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

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 However, in the function we try to get its address (an address of a local variable) which is no longer valid once function is finished and stack is cleaned.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(a)redhat.com&gt; --- 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.