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Re: [Qemu-devel] [PATCH v3 3/3] net: tap: refactor net_bridge_run_helper


From: P J P
Subject: Re: [Qemu-devel] [PATCH v3 3/3] net: tap: refactor net_bridge_run_helper routine
Date: Fri, 5 Jul 2019 17:55:12 +0530 (IST)

+-- On Tue, 2 Jul 2019, P J P wrote --+
| |    -netdev bridge,helper="/path/to/helper myarg otherarg"
| | 
| | In theory any parts could contain shell meta characters, but even if
| | they don't we'll have slightly broken compat with this change.
| 
| I wonder if anybody uses it like that. Because of the 3 arguments that 
| qemu-bridge-helper takes
| 
|   --use-vnet --fd=sv[1] --br=bridge
| 
| only bridge name is supplied by user; Which is anyway comming without 
'helper' 
| having to include '--br=bridge' argument, as is looked for before shell 
| invocation
| 
|   if (strstr(helper, "--br=") == NULL) {
|       snprintf(br_buf, sizeof(br_buf), "%s%s", "--br=", bridge);
|   }
| 
| '--br=bridge' has limited scope to use shell meta characters, ie. other than 
| space(' ') and tab('\t').
| 
| 
| | The QEMU man page has never documented that you can pass a command
| | and args, which get sent via the shell though. It only ever documented
| | the helper arg as being a plain qualified binary path.
| | 
| | So the question is how strictly we need to consider compatibility.
| | 
| | The "if it isn't documented it never existed" option is to use your
| | patch here.
| 
| We don't know if "/path/to/helper arg1 arg2" usage exists in practice. And 
| considering user would still be able to supply 'bridge' argument, I wonder if 
| we are breaking compatibility.
| 
| | The safest option is to put in a place a deprecation saying we'll
| | drop use of shell in future, only implementing the aggressive
| | option in a later release.
| 
| ie. for Qemu > v4.0.0? How do we do this?
| 
| | Perhaps from your POV, the easy thing is to avoid this entire
| | question - just leave the code calling shell, but switch to
| | g_strdup_printf instead of snprintf.
| 
| Okay, this will be for Qemu <= v4.0.0?
| 

@Dan:...ping!?
--
Prasad J Pandit / Red Hat Product Security Team
47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F



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