* [PATCH] virtio-rng: return available data with O_NONBLOCK
@ 2020-07-14 22:00 mwilck
2020-07-15 7:05 ` Martin Wilck
0 siblings, 1 reply; 2+ messages in thread
From: mwilck @ 2020-07-14 22:00 UTC (permalink / raw)
To: Michael S. Tsirkin, Jason Wang; +Cc: Martin Wilck, qemu-devel, virtualization
From: Martin Wilck <mwilck@suse.com>
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 <mwilck@suse.com>
---
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
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH] virtio-rng: return available data with O_NONBLOCK
2020-07-14 22:00 [PATCH] virtio-rng: return available data with O_NONBLOCK mwilck
@ 2020-07-15 7:05 ` Martin Wilck
0 siblings, 0 replies; 2+ messages in thread
From: Martin Wilck @ 2020-07-15 7:05 UTC (permalink / raw)
To: Michael S. Tsirkin, Jason Wang; +Cc: qemu-devel, virtualization
On Wed, 2020-07-15 at 00:00 +0200, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@suse.com>
>
> 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.
...
> 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 <mwilck@suse.com>
> ---
> 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);
This patch was nonsense. I'm sorry. Looking into it again.
Martin
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