Avoid double unmount on FUSE_DESTROY

This is a long standing issue, a system could have unmounted
/path/to/mnt and then fuse-client/kernel would send FUSE_DESTROY,
which would then again try a umount.

Given that FUSE_DESTROY is async, that umount might arrive
any time later and might possibly unmount a wrong mount point.
A warning as in issue #1286 is just minor to that.

Code wise this uses atomics to free the char *, as FUSE_DESTROY
might race with a signal and a double free might be possible
without proctection. A lock might run into the same issue,
if the signal would arrive at the wrong time a double lock
would be possible.

Additionally, fuse_session_mount() is updated, to first
duplicatate the pointer and to then do the kernel mount -
reverting the kernel mount in case of strdup() failure
is much harder.

Closes: https://github.com/libfuse/libfuse/issues/1286
Signed-off-by: Bernd Schubert <bernd@bsbernd.com>
(cherry picked from commit d8253770ac2cf4b8769e8cf41eb3c629f30ee80f)

1 files changed, 2 insertions, 1 deletions

diff --git a/lib/fuse_i.h b/lib/fuse_i.h
index 718fa14..acf9d5a 100644
--- a/lib/fuse_i.h
+++ b/lib/fuse_i.h
@@ -13,6 +13,7 @@
 #include <stdint.h>
 #include <stdbool.h>
 #include <errno.h>
+#include <stdatomic.h>
 
 #define MIN(a, b) \
 ({									\
@@ -54,7 +55,7 @@ struct fuse_notify_req {
 };
 
 struct fuse_session {
-	char *mountpoint;
+	_Atomic(char *)mountpoint;
 	volatile int exited;
 	int fd;
 	struct fuse_custom_io *io;