From mboxrd@z Thu Jan 1 00:00:00 1970 From: mwilck@suse.com Subject: [PATCH] virtio-rng: return available data with O_NONBLOCK Date: Wed, 15 Jul 2020 00:00:19 +0200 Message-ID: <20200714220019.10854-1-mwilck@suse.com> Mime-Version: 1.0 Content-Transfer-Encoding: 8bit Return-path: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+gceq-qemu-devel2=m.gmane-mx.org@nongnu.org Sender: "Qemu-devel" To: "Michael S. Tsirkin" , Jason Wang Cc: Martin Wilck , qemu-devel@nongnu.org, virtualization@lists.linux-foundation.org List-Id: virtualization@lists.linuxfoundation.org From: Martin Wilck If a program opens /dev/hwrng with O_NONBLOCK and uses poll() and non-blocking read() to retrieve random data, it ends up in a tight loop with poll() always returning POLLIN and read() returning EAGAIN. This repeats forever until some process makes a blocking read() call. The reason is that virtio_read() always returns 0 in non-blocking mode, even if data is available. The following test program illustrates the behavior. void loop(int fd) { struct pollfd pfd0 = { .fd = fd, .events = POLLIN, }; int rc; unsigned int n; for (n = LOOPS; n > 0; n--) { struct pollfd pfd = pfd0; char buf[SIZE]; rc = poll(&pfd, 1, 1); if (rc > 0) { int rd = read(fd, buf, sizeof(buf)); if (rd == -1) perror("read"); else printf("read %d bytes\n", rd); } else if (rc == -1) perror("poll"); else fprintf(stderr, "timeout\n"); } } int main(void) { int fd; fd = open("/dev/hwrng", O_RDONLY|O_NONBLOCK); if (fd == -1) { perror("open"); return 1; }; loop(fd); close(fd); return 0; } This can be observed in the real word e.g. with nested qemu/KVM virtual machines, if both the "outer" and "inner" VMs have a virtio-rng device. If the "inner" VM requests random data, qemu running in the "outer" VM uses this device in a non-blocking manner like the test program above. Fix it by returning available data if it exists. Signed-off-by: Martin Wilck --- drivers/char/hw_random/virtio-rng.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c index 79a6e47b5fbc..94806308d814 100644 --- a/drivers/char/hw_random/virtio-rng.c +++ b/drivers/char/hw_random/virtio-rng.c @@ -59,6 +59,9 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait) if (vi->hwrng_removed) return -ENODEV; + if (vi->data_avail >= size || (vi->data_avail && !wait)) + return vi->data_avail; + if (!vi->busy) { vi->busy = true; reinit_completion(&vi->have_data); -- 2.26.2