Re: [Qemu-ppc] [Qemu-devel] [PATCH] Allow -sandbox off with --disable-seccomp

[Eric Blake]

On 2/27/19 5:01 AM, Daniel P. Berrangé wrote:
> On Wed, Feb 27, 2019 at 12:21:32PM +1100, David Gibson wrote:
>> At present, when seccomp support is compiled out with --disable-seccomp
>> we fail with an error if the user puts -sandbox on the command line.
>>
>> That kind of makes sense, but it's a bit strange that we reject a request
>> to disable sandboxing with "-sandbox off" saying we don't support
>> sandboxing.
>>
>> This puts in a small sandbox to (correctly) silently ignore -sandbox off
>> when we don't have sandboxing support compiled in.  This makes life easier
>> for testcases, since they can safely specify "-sandbox off" without having
>> to care if the qemu they're using is compiled with sandbox support or not.
>>
>> Signed-off-by: David Gibson <address@hidden>

> '-sandbox off'  is just syntax sugar for '-sandbox enable=off', with
> the default arg name handled by QemuOpts.

Except that libvirt probes, via query-command-line-options, whether the
'-sandbox' option supports the 'enable' key.  You risk breaking
introspection (where libvirt knows NOT to use enable=on|off) if -sandbox
enable=off is advertised even when the feature is not compiled in.

> 
> If we want to keep ability to run "-sandbox off" we should do it via
> the QEMU opts code we already have in the first part of the conditional,
> so that "-sandbox enable=off" also works as normal.
> 
> Essentially we need to push the #ifdef CONFIG_SECCOMP down into the
> parse_sandbox method, so that it parses the "enable" option normally
> but rejects any value except disabled..
> 
> The QemuOptsList should also be conditional to only register the
> "enable"  arg and not the other bits.

Whatever we do here had better be careful that the introspection used by
libvirt doesn't break, when libvirt is trying to learn when seccomp was
not compiled in.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org