Re: [PATCH] util: netdevbridge: fall back to ioctl from sysfs

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On Tue, Nov 20, 2018 at 11:17:07AM +0100, Christian Ehrhardt wrote:
> There are certain cases e.g. containers where the sysfs path might
> exists, but might fail. Unfortunately the exact restrictions are only
> known to libvirt when trying to write to it so we need to try it.
> 
> But in case it fails there is no need to fully abort, in those cases try
> to fall back to the older ioctl interface which can still work.
> 
> That makes setting up a bridge in unprivileged LXD containers work.
> 
> Fixes: https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/1802906
> 
> Signed-off-by: Christian Ehrhardt <christian.ehrhardt@xxxxxxxxxxxxx>
> ---
>  src/util/virnetdevbridge.c | 48 +++++++++++++++++++++-----------------
>  1 file changed, 26 insertions(+), 22 deletions(-)
> 
> diff --git a/src/util/virnetdevbridge.c b/src/util/virnetdevbridge.c
> index 071ebb7b35..cbba3c9652 100644
> --- a/src/util/virnetdevbridge.c
> +++ b/src/util/virnetdevbridge.c
> @@ -113,6 +113,8 @@ static int virNetDevBridgeCmd(const char *brname,
>   * or by  ioctl on older kernels. Perhaps we could just use
>   * ioctl for every kernel, but its not clear what the long
>   * term lifespan of the ioctl interface is...
> + * Fall back to ioctl if sysfs interface is not available or
> + * failing (e.g. due to container isolation).
>   */
>  static int virNetDevBridgeSet(const char *brname,
>                                const char *paramname,  /* sysfs param name */
> @@ -128,29 +130,31 @@ static int virNetDevBridgeSet(const char *brname,
>      if (virFileExists(path)) {
>          char valuestr[INT_BUFSIZE_BOUND(value)];
>          snprintf(valuestr, sizeof(valuestr), "%lu", value);
> -        if (virFileWriteStr(path, valuestr, 0) < 0) {
> -            virReportSystemError(errno,
> -                                 _("Unable to set bridge %s %s"), brname, paramname);
> -            return -1;
> -        }
> +        if (virFileWriteStr(path, valuestr, 0) >= 0)
> +            return 0;
> +        virReportSystemError(errno,
> +                             _("Unable to set bridge %s %s via sysfs"),
> +                             brname, paramname);

The virReportSystemError line should be dropped, since we're not
returning an error - we're moving onto the second code path instead.

> +    }
> +
> +    unsigned long paramid;
> +    if (STREQ(paramname, "stp_state")) {
> +        paramid = BRCTL_SET_BRIDGE_STP_STATE;
> +    } else if (STREQ(paramname, "forward_delay")) {
> +        paramid = BRCTL_SET_BRIDGE_FORWARD_DELAY;
>      } else {
> -        unsigned long paramid;
> -        if (STREQ(paramname, "stp_state")) {
> -            paramid = BRCTL_SET_BRIDGE_STP_STATE;
> -        } else if (STREQ(paramname, "forward_delay")) {
> -            paramid = BRCTL_SET_BRIDGE_FORWARD_DELAY;
> -        } else {
> -            virReportSystemError(EINVAL,
> -                                 _("Unable to set bridge %s %s"), brname, paramname);
> -            return -1;
> -        }
> -        unsigned long args[] = { paramid, value, 0, 0 };
> -        ifr->ifr_data = (char*)&args;
> -        if (ioctl(fd, SIOCDEVPRIVATE, ifr) < 0) {
> -            virReportSystemError(errno,
> -                                 _("Unable to set bridge %s %s"), brname, paramname);
> -            return -1;
> -        }
> +        virReportSystemError(EINVAL,
> +                             _("Unable to set bridge %s %s via ioctl"),
> +                             brname, paramname);
> +        return -1;
> +    }
> +    unsigned long args[] = { paramid, value, 0, 0 };
> +    ifr->ifr_data = (char*)&args;
> +    if (ioctl(fd, SIOCDEVPRIVATE, ifr) < 0) {
> +        virReportSystemError(errno,
> +                             _("Failed to set bridge %s %s via ioctl"),
> +                             brname, paramname);
> +        return -1;
>      }
>  
>      return 0;

Regards,
Daniel
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