openat2.h>
+#endif
+
 #define MAX_GROUPS 1024
 #define MAXBUF 4098
 #define EMPTY_STRING ("")
@@ -1157,46 +1162,57 @@ void disable_file_path(const char *path, const char *file) {
 	free(fname);
 }
 
-// open file without following any symbolic link
-// returns a file descriptor on success, or -1 if a symlink is found
+// open an existing file without following any symbolic link
 int safe_fd(const char *path, int flags) {
 	assert(path);
-	if (*path != '/')
-		goto errexit;
-	if (strstr(path, ".."))
-		goto errexit;
-
-	int parentfd = open("/", O_PATH|O_DIRECTORY|O_CLOEXEC);
-	if (parentfd == -1)
-		errExit("open");
+	if (*path != '/' || strstr(path, "..")) {
+		fprintf(stderr, "Error: invalid path %s\n", path);
+		exit(1);
+	}
+	flags |= O_NOFOLLOW;
 	int fd = -1;
 
-	char *last_tok = EMPTY_STRING;
+#ifdef __NR_openat2 // kernel 5.6 or better
+	struct open_how oh;
+	memset(&oh, 0, sizeof(oh));
+	oh.flags = flags;
+	oh.resolve = RESOLVE_NO_SYMLINKS;
+	fd = syscall(__NR_openat2, -1, path, &oh, sizeof(struct open_how));
+	if (fd != -1 || errno != ENOSYS)
+		return fd;
+#endif
+
+	// openat2 syscall is not available, traverse path and
+	// check each component if it is a symbolic link or not
 	char *dup = strdup(path);
 	if (!dup)
 		errExit("strdup");
 	char *tok = strtok(dup, "/");
 	if (!tok) { // root directory
 		free(dup);
-		return parentfd;
+		return open("/", flags);
 	}
+	char *last_tok = EMPTY_STRING;
+	int parentfd = open("/", O_PATH|O_CLOEXEC);
+	if (parentfd == -1)
+		errExit("open");
 
 	while(1) {
-		// open the element, assuming it is a directory; this fails with ENOTDIR if it is a symbolic link
+		// open path component, assuming it is a directory; this fails with ENOTDIR if it is a symbolic link
 		// if token is a single dot, the previous directory is reopened
 		fd = openat(parentfd, tok, O_PATH|O_DIRECTORY|O_NOFOLLOW|O_CLOEXEC);
 		if (fd == -1) {
-			// if the following token is NULL, the current token is the final path element
+			// if the following token is NULL, the current token is the final path component
 			// try again to open it, this time using the passed flags, and return -1 or the descriptor
 			last_tok = tok;
 			tok = strtok(NULL, "/");
 			if (!tok)
-				fd = openat(parentfd, last_tok, flags|O_NOFOLLOW);
+				fd = openat(parentfd, last_tok, flags);
 			close(parentfd);
 			free(dup);
-			return fd; // -1 if open failed
+			return fd;
 		}
-		// move on to next path segment
+		// move on to next path component
 		last_tok = tok;
 		tok = strtok(NULL, "/");
 		if (!tok)
@@ -1204,18 +1220,13 @@ int safe_fd(const char *path, int flags) {
 		close(parentfd);
 		parentfd = fd;
 	}
-
-	// we are here because the last path element exists and is of file type directory
+	// getting here when the last path component exists and is of file type directory
 	// reopen it using the passed flags
 	close(fd);
-	fd = openat(parentfd, last_tok, flags|O_NOFOLLOW);
+	fd = openat(parentfd, last_tok, flags);
 	close(parentfd);
 	free(dup);
-	return fd; // -1 if open failed
-
-errexit:
-	fprintf(stderr, "Error: cannot open \"%s\": invalid path\n", path);
-	exit(1);
+	return fd;
 }
 
 int has_handler(pid_t pid, int signal) {
@@ -1321,7 +1332,7 @@ static int has_link(const char *dir) {
 	assert(dir);
 	int fd = safe_fd(dir, O_PATH|O_DIRECTORY|O_NOFOLLOW|O_CLOEXEC);
 	if (fd == -1) {
-		if (errno == ENOTDIR && is_dir(dir))
+		if ((errno == ELOOP || errno == ENOTDIR) && is_dir(dir))
 			return 1;
 	}
 	else
-- 
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