Re: [PATCH for-9.1 v2 02/11] libvhost-user: fail vu_message_write() if sendmsg() is failing
I was wondering if we could see some partial sendmsg()/write succeeding. Meaning, we transferred some bytes but not all, and we'd actually need to loop ... Yep, true, but I would fix it in another patch/series if you agree. Absolutely. -- Cheers, David / dhildenb
Re: [PATCH for-9.1 v2 02/11] libvhost-user: fail vu_message_write() if sendmsg() is failing
On Tue, Mar 26, 2024 at 03:36:52PM +0100, David Hildenbrand wrote:
On 26.03.24 15:34, Eric Blake wrote:
On Tue, Mar 26, 2024 at 02:39:27PM +0100, Stefano Garzarella wrote:
In vu_message_write() we use sendmsg() to send the message header,
then a write() to send the payload.
If sendmsg() fails we should avoid sending the payload, since we
were unable to send the header.
Discovered before fixing the issue with the previous patch, where
sendmsg() failed on macOS due to wrong parameters, but the frontend
still sent the payload which the backend incorrectly interpreted
as a wrong header.
Signed-off-by: Stefano Garzarella
---
subprojects/libvhost-user/libvhost-user.c | 5 +
1 file changed, 5 insertions(+)
diff --git a/subprojects/libvhost-user/libvhost-user.c
b/subprojects/libvhost-user/libvhost-user.c
index 22bea0c775..a11afd1960 100644
--- a/subprojects/libvhost-user/libvhost-user.c
+++ b/subprojects/libvhost-user/libvhost-user.c
@@ -639,6 +639,11 @@ vu_message_write(VuDev *dev, int conn_fd, VhostUserMsg
*vmsg)
rc = sendmsg(conn_fd, &msg, 0);
} while (rc < 0 && (errno == EINTR || errno == EAGAIN));
+if (rc <= 0) {
Is rejecting a 0 return value correct? Technically, a 0 return means
a successful write of 0 bytes - but then again, it is unwise to
attempt to send an fd or other auxilliary ddata without at least one
regular byte, so we should not be attempting a write of 0 bytes. So I
guess this one is okay, although I might have used < instead of <=.
I blindly copied what they already do at the end of the same function.
However, your observation is correct. That said they have a sendmsg()
to send the header. After this we have a write() loop to send the
payload.
Now if sendmsg() returns 0, but we have not sent all the header, what
should we do? Try sendmsg() again? For how many times?
I was wondering if we could see some partial sendmsg()/write
succeeding. Meaning, we transferred some bytes but not all, and we'd
actually need to loop ...
Yep, true, but I would fix it in another patch/series if you agree.
Thanks,
Stefano
Re: [PATCH for-9.1 v2 02/11] libvhost-user: fail vu_message_write() if sendmsg() is failing
On 26.03.24 15:34, Eric Blake wrote:
On Tue, Mar 26, 2024 at 02:39:27PM +0100, Stefano Garzarella wrote:
In vu_message_write() we use sendmsg() to send the message header,
then a write() to send the payload.
If sendmsg() fails we should avoid sending the payload, since we
were unable to send the header.
Discovered before fixing the issue with the previous patch, where
sendmsg() failed on macOS due to wrong parameters, but the frontend
still sent the payload which the backend incorrectly interpreted
as a wrong header.
Signed-off-by: Stefano Garzarella
---
subprojects/libvhost-user/libvhost-user.c | 5 +
1 file changed, 5 insertions(+)
diff --git a/subprojects/libvhost-user/libvhost-user.c
b/subprojects/libvhost-user/libvhost-user.c
index 22bea0c775..a11afd1960 100644
--- a/subprojects/libvhost-user/libvhost-user.c
+++ b/subprojects/libvhost-user/libvhost-user.c
@@ -639,6 +639,11 @@ vu_message_write(VuDev *dev, int conn_fd, VhostUserMsg
*vmsg)
rc = sendmsg(conn_fd, &msg, 0);
} while (rc < 0 && (errno == EINTR || errno == EAGAIN));
+if (rc <= 0) {
Is rejecting a 0 return value correct? Technically, a 0 return means
a successful write of 0 bytes - but then again, it is unwise to
attempt to send an fd or other auxilliary ddata without at least one
regular byte, so we should not be attempting a write of 0 bytes. So I
guess this one is okay, although I might have used < instead of <=.
I was wondering if we could see some partial sendmsg()/write succeeding.
Meaning, we transferred some bytes but not all, and we'd actually need
to loop ...
--
Cheers,
David / dhildenb
Re: [PATCH for-9.1 v2 02/11] libvhost-user: fail vu_message_write() if sendmsg() is failing
On Tue, Mar 26, 2024 at 02:39:27PM +0100, Stefano Garzarella wrote:
> In vu_message_write() we use sendmsg() to send the message header,
> then a write() to send the payload.
>
> If sendmsg() fails we should avoid sending the payload, since we
> were unable to send the header.
>
> Discovered before fixing the issue with the previous patch, where
> sendmsg() failed on macOS due to wrong parameters, but the frontend
> still sent the payload which the backend incorrectly interpreted
> as a wrong header.
>
> Signed-off-by: Stefano Garzarella
> ---
> subprojects/libvhost-user/libvhost-user.c | 5 +
> 1 file changed, 5 insertions(+)
>
> diff --git a/subprojects/libvhost-user/libvhost-user.c
> b/subprojects/libvhost-user/libvhost-user.c
> index 22bea0c775..a11afd1960 100644
> --- a/subprojects/libvhost-user/libvhost-user.c
> +++ b/subprojects/libvhost-user/libvhost-user.c
> @@ -639,6 +639,11 @@ vu_message_write(VuDev *dev, int conn_fd, VhostUserMsg
> *vmsg)
> rc = sendmsg(conn_fd, &msg, 0);
> } while (rc < 0 && (errno == EINTR || errno == EAGAIN));
>
> +if (rc <= 0) {
Is rejecting a 0 return value correct? Technically, a 0 return means
a successful write of 0 bytes - but then again, it is unwise to
attempt to send an fd or other auxilliary ddata without at least one
regular byte, so we should not be attempting a write of 0 bytes. So I
guess this one is okay, although I might have used < instead of <=.
> +vu_panic(dev, "Error while writing: %s", strerror(errno));
> +return false;
> +}
At any rate, noticing the error is the correct thing to do.
> +
> if (vmsg->size) {
> do {
> if (vmsg->data) {
> --
> 2.44.0
>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization: qemu.org | libguestfs.org
