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Date:   Wed, 18 Apr 2018 12:42:51 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Chengguang Xu <cgxu519@gmx.com>
Cc:     linux-fsdevel@vger.kernel.org, dhowells@redhat.com,
        kstewart@linuxfoundation.org, gregkh@linuxfoundation.org,
        tglx@linutronix.de, pombredanne@nexb.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] hfs: fix potential refcnt problem of nls module
Message-Id: <20180418124251.d66a36cb23673f0d8b152910@linux-foundation.org>
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On Tue, 17 Apr 2018 15:05:32 +0800 Chengguang Xu <cgxu519@gmx.com> wrote:

> When specifying iocharset/codepage multiple times in a mount,
> current option parsing will cause inaccurate refcount of nls
> module. Hence, call unload_nls for previous one in this case.
> 
> ...
>
> --- a/fs/hfs/super.c
> +++ b/fs/hfs/super.c
> @@ -329,8 +329,10 @@ static int parse_options(char *options, struct hfs_sb_info *hsb)
>  				return 0;
>  			}
>  			p = match_strdup(&args[0]);
> -			if (p)
> +			if (p) {
> +				unload_nls(hsb->nls_disk);
>  				hsb->nls_disk = load_nls(p);
> +			}
>  			if (!hsb->nls_disk) {
>  				pr_err("unable to load codepage \"%s\"\n", p);
>  				kfree(p);
> @@ -344,8 +346,10 @@ static int parse_options(char *options, struct hfs_sb_info *hsb)
>  				return 0;
>  			}
>  			p = match_strdup(&args[0]);
> -			if (p)
> +			if (p) {
> +				unload_nls(hsb->nls_io);
>  				hsb->nls_io = load_nls(p);
> +			}
>  			if (!hsb->nls_io) {
>  				pr_err("unable to load iocharset \"%s\"\n", p);
>  				kfree(p);

Confused.

			break;
: 		case opt_codepage:
: 			if (hsb->nls_disk) {
: 				pr_err("unable to change codepage\n");
: 				return 0;
: 			}

Here, hsb->nls_disk is known to be zero.

: 			p = match_strdup(&args[0]);
: 			if (p) {
: 				unload_nls(hsb->nls_disk);

So this will always do unload_nls(0).

: 				hsb->nls_disk = load_nls(p);
: 			}

And the same applies to your opt_iocharset change.