[libvirt] [PATCH] lxc: Don't pass a local variable address randomly

Peter Krempa pkrempa at redhat.com
Wed Jul 1 15:48:32 UTC 2015

On Wed, Jul 01, 2015 at 17:36:06 +0200, Michal Privoznik wrote:
> So, recently I was testing the LXC driver. You know, startup some
> domains. But to my surprise, I was not able to start a single one:
>   virsh # start --console test
>   error: Reconnected to the hypervisor
>   error: Failed to start domain test
>   error: internal error: guest failed to start: unexpected exit status 125
> So I've start digging. It turns out, that in virExec(), when I printed
> out the @cmd, I got strange values: *(cmd->outfdptr) was certainly not
> valid FD number: it has random value of several millions. This
> obviously made prepareStdFd(childout, STDOUT_FILENO) fail (line 611).
> But outfdptr is set in virCommandSetOutputFD(). The only place within
> LXC driver where the function is called is in
> virLXCProcessBuildControllerCmd(). If you take a closer look at the
> function it looks like this:
> static virCommandPtr
> virLXCProcessBuildControllerCmd(virLXCDriverPtr driver,
>                                 ..
>                                 int logfd,
>                                 const char *pidfile)
> {
>     ...
>     virCommandSetOutputFD(cmd, &logfd);
>     virCommandSetErrorFD(cmd, &logfd);
>     ...
> }
> Yes, you guessed it. @logfd is passed into the function by value.
> However, in the function we try to get its address (an address of a
> local variable) which is no longer valid once function is finished and
> stack is cleaned. Therefore when cmd->outfdptr is evaluated at any
> point after this function, we may get a random number, depending on
> what's currently on the stack. Of course, this may work sometimes too
> - it depends on the compiler how it arranges the code, when the stack
> is wiped out.
> In order to fix this, lets pass a pointer to @logfd instead of
> figuring out (wrong) its value in a function.
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
> I'd love to target this into the current release - in my case LXC
> driver is unusable without this patch. There might be other users
> affected too.

ACK in freeze. BTW it was caused by commit e1de5521.


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