[RFC] fuse: do not generate interrupt requests for fatal signals

[Haifeng Xu <haifeng.xu@shopee.com>]

When the child reaper of a pid namespace exits, it invokes
zap_pid_ns_processes() to send SIGKILL to all processes in the
namespace and wait them exit. But one of the child processes get
stuck and its call trace like this:

[<0>] request_wait_answer+0x132/0x210 [fuse]
[<0>] fuse_simple_request+0x1a8/0x2e0 [fuse]
[<0>] fuse_flush+0x193/0x1d0 [fuse]
[<0>] filp_close+0x34/0x70
[<0>] close_fd+0x38/0x50
[<0>] __x64_sys_close+0x12/0x40
[<0>] do_syscall_64+0x59/0xc0
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xae

The flags of fuse request is (FR_ISREPLY | FR_FORCE | FR_WAITING
| FR_INTERRUPTED | FR_SENT). For interrupt requests, fuse_dev_do_write()
doesn't invoke fuse_request_end() to wake the client thread, so it will
get stuck forever and the child reaper can't exit.

In order to write reply to the client thread and make it exit the
namespace, so do not generate interrupt requests for fatal signals.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
---
 fs/fuse/dev.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 9eb191b5c4de..5fb830ad860d 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -374,11 +374,14 @@ static void request_wait_answer(struct fuse_req *req)
 		if (!err)
 			return;
 
-		set_bit(FR_INTERRUPTED, &req->flags);
-		/* matches barrier in fuse_dev_do_read() */
-		smp_mb__after_atomic();
-		if (test_bit(FR_SENT, &req->flags))
-			queue_interrupt(req);
+		/* Any signal except fatal can generate an interrupt request */
+		if (!__fatal_signal_pending(current)) {
+			set_bit(FR_INTERRUPTED, &req->flags);
+			/* matches barrier in fuse_dev_do_read() */
+			smp_mb__after_atomic();
+			if (test_bit(FR_SENT, &req->flags))
+				queue_interrupt(req);
+		}
 	}
 
 	if (!test_bit(FR_FORCE, &req->flags)) {
-- 
2.25.1