[PATCH v1.1 1/2] libfuse: don't use SYNC_INIT unless asked for

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

fuse2fs calls fuse_main, then starts threads from ->init.  It doesn't
call fuse_daemonize_early_start because I haven't ported it to use any
of the new APIs.  I don't have io_uring enabled for fuse on my dev box.

fuse_main calls fuse_session_mount calls fuse_session_mount_new_api
calls session_start_sync_init.  At the start of the function,
se->want_sync_init, se->uring.enable, and daemonize.active are all
false.  The first branch is not taken, so we call FUSE_DEV_IOC_SYNC_INIT
and enable sync_init even though the user didn't ask for that and didn't
prepare for it either.

FUSE_DEV_IOC_SYNC_INIT succeeds, so we send the synchronous FUSE_INIT
from mount, which calls fuse2fs' init() method.  That starts the
background threads and returns.  Upon return to the kernel, the mount()
now succeeds, and the next thing that fuse_main does is call
fuse_daemonize().  Since we didn't call fuse_daemonize_early_start, the
daemonize forks the process and the threads die with the parent.

If we didn't ask for SYNC_INIT, don't enable it.  This is needed to
maintain compatibility with older fuse servers that only support
asynchronous FUSE_INIT.

Fixes: 3e1101057aea57 ("fuse mount: Support synchronous FUSE_INIT (privileged daemon)")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
v1.1: improve commit message, refine logging logic
---
 lib/fuse_lowlevel.c |   18 +++++++++++++++---
 1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/lib/fuse_lowlevel.c b/lib/fuse_lowlevel.c
index 0e16845d2f14ff..83158d02bb8827 100644
--- a/lib/fuse_lowlevel.c
+++ b/lib/fuse_lowlevel.c
@@ -4475,9 +4475,21 @@ static int session_start_sync_init(struct fuse_session *se, int fd)
 {
 	int err, res;
 
-	if (!se->want_sync_init &&
-		(se->uring.enable && !fuse_daemonize_is_used())) {
-		if (se->debug)
+	if (!se->want_sync_init) {
+		/*
+		 * SYNC_INIT is required for io_uring to initialize without
+		 * deadlocking the kernel if the fuse server crashes.
+		 *
+		 * !fuse_daemonize_is_used implies the fuse server doesn't know
+		 * about any of the SYNC_INIT APIs, so we don't enable sync
+		 * init or generate log messages.
+		 */
+		if (se->uring.enable)
+			fuse_log(FUSE_LOG_DEBUG,
+					"fuse: io_uring broken with async init\n");
+		else if (!fuse_daemonize_is_used())
+			; /* empty */
+		else if (se->debug)
 			fuse_log(FUSE_LOG_DEBUG,
 					"fuse: sync init not enabled\n");
 		return 0;