linux-kernel - Re: dm: change dm_get_target_type() to check for module load error

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Date:   Wed, 6 Oct 2021 16:35:05 -0400
From:   Mike Snitzer <snitzer@...hat.com>
To:     Shuah Khan <skhan@...uxfoundation.org>
Cc:     agk@...hat.com, dm-devel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: dm: change dm_get_target_type() to check for module load error

On Mon, Oct 04 2021 at  4:06P -0400,
Shuah Khan <skhan@...uxfoundation.org> wrote:

> dm_get_target_type() doesn't check error return from request_module().
> Change to check for error and return NULL instead of trying to get
> target type again which would fail.
> 
> Signed-off-by: Shuah Khan <skhan@...uxfoundation.org>
> ---
>  drivers/md/dm-target.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/dm-target.c b/drivers/md/dm-target.c
> index 64dd0b34fcf4..0789e9f91d3a 100644
> --- a/drivers/md/dm-target.c
> +++ b/drivers/md/dm-target.c
> @@ -41,17 +41,22 @@ static struct target_type *get_target_type(const char *name)
>  	return tt;
>  }
>  
> -static void load_module(const char *name)
> +static int load_module(const char *name)
>  {
> -	request_module("dm-%s", name);
> +	return request_module("dm-%s", name);
>  }
>  
>  struct target_type *dm_get_target_type(const char *name)
>  {
>  	struct target_type *tt = get_target_type(name);
> +	int ret;
>  
>  	if (!tt) {
> -		load_module(name);
> +		ret = load_module(name);
> +		if (ret < 0) {
> +			pr_err("Module %s load failed %d\n", name, ret);
> +			return NULL;
> +		}
>  		tt = get_target_type(name);
>  	}
>  
> -- 
> 2.30.2
> 

While I appreciate your intent, the reality is that multiple targets
may be made available in a given module.  And so loading one dm module
may bring in access to N targets.  There isn't a rigid 1:1 mapping of
target modules to names.  And there may not even be a loadable module
that has the name dm-${name} -- but that doesn't mean the target_type
won;t have been loaded into DM for it to access.

So all said, your patch is bogus and would break DM and user
experience:

Nacked-by: Mike Snitzer <snitzer@...hat.com>

But thanks for raising your concern.