[PATCH v2 3/3] sandbox/seunshare: pin_dir: use openat2 if available

Update seunshare's pin_dir() helper to use openat2() if defined to
optimize the lookup of the directory and prevent following any
intermediate symlinks. Fall back to the existing open() on
ENOSYS/EINVAL so that pre-5.6 and seccomp-filtered environments are
unaffected.

Signed-off-by: Stephen Smalley <[email protected]>
---
 sandbox/seunshare.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/sandbox/seunshare.c b/sandbox/seunshare.c
index 3fa42f9a..f2d637e1 100644
--- a/sandbox/seunshare.c
+++ b/sandbox/seunshare.c
@@ -10,8 +10,12 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <sys/syscall.h>
 #include <syslog.h>
 #include <sys/mount.h>
+#ifdef __NR_openat2
+#include <linux/openat2.h>
+#endif
 #include <glob.h>
 #include <pwd.h>
 #include <sched.h>
@@ -219,10 +223,19 @@ static int check_owner_gid(gid_t gid, const char *file, struct stat *st)
  */
 static int pin_dir(const char *dir, struct stat *st_out)
 {
-	int fd;
+	int fd = -1;
 	struct stat sb;
 
-	fd = open(dir, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
+#ifdef __NR_openat2
+	struct open_how how = {
+		.flags = O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC,
+		.resolve = RESOLVE_NO_SYMLINKS | RESOLVE_NO_MAGICLINKS,
+	};
+
+	fd = syscall(__NR_openat2, AT_FDCWD, dir, &how, sizeof(how));
+	if (fd < 0 && (errno == ENOSYS || errno == EINVAL))
+#endif
+		fd = open(dir, O_RDONLY | O_DIRECTORY | O_NOFOLLOW | O_CLOEXEC);
 	if (fd < 0) {
 		fprintf(stderr, _("Failed to open %s: %m\n"), dir);
 		return -1;
-- 
2.54.0