Subject: Re: linux-next: umh: fix processed error when UMH_WAIT_PROC is used seems to break linux bridge on s390x (bisected)

From: Luis Chamberlain <[email-masked]>
Date: Tue, 30 Jun 2020 17:57:04 +0000

On Fri, Jun 26, 2020 at 02:54:10AM +0000, Luis Chamberlain wrote:
> On Wed, Jun 24, 2020 at 08:37:55PM +0200, Christian Borntraeger wrote:
> > 
> > 
> > On 24.06.20 20:32, Christian Borntraeger wrote:
> > [...]> 
> > > So the translations look correct. But your change is actually a sematic change
> > > if(ret) will only trigger if there is an error
> > > if (KWIFEXITED(ret)) will always trigger when the process ends. So we will always overwrite -ECHILD
> > > and we did not do it before. 
> > > 
> > 
> > So the right fix is
> > 
> > diff --git a/kernel/umh.c b/kernel/umh.c
> > index f81e8698e36e..a3a3196e84d1 100644
> > --- a/kernel/umh.c
> > +++ b/kernel/umh.c
> > @@ -154,7 +154,7 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
> >                  * the real error code is already in sub_info->retval or
> >                  * sub_info->retval is 0 anyway, so don't mess with it then.
> >                  */
> > -               if (KWIFEXITED(ret))
> > +               if (KWEXITSTATUS(ret))
> >                         sub_info->retval = KWEXITSTATUS(ret);
> >         }
> >  
> > I think.
> 
> Nope, the right form is to check for WIFEXITED() before using WEXITSTATUS().
> I'm not able to reproduce this on x86 with a bridge. What type of bridge
> are you using on a guest, or did you mean using KVM so that the *host*
> can spawn kvm guests?
> 
> It would be good if you can try to add a bridge manually and see where
> things fail. Can you do something like this:
> 
> brctl addbr br0
> brctl addif br0 ens6 
> ip link set dev br0 up
> 
> Note that most callers are for modprobe. I'd be curious to see which
> umh is failing which breaks bridge for you. Can you trut this so we can
> see which umh call is failing?

Christian, any luck getting to test the code below to see what this
reveals?

  Luis

> 
> diff --git a/kernel/umh.c b/kernel/umh.c
> index f81e8698e36e..5ad74bc301d8 100644
> --- a/kernel/umh.c
> +++ b/kernel/umh.c
> @@ -2,6 +2,9 @@
>  /*
>   * umh - the kernel usermode helper
>   */
> +
> +#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
> +
>  #include <linux/module.h>
>  #include <linux/sched.h>
>  #include <linux/sched/task.h>
> @@ -154,8 +157,12 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info)
>  		 * the real error code is already in sub_info->retval or
>  		 * sub_info->retval is 0 anyway, so don't mess with it then.
>  		 */
> -		if (KWIFEXITED(ret))
> +		printk("== ret: %02x\n", ret);
> +		printk("== KWIFEXITED(ret): %02x\n", KWIFEXITED(ret));
> +		if (KWIFEXITED(ret)) {
> +			printk("KWEXITSTATUS(ret): %d\n", KWEXITSTATUS(ret));
>  			sub_info->retval = KWEXITSTATUS(ret);
> +		}
>  	}
>  
>  	/* Restore default kernel sig handler */
> @@ -383,6 +390,7 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
>  		void *data)
>  {
>  	struct subprocess_info *sub_info;
> +	unsigned int i = 0;
>  	sub_info = kzalloc(sizeof(struct subprocess_info), gfp_mask);
>  	if (!sub_info)
>  		goto out;
> @@ -394,6 +402,11 @@ struct subprocess_info *call_usermodehelper_setup(const char *path, char **argv,
>  #else
>  	sub_info->path = path;
>  #endif
> +	pr_info("sub_info->path: %s\n", sub_info->path);
> +	while (argv[i])
> +		printk(KERN_INFO "%s ", argv[i++]);
> +	printk(KERN_INFO  "\n");
> +
>  	sub_info->argv = argv;
>  	sub_info->envp = envp;
>  
>