[PATCHv2 2/2] util:veth: Create veth device pair by netlink

[Laine Stump]

On 11/22/20 10:28 PM, Shi Lei wrote:
> When netlink is supported, use netlink to create veth device pair
> rather than 'ip link' command.
>
> Signed-off-by: Shi Lei <shi_lei at massclouds.com>
> ---
>   src/util/virnetdevveth.c | 85 ++++++++++++++++++++++++++--------------
>   1 file changed, 56 insertions(+), 29 deletions(-)
>
> diff --git a/src/util/virnetdevveth.c b/src/util/virnetdevveth.c
> index b3eee1af..b4074371 100644
> --- a/src/util/virnetdevveth.c
> +++ b/src/util/virnetdevveth.c
> @@ -27,6 +27,7 @@
>   #include "virfile.h"
>   #include "virstring.h"
>   #include "virnetdev.h"
> +#include "virnetlink.h"
>   
>   #define VIR_FROM_THIS VIR_FROM_NONE
>   
> @@ -75,6 +76,55 @@ static int virNetDevVethGetFreeNum(int startDev)
>       return -1;
>   }
>   
> +#if defined(WITH_LIBNL)
> +static int
> +virNetDevVethCreateInternal(const char *veth1, const char *veth2, int *status)
> +{
> +    virNetlinkNewLinkData data = { .veth_peer = veth2 };
> +
> +    return virNetlinkNewLink(veth1, "veth", &data, status);
> +}

The only thing that makes me uncomfortable in this patch is that the two 
versions of virNetDevVethCreateInternal() each return something 
different for "status". In this first case, it is returning 0 on 
success, and -errno on failure...


> [...]

> +#else
> +static int
> +virNetDevVethCreateInternal(const char *veth1, const char *veth2, int *status)
> +{
> +    g_autoptr(virCommand) cmd = virCommandNew("ip");
> +    virCommandAddArgList(cmd, "link", "add", veth1, "type", "veth",
> +                         "peer", "name", veth2, NULL);
> +
> +    return virCommandRun(cmd, status);
> +}


But in this case it is returning the exit code of the "ip link add" command.


It turns out that the value of status is only checked for 0 / not-0, so 
in practice it doesn't matter, but it could lead to confusion in the future.


If we want these patches to be applied before your "netdevveth: Simplify 
virNetDevVethCreate by using virNetDevGenerateName" patch (which I'll 
get to as soon as I send this mail), then we do still need a way to 
differentiate between "The requested device already exists" and 
"Permanent Failure", and in that case I would suggest that "status" be 
replaced with a variable called "retry" which would be set to true if 
retrying with a different device name might lead to success (it would be 
set based on an interpretation of status made by each of the 
vir*Internal() functions)


However, if we decide to apply the *other* patchset first, then we will 
never retry anyway (because we've already checked if the device exists 
before we try to create it, and there is therefore no loop) and so we 
could just eliminate the final argument completely, and keep each 
vir*Internal() function's status internal to itself. (As a matter of 
fact, status in the virCommandRun() version could simply be replaced 
with NULL in the arglist to virCommandRun(), and not defined at all; 
status in the case of virNetlinkNewLink() would still need to be defined 
and passed into the function, but its value would just be ignored).


I think it may be cleaner to do the latter, and it looks like your other 
series was written to be applied without this series anyway; let me look 
at those patches and I'll reply a 2nd time to this based on the results 
of reviewing the other series...


BTW, I appreciate your patience - I had looked at these patches nearly 
two weeks ago (soon after you sent them), but then a holiday got in the 
way, and I forgot to post my reply after I returned to work :-/


> [...]
> -
> -        if (virCommandRun(cmd, &status) < 0)
> +        if (virNetDevVethCreateInternal(*veth1 ? *veth1 : veth1auto,
> +                                        *veth2 ? *veth2 : veth2auto,
> +                                        &status) < 0)
>               goto cleanup;
>   
>           if (status == 0) {


This spot here ^ is the only place that status is examined, and I 
actually think it's not going to work as you'd like in the case of 
netlink - status will always be 0 if virNetDevVethCreateInternal() 
returns 0, and when the return is < 0, status may or may not be set to 
-errno of the attempted operation - if status is 0, that means there was 
a serious error before even trying to create the device (e.g. OOM), and 
if it is non-0, then it might be because a device with the desired name 
already exists, or it might be because we don't have enough privilege to 
create a new device.


Anyway, let me look at the other patchset...