Re: [Qemu-devel] [PATCH v2 2/4] linux-user: pass environment arguments in execve

[Joel Holdsworth <joel.holdsworth@vcatechnology.com>]

On 15/06/16 20:59, Laurent Vivier wrote:
>
> Le 14/06/2016 à 21:26, Joel Holdsworth a écrit :
>> Previously, when emulating execve(2), qemu would execute a child
>> instance of the emulator with the environment variables provided by
>> the parent process. This caused problems with qemu if any of the
>> variables affected the child emulator's behaviour e.g.
>> LD_LIBRARY_PATH.
> The best way to avoid that is to use a statically linked qemu.

Stepping back a bit; the problem I'm trying to solve is this...

There are some processes that invoke a helper process to do some work 
for them e.g. gstreamer's gst-plugin-scanner. Previously qemu would 
attempt to execute the helper executable as if it were machine-native, 
which won't work. These patches modify qemu so that it will (optionally) 
run the child process inside a child instance of qemu.

My experience as a user was that it took a couple of hours of searching 
through strace logs to figure out what the issue was. gstreamer would 
just fail with a generic error about the helper. These patches are meant 
to make qemu do the right thing.

Saying to the user that they should make a static linked build of qemu 
isn't very practical. Having a command line argument is a much easier 
solution for the user, that doesn't force them not to used 
shared-library builds. The distros aren't going to go for that.

Moreover, LD_LIBRARY_PATH is just one example. LD_PRELOAD is another. 
Timezone and locale environment variables are also an issue.

In an ideal world, it wouldn't even be necessary to add an argument - 
qemu would just do the right thing automatically. Though I'm not sure 
how that could be done correctly, so a command line option is a good 
compromise for a starting point.


>
>> This patch solves this issue by passing the environment variables
>> with '-E' arguments to the child qemu instance. The call to
>> execve(2) is replaced by a call to execv(2) so that the parent
>> emulator's environment variable state is propagated into the child.
>>
>> Any variables from the host environment that are not in the in the
>> execve() call are removed with a '-U' argument.
> Run ./scripts/checkpatch.pl on your patch...
>
> and add your Signed-off-by here.
Sure ok.


> The environment is already managed in linux-user/main.c:main(), I don't
> understand why the qemu_execve() special case should differ from the
> general case.
Maybe I've missed something, but the problem I'm trying to solve here is 
the issue of correctly propagating the guest environment variables into 
the child process.

The parent guest process (running inside qemu-user) constructs a set of 
environment variables and passes them to execve. However, if the parent 
qemu-user decides to run a child instance of qemu-user it should run 
with the same environment variables as the parent, but with environment 
variables the parent-guest passed through the arguments for the child-guest.

If gstreamer decides to run gst-plugin-scanner with a certain environ, 
that is for the child qemu guest, not the child qemu instance itself.