diff mbox series

[for-9.1,v2,02/11] libvhost-user: fail vu_message_write() if sendmsg() is failing

Message ID 20240326133936.125332-3-sgarzare@redhat.com (mailing list archive)
State New, archived
Headers show
Series vhost-user: support any POSIX system (tested on macOS, FreeBSD, OpenBSD) | expand

Commit Message

Stefano Garzarella March 26, 2024, 1:39 p.m. UTC
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 <sgarzare@redhat.com>
---
 subprojects/libvhost-user/libvhost-user.c | 5 +++++
 1 file changed, 5 insertions(+)

Comments

Eric Blake March 26, 2024, 2:34 p.m. UTC | #1
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 <sgarzare@redhat.com>
> ---
>  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
>
David Hildenbrand March 26, 2024, 2:36 p.m. UTC | #2
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 <sgarzare@redhat.com>
>> ---
>>   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 ...
Stefano Garzarella March 27, 2024, 9:26 a.m. UTC | #3
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 <sgarzare@redhat.com>
>>>---
>>>  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
David Hildenbrand March 27, 2024, 9:29 a.m. UTC | #4
>>
>> 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.
diff mbox series

Patch

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) {
+        vu_panic(dev, "Error while writing: %s", strerror(errno));
+        return false;
+    }
+
     if (vmsg->size) {
         do {
             if (vmsg->data) {