use openat2 syscall when available

1 files 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