On Wed, Jul 01, 2015 at 05:36:06PM +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 local variable) which is no longer valid once function is finished and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.
Nice catch and it's a tricky bug hard to find. ACK and safe for release.
Pavel
>
> Signed-off-by: Michal Privoznik <mprivozn@xxxxxxxxxx>
> ---
>
> 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.
>
>
> src/lxc/lxc_process.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/src/lxc/lxc_process.c b/src/lxc/lxc_process.c
> index 6c23a0b..2bdce3b 100644
> --- a/src/lxc/lxc_process.c
> +++ b/src/lxc/lxc_process.c
> @@ -750,7 +750,7 @@ virLXCProcessBuildControllerCmd(virLXCDriverPtr driver,
> int *files,
> size_t nfiles,
> int handshakefd,
> - int logfd,
> + int * const logfd,
> const char *pidfile)
> {
> size_t i;
> @@ -820,8 +820,8 @@ virLXCProcessBuildControllerCmd(virLXCDriverPtr driver,
> virCommandPassFD(cmd, handshakefd, 0);
> virCommandDaemonize(cmd);
> virCommandSetPidFile(cmd, pidfile);
> - virCommandSetOutputFD(cmd, &logfd);
> - virCommandSetErrorFD(cmd, &logfd);
> + virCommandSetOutputFD(cmd, logfd);
> + virCommandSetErrorFD(cmd, logfd);
> /* So we can pause before exec'ing the controller to
> * write the live domain status XML with the PID */
> virCommandRequireHandshake(cmd);
> @@ -1208,7 +1208,7 @@ int virLXCProcessStart(virConnectPtr conn,
> ttyFDs, nttyFDs,
> files, nfiles,
> handshakefds[1],
> - logfd,
> + &logfd,
> pidfile)))
> goto cleanup;
>
> --
> 2.3.6
>
> --
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