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Re: [PATCH 2/2] net/tap: fix error reporting when bridge helper forgets
From: |
Philippe Mathieu-Daudé |
Subject: |
Re: [PATCH 2/2] net/tap: fix error reporting when bridge helper forgets to send an FD |
Date: |
Wed, 5 May 2021 14:32:04 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 |
On 5/5/21 2:12 PM, Daniel P. Berrangé wrote:
> The recv_fd() method returns -1 on error, 0 on end of file, or an FD
> number on success.
>
> Technically 0 is also a valid FD number, so the return value is
> ambiguous.
>
> The caller doesn't even consider the possibility of 0 meaning end
> of file, so just blindly assume it is a valid FD.
>
> IOW if the bridge helper exits with 0 status code and forgets to
> send an FD, QEMU will accidentally try to use STDIN as a TAP FD.
>
> Fortunately we get an error shortly thereafter
>
> $ qemu-system-x86_64 -netdev bridge,br=br99,helper=/bin/true,id=ns0
> qemu-system-x86_64: -netdev bridge,br=br99,helper=/bin/true,id=ns0:
> Unable to query TUNGETIFF on FD 0: Inappropriate ioctl for device
>
> It is better if we correctly diagnose this broken bridge helper
> though. To do this we need to return the FD in an output parameter
> to remove the ambiguity and then fix the caller to check for the
> end of file condition. With this done we now get
>
> $ qemu-system-x86_64 -netdev bridge,br=br99,helper=/bin/true,id=ns0
> qemu-system-x86_64: -netdev bridge,br=br99,helper=/bin/true,id=ns0:
> bridge helper did not send a file descriptor
>
> Fixes: https://gitlab.com/qemu-project/qemu/-/issues/166
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> ---
> net/tap.c | 32 ++++++++++++++++++++++----------
> 1 file changed, 22 insertions(+), 10 deletions(-)
> + /*
> + * ret == 0 means EOF, and if status == 0 then helper
> + * exited cleanly but forgot to send us an FD. Opps...
Opps or Oops?
Otherwise:
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>