linux-kernel - Re: [PATCH v2 2/4] fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn

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Date:   Fri, 13 Mar 2020 09:06:46 +1100
From:   NeilBrown <neilb@...e.de>
To:     Eric Biggers <ebiggers@...nel.org>, linux-kernel@...r.kernel.org
Cc:     linux-fsdevel@...r.kernel.org, Alexei Starovoitov <ast@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeff Vander Stoep <jeffv@...gle.com>,
        Jessica Yu <jeyu@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        NeilBrown <neilb@...e.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 2/4] fs/filesystems.c: downgrade user-reachable WARN_ONCE() to pr_warn_once()

On Thu, Mar 12 2020, Eric Biggers wrote:

> From: Eric Biggers <ebiggers@...gle.com>
>
> After request_module(), nothing is stopping the module from being
> unloaded until someone takes a reference to it via try_get_module().
>
> The WARN_ONCE() in get_fs_type() is thus user-reachable, via userspace
> running 'rmmod' concurrently.
>
> Since WARN_ONCE() is for kernel bugs only, not for user-reachable
> situations, downgrade this warning to pr_warn_once().
>
> Cc: stable@...r.kernel.org
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jeff Vander Stoep <jeffv@...gle.com>
> Cc: Jessica Yu <jeyu@...nel.org>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Luis Chamberlain <mcgrof@...nel.org>
> Cc: NeilBrown <neilb@...e.com>
> Signed-off-by: Eric Biggers <ebiggers@...gle.com>
> ---
>  fs/filesystems.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/fs/filesystems.c b/fs/filesystems.c
> index 77bf5f95362da..90b8d879fbaf3 100644
> --- a/fs/filesystems.c
> +++ b/fs/filesystems.c
> @@ -272,7 +272,9 @@ struct file_system_type *get_fs_type(const char *name)
>  	fs = __get_fs_type(name, len);
>  	if (!fs && (request_module("fs-%.*s", len, name) == 0)) {
>  		fs = __get_fs_type(name, len);
> -		WARN_ONCE(!fs, "request_module fs-%.*s succeeded, but still no fs?\n", len, name);
> +		if (!fs)
> +			pr_warn_once("request_module fs-%.*s succeeded, but still no fs?\n",
> +				     len, name);

I strongly support the replacement of "WARN" by "pr_warn".
I wonder if we really want the "once" now.  Possibly using rate_limited
would be justified, but I think that in general we should see a warning
every time this event happens.

Thanks,
NeilBrown


>  	}
>  
>  	if (dot && fs && !(fs->fs_flags & FS_HAS_SUBTYPE)) {
> -- 
> 2.25.1

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