A F_GETLK sent to lockd that returns a conflicting lock can cause a crash because the fl_owner is not guaranteed to be struct nlm_lockowner.  We need provisionally pending patch:

https://lore.kernel.org/linux-nfs/f94e02c019495fea4495fbef7498f342d5848dac.1627217317.git.bcodding@redhat.com/T/#u

Here's a simple demonstration for a system that has exported /exports and mounted that export on a NFSv3 mount on /mnt/localhost:

#define _GNU_SOURCE
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>

#define LOCALFILE    "/exports/foo"
#define NFSFILE    "/mnt/localhost/foo"

int
main(int argc, char **argv)
{
	int local_fd, nfs_fd, ret;

	struct flock lck_one = {
		.l_whence = SEEK_SET,
		.l_start  = 0,
		.l_len    = 4,
	};

	struct flock lck_two = {
		.l_whence = SEEK_SET,
		.l_start  = 0,
		.l_len    = 4,
	};

	local_fd = open(LOCALFILE, O_RDWR|O_CREAT, 0666);
	nfs_fd = open(NFSFILE, O_RDWR|O_CREAT, 0666);

	lck_one.l_type = F_WRLCK;
	ret = fcntl(local_fd, F_SETLKW, &lck_one);
	ret = fcntl(nfs_fd, F_GETLK, &lck_two);
}